In light of the current models for an early RNA-based universe, the potential influence of simple amino acids on tertiary folding of ribozymal RNA into biochemically competent structures is speculated to be of significant evolutionary importance. In the present work, the folding–unfolding kinetics of a ubiquitous tertiary interaction motif, the GAAA tetraloop–tetraloop receptor (TL–TLR), is investigated by single-molecule fluorescence resonance energy transfer spectroscopy in the presence of natural amino acids both with (e.g., lysine, arginine) and without (e.g., glycine) protonated side chain residues. By way of control, we also investigate the effects of a special amino acid (e.g., proline) and amino acid mimetic (e.g., betaine) that contain secondary or quaternary amine groups rather than a primary amine group. This combination permits systematic study of amino acid induced (or amino acid like) RNA folding dynamics as a function of side chain complexity, pKa, charge state, and amine group content. Most importantly, each of the naturally occurring amino acids is found to destabilize the TL–TLR tertiary folding equilibrium, the kinetic origin of which is dominated by a decrease in the folding rate constant (kdock), also affected by a strongly amino acid selective increase in the unfolding rate constant (kundock). To further elucidate the underlying thermodynamics, single-molecule equilibrium constants (Keq) for TL–TLR folding have been probed as a function of temperature, which reveal an amino acid dependent decrease in both overall exothermicity (ΔΔH° > 0) and entropic cost (−TΔΔS° < 0) for the overall folding process. Temperature-dependent studies on the folding/unfolding kinetic rate constants reveal analogous amino acid specific changes in both enthalpy (ΔΔH⧧) and entropy (ΔΔS⧧) for accessing the transition state barrier. The maximum destabilization of the TL–TLR tertiary interaction is observed for arginine, which is consistent with early studies of arginine and guanidine ion-inhibited self-splicing kinetics for the full Tetrahymena ribozyme [Yarus, M.; Christian, E. L. Nature 1989, 342, 349−350; Yarus, M. Science 1988, 240, 1751–1758].

Thermal and hyperthermal HCl (v = 0, J = 0) collision dynamics at the surface of methyl-terminated self-assembled monolayers (SAMs) are probed by state-selective ionization followed by velocity-map imaging (VMI) to yield a full 2π steradian map of final 3D velocity distributions (vx, vy, vz) as a function of rovibrational (v, J) quantum state. “DC slicing” of the scattered HCl flux normal to the surface (vz) provides a powerful tool for eliminating incident beam contamination, as well as access to fully correlated, 3D flux weighted rovibrational quantum state + translational scattering dynamics in unprecedented detail. At low collision energies (Einc ≈ 0.7(1) kcal/mol), the scattering dynamics are completely dominated by trapping-desorption (TD) events, for which both external (i.e., translational) and internal (i.e., rovibrational) degrees of freedom quantitatively track the SAM surface temperature (TS). Hyperthermal scattering data at high collision energies (Einc ≈ 17(1) kcal/mol) provide direct evidence for growth of an additional nonequilibrium, impulsive scattering (IS) channel, with a strong forward scattering propensity broadly distributed around the specular angle. The competition between linear and angular momentum transfer for such a rapidly rotating hydride species (BHCl ≈ 10 cm–1) is investigated in the IS channel, which reveals strong retention of translational energy with only modest rotational excitation (κtrans ≈ 48(7)%, κrot ≈ 6(2)%) and in clear contrast with studies of more slowly tumbling species (BCO2 ≈ 0.4 cm–1) such as CO2 (κtrans ≈ 6(2)%, κrot ≈ 20(4)%). Most importantly, the combination of (i) full 2π steradian angular data with (ii) full quantum state resolution permits a model free deconstruction of the experimental velocity map images into TD and IS components, which provides striking, independent confirmation of the hyperthermal yet Boltzmann-like nature of both the (i) IS quantum state and the (ii) out-of-plane momentum distributions. In summary, this novel combination of VMI with quantum state resolved scattering techniques provides powerful synergistic opportunities for correlated investigation of quantum state resolved reactive and inelastic energy transfer dynamics at gas–liquid-like interfaces with chemically “tunable” surface moieties.

Two-photon fluorescence microscopy of single quantum dots conditions has been reported by several groups,1−3 with contrasting observations regarding the kinetics and dynamics of fluorescence intermittency or “blinking”. Here, we investigate the power dependence, kinetics, and statistics of two photon-excited fluorescence intermittency from single CdSe/ZnS quantum dots in a solid PMMA film as a function of sub-bandgap laser intensity at 800 nm. Fluorescence intermittency is observed at all excitation powers and a quadratic (n = 1.97(3)) dependence of the shot noise-limited fluorescence intensity on the incident laser power is verified, confirming essentially zero background contribution from one-photon excitation processes. Such analyses permit two photon absorption cross sections for single quantum dots to be extracted quantitatively from the data, which reveal good agreement with those obtained from previous two-photon FCS measurements. Strictly inverse power law-distributed off-state dwell times are observed for all excitation powers, with a mean power law exponent ⟨moff⟩ = 1.65(4) in excellent agreement with the behavior observed under one-photon excitation conditions. Finally, a superquadratic (n = 2.3(2)) rather than quartic (n = 4) power dependence is observed for the on-state blinking dwell times, which we kinetically analyze and interpret in terms of a novel 2 + 1 “hot” exciton ionization/blinking mechanism due to partially saturated 1-photon sub-bandgap excitation out of the two-photon single exciton state. The kinetic results are consistent with quantum dot photoionization quantum yields from “hot” exciton states (4(1) × 10–6) comparable with experimental estimates (10–6–10–5) of Auger ionization efficiencies out of the biexcitonic state.

Fundamental, bending (ν6, ν7, ν8, ν9), and CC-stretch (ν2, ν3) hot band spectra in the antisymmetric CH stretch (ν4) region near 3330 cm–1 have been observed and analyzed for jet cooled diacetylene (HC≡C–C≡CH) under sub-Doppler conditions. Diacetylene is generated in situ in the throat of a pulsed supersonic slit expansion by discharge dissociation of acetylene to form ethynyl (C≡CH) + H, followed by radical attack (HC≡CH + C≡C–H) to form HC≡C–C≡CH + H. The combination of (i) sub-Doppler line widths and (ii) absence of spectral congestion permits rotational structure and Coriolis interactions in the ν4 CH stretch fundamental to be observed and analyzed with improved precision. Of particular dynamical interest, the spectra reveal diacteylene formation in highly excited internal vibrational states. Specifically, multiple Π ← Π and Δ ← Δ hot bands built on the ν4 CH stretch fundamental are observed, due to doubly degenerate bending vibrations [cis C≡C–H bend (ν6), trans C–C≡C bend (ν7), trans C≡C–H bend (ν8) and cis C–C≡C bend (ν9)], as well as a heretofore unobserved Σ ← Σ band assigned to excitation of ν2 or 2ν3 CC stretch. Boltzmann analysis yields populations consistent with universally cold rotations (Trot ≈ 15 ± 5 K) and yet superthermal vibrations (Tvib ≈ 85–430 K), the latter of which is quite anomalous for the high collision densities in a slit jet expansion. In order to elucidate the physical mechanism for this excess vibrational excitation, high level ab initio CCSD(T) calculations have been pursued with explicitly correlated basis sets (VnZ-f12; n = 2,3) and extrapolated to the complete basis set (CBS) limit using MOLPRO quantum chemistry software. The results suggest that the extensive hot band structure observed arises from (i) highly exothermic CCH + HCCH addition to yield a strongly bent HCCHCCH radical intermediate (ΔH = −62.6 kcal/mol), followed by (ii) rapid fragmentation over a submerged transition state barrier (ΔH = −18.9 kcal/mol) to form vibrationally hot diacetylene + H products (ΔH = −25.6 kcal/mol), and consistent with crossed molecular beam studies by Kaiser et al. [Phys. Chem. Chem. Phys. 2002, 4, 2950.] Finally, RRKM fragmentation rates for this complex are calculated, which exceed collision frequencies in the slit jet expansion and suggest near unity quantum efficiency for diacetylene formation.

Room temperature ionic liquids (RTILs) offer an extremely promising new class of solvents with chemical control of bulk gas solubility, but surprisingly little is known about detailed molecular scale interactions at the gas–liquid interface. In this work, quantum state-to-state resolved collision dynamics at the gas–liquid interface are studied by scattering a jet-cooled molecular beam of ground state NO(2Π1/2; N = 0) molecules from 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (i.e., [bmim]+[Tf2N]−) RTIL, with the resulting rovibronic state distributions probed via laser-induced fluorescence as a function of incident collision energy (Einc) and surface temperature (Ts). Significant excitation is observed from ground (2Π1/2) to excited (2Π3/2) spin–orbit states, highlighting the presence of electronically nonadiabatic effects at the gas–RTIL interface sensitive to both Einc and Ts. At low collision energies (Einc = 2.7(9) kcal/mol), the two spin–orbit manifold rotational distributions are well described by a single temperature, but with (i) Trot(2Π1/2) consistently 30 K lower than Trot(2Π3/2), and (ii) both temperatures lower than Ts. At high collision energies (Einc = 20(6) kcal/mol), the rotational populations are well fit to two-temperature “trapping-desorption” (TD) and “impulsively scattered” (IS) distributions, with the branching ratio into the TD channel (α) for 2Π1/2 consistently higher than that for the spin–orbit excited 2Π3/2 state. From detailed balance considerations these rotational temperatures, in both the low collision energy and TD component of the high collision energy scattered flux, imply the presence of electronic and rotational state dependent trapping-desorption probabilities and provide new theoretical challenges to high level modeling of collision dynamics at the gas–RTIL interface.

Fluorescence intermittency in single semiconductor nanocrystals has been shown to follow power law statistics over many decades in time and in probability. Recently, several studies have shown that, while “off” dwell times are insensitive to almost all experimental parameters, “on” dwell times exhibit a pump-power dependent exponential truncation at long times, suggestive of enhanced biexciton photoionization probabilities at high excitation powers. Here we report the dependence of this on-time truncation on nanocrystal radius. We observe a decrease in the per-pulse photoionization probability from 1.8(2) × 10–4 to 2.0(7) × 10–6 as the CdSe core radius increases from 1.3 to 3.5 nm, with a radius scaling for the probability for charge ejection arising from biexciton formation Pionize(r) ∝ 1/r3.5(5). Effective mass calculations of the exciton wave functions show that the product of fractional electron and hole probabilities in the trap-rich ZnS shell scale similarly with nanocrystal radius. Possible charge ejection mechanisms from such a surface-localized state are discussed.

Multiphoton photoelectron emission from individual Au nanorods deposited on indium tin oxide (ITO) substrates is studied via scanning photoionization microscopy, based on femtosecond laser excitation at frequencies near the rod longitudinal surface plasmon resonance (LSPR). The observed resonances in photoemission correlate strongly with plasmon resonances measured in dark field microscopy (DFM), thus establishing a novel scheme for wavelength-resolved study of plasmons in isolated metallic nanoparticles based on highly sensitive electron counting methods. In this work, we explore experimental and theoretical effects of (i) morphology and (ii) aspect ratio (AR) for longitudinal plasmon resonance behavior in Au nanorods. A quasilinear dependence between LSPR and aspect ratio (AR) is experimentally determined [Δλ ≈ +100(10) nm/AR unit] for Au nanorods on ITO, in excellent agreement with the first principles value from finite element computer modeling [Δλ = +108(5) nm/AR unit]. Interestingly, however, LSPR values for larger vs. smaller diameter rods (w ≈ 20 nm and 10 nm) are systematically red-shifted [ΔE ≈ −0.03(1) eV; Δλ ≈ +15(5) nm at λ ≈ 800 nm], indicating that electromagnetic retardation effects must also be considered for highest accuracy in LSPR position. To augment these results, the influence of the dielectric environment on the rod LSPR has been explored both experimentally and numerically. In particular, detailed finite-element simulations for ITO supported Au nanorods are found to yield plasmon resonances in near quantitative agreement (ΔE ≈ ±0.04 eV) with experiment, with residual differences arising from uncertainty in the refractive index of the ITO thin film. Furthermore, the results indicate that plasmon resonance predictions based on infinitely thick ITO substrates are reliable to a few meV for film thicknesses larger than approximately twice the rod width.

Phenyl radical has been studied via sub-Doppler infrared spectroscopy in a slit supersonic discharge expansion source, with assignments for the highest frequency b2 out-of-phase C–H symmetric stretch vibration (ν19) unambiguously confirmed by ≤6 MHz (0.0002 cm–1) agreement with microwave ground state combination differences of McMahon et al. [Astrophys. J. 2003, 590, L61–64]. Least squares analysis of over 100 resolved rovibrational peaks in the sub-Doppler spectrum to a Watson Hamiltonian yields precision excited-state rotational constants and a vibrational band origin (ν0 = 3071.8915(4) cm–1) consistent with a surprisingly small red-shift (0.9 cm–1) with respect to Ar matrix isolation studies of Ellison and co-workers [J. Am. Chem. Soc. 2001, 123, 1977]. Nuclear spin weights and inertial defects confirm the vibrationally averaged planarity and 2A1 rovibronic symmetry of phenyl radical, with analysis of the rotational constants consistent with a modest C2v distortion of the carbon backbone frame due to partial sp rehybridization of the σ C radical-center. Most importantly, despite the number of atoms (N = 11) and vibrational modes (3N – 6 = 27), phenyl radical exhibits a remarkably clean jet cooled high-resolution IR spectrum that shows no evidence of intramolecular vibrational relaxation (IVR) phenomena such as local or nonlocal perturbations due to strongly coupled nearby dark states. This provides strong support for the feasibility of high-resolution infrared spectroscopy in other aromatic hydrocarbon radical systems.

Multiphoton photoelectron emission from individual SiO2 core–Au shell nanoparticles supported on an ITO substrate is studied with ultrafast scanning photoemission imaging microscopy. Higher than expected photoemission yields (∼105-fold) and a strong sensitivity to excitation laser polarization direction indicate the presence of anomalously high electromagnetic field enhancement areas (i.e., “hot spots”) on the surface of Au nanoshells. The measured magnitude of the photoelectron current is consistent with 1–2 localized hot spots on each nanoparticle exhibiting electric near-field enhancement factors of nominally |E|/|E0| ∼ 50–100. Secondary electron microscopy (SEM) studies reveal asperities on the surface of each nanoparticle that most likely arise due to postsynthetic Ostwald ripening of the Au shell layer. However, no correlation is found between these features and the laser polarization that yields the maximum photoelectron emissivity, indicating that the hot spots responsible for the observed high electron emission rates are smaller than our SEM resolution of ∼3–5 nm. Numerical electrodynamics simulations of near-field enhancements (|E|/|E0| ∼ 20) for the two most commonly observed defect geometries (i.e., asperities and pinholes) can account for <20–50% of the experimentally inferred values. The larger near-field enhancements observed experimentally thus provide indirect evidence for sharp asperities and crevices in Au nanoshells considerably below the optical diffraction limit.

Electron emission from individual Au nanorods deposited on indium–tin–oxide (ITO) following excitation with femtosecond laser pulses near the rod longitudinal plasmon resonance is studied via scanning photoionization microscopy. The measured electron signal is observed to strongly depend on the excitation laser polarization and wavelength. Correlated secondary electron microscopy (SEM) and dark-field microscopy (DFM) studies of the same nanorods unambiguously confirm that maximum electron emission results from (i) laser polarization aligned with the rod long axis and (ii) laser wavelength resonant with the localized surface plasmon resonance. The experimental results are in good agreement with quantitative predictions for a coherent multiphoton photoelectric effect, which is identified as the predominant electron emission mechanism for metal nanoparticles under employed excitation conditions. According to this mechanism, the multiphoton photoemission rate is increased by over 10 orders of magnitude in the vicinity of a localized surface plasmon resonance, due to enhancement of the incident electromagnetic field in the particle near-field. These findings identify multiphoton photoemission as an extremely sensitive metric of local electric fields (i.e., “hot spots”) in plasmonic nanoparticles/structures that can potentially be exploited for direct quantitation of local electric field enhancement factors.Keywords: Au nanorods; coherent multiphoton photoelectron emission; localized surface plasmon resonance; scanning photoionization microscopy; single-particle dark-field scattering; ultrafast excitation

Electron emission from single, supported Ag nanocubes excited with ultrafast laser pulses (λ = 800 nm) is studied via spatial and polarization correlated (i) dark field scattering microscopy (DFM), (ii) scanning photoionization microscopy (SPIM), and (iii) high-resolution transmission electron microscopy (HRTEM). Laser-induced electron emission is found to peak for laser polarization aligned with cube diagonals, suggesting the critical influence of plasmonic near-field enhancement of the incident electric field on the overall electron yield. For laser pulses with photon energy below the metal work function, coherent multiphoton photoelectron emission (MPPE) is identified as the most probable mechanism responsible for electron emission from Ag nanocubes and likely metal nanoparticles/surfaces in general.

This work describes a novel surface-scattering technique which combines resonance enhanced multiphoton ionization (REMPI) with velocity-map imaging (VMI) to yield quantum-state and 2D velocity component resolved distributions in the scattered molecular flux. As an initial test system, we explore hyperthermal scattering (Einc = 21(5) kcal mol−1) of jet cooled HCl from Au(111) on atomically flat mica surfaces at 500 K. The resulting images reveal 2D (vin-plane and vout-of-plane) velocity distributions dominated by two primary features: trapping/thermal-desorption (TD) and a hyperthermal, impulsively scattering (IS) distribution. In particular, the IS component is strongly forward scattered and largely resolved in the velocity map images, which allows us to probe correlations between rotational and translational degrees of freedom in the IS flux without any model dependent deconvolution from the TD fraction. These correlations reveal that HCl molecules which have undergone a large decrease in velocity parallel to scattering plane have actually gained the most rotational energy, reminiscent of a dynamical energy constraint between these two degrees of freedom. The data are reduced to a rotational energy map that correlates 〈Erot〉 with velocity along and normal to the scattering plane, revealing that exchange occurs primarily between rotation and the in-plane kinetic energy component, with vout-of-plane playing a relatively minor role.

The lysine riboswitch is a cis-acting RNA genetic regulatory element found in the leader sequence of bacterial mRNAs coding for proteins related to biosynthesis or transport of lysine. Structural analysis of the lysine-binding aptamer domain of this RNA has revealed that it completely encapsulates the ligand and therefore must undergo a structural opening/closing upon interaction with lysine. In this work, single-molecule fluorescence resonance energy transfer (FRET) methods are used to monitor these ligand-induced structural transitions that are central to lysine riboswitch function. Specifically, a model FRET system has been developed for characterizing the lysine dissociation constant as well as the opening/closing rate constants for the Bacillus subtilis lysC aptamer domain. These techniques permit measurement of the dissociation constant (KD) for lysine binding of 1.7(5) mM and opening/closing rate constants of 1.4(3) s–1 and 0.203(7) s–1, respectively. These rates predict an apparent dissociation constant for lysine binding (KD,apparent) of 0.25(9) mM at near physiological ionic strength, which differs markedly from previous reports.

Quantum mechanics/molecular mechanics (QM/MM) molecular dynamics simulations have been carried out to model the scattering of hyperthermal (15 kcal/mol) CO2 on the surfaces of two common imidazolium based room-temperature ionic liquids (RTILs) [bmim][BF4] and [bmim][Tf2N]. Good agreement was achieved in comparison with experiment. The [bmim][BF4] surface is found to be more absorptive of CO2 than [bmim][Tf2N], which leads to greater loss in translational energy and less rotational excitation of CO2’s that scatter from [bmim][BF4]. These differences are found to result from a interplay of differences in the structure of the interface and the strength of interactions that depend on anion identity. Our results also suggest that CO2 interacts strongly with ionic species on the RTIL surfaces due to the large induced dipole moments on CO2 during the collisions. The inclusion of electronic polarization is critical in determining the final rotational excitation of CO2 compared to results from an MM model with fixed charge.

Mg2+ is essential for the proper folding and function of RNA, though the effect of Mg2+ concentration on the free energy, enthalpy, and entropy landscapes of RNA folding is unknown. This work exploits temperature-controlled single-molecule FRET methods to address the thermodynamics of RNA folding pathways by probing the intramolecular docking/undocking kinetics of the ubiquitous GAAA tetraloop−receptor tertiary interaction as a function of [Mg2+]. These measurements yield the barrier and standard state enthalpies, entropies, and free energies for an RNA tertiary transition, in particular, revealing the thermodynamic origin of [Mg2+]-facilitated folding. Surprisingly, these studies reveal that increasing [Mg2+] promotes tetraloop–receptor interaction by reducing the entropic barrier () and the overall entropic penalty () for docking, with essentially negligible effects on both the activation enthalpy () and overall exothermicity (). These observations contrast with the conventional notion that increasing [Mg2+] facilitates folding by minimizing electrostatic repulsion of opposing RNA helices, which would incorrectly predict a decrease in and with [Mg2+]. Instead we propose that higher [Mg2+] can aid RNA folding by decreasing the entropic penalty of counterion uptake and by reducing disorder of the unfolded conformational ensemble.

This work investigates plasmon-enhanced multiphoton scanning photoelectron emission microscopy (SPIM) of single gold nanorods under vacuum conditions. Striking differences in their photoemission properties are observed for nanorods deposited either on 2 nm thick Pt films or 10 nm thick indium tin oxide (ITO) films. On a Pt support, the Au nanorods display fourth-order photoionization when excited at 800 nm, a wavelength corresponding to their plasmon resonance in aqueous solution. A cos8(θ) dependence of the photoelectron flux on laser polarization implies photoemission mediated by the dipolar plasmon; however, no plasmon resonance signature is exhibited over the 750−880 nm range. Electromagnetic simulations confirm that the resonance is severely broadened compared to aqueous solution, indicative of strong interactions between the Au nanorod and propagating surface plasmon modes in the Pt substrate. On ITO substrates, by way of contrast, sharp plasmon resonances in the photoemission from individual Au nanorods are observed, with widths limited only by fundamental internal electron collision processes. Furthermore, the ensemble-averaged plasmon resonance for Au nanorods on ITO is almost unshifted compared to its frequency in solution. Both findings suggest that plasmonic particle−substrate interactions are suppressed in the Au/ITO system. However, Au nanorods on ITO exhibit a surprising third-order photoemission (observed neither in Au nor ITO by itself), indicating that electrostatic interactions introduce a substantial shift in the work function for this fundamental nanoparticle−substrate system.Keywords: gold nanorod; multiphoton photoemission; photoelectron emission; plasmon resonance; polarization dependence; ultrafast electron dynamics

Stable, intensely Raman active silver nanoparticles are photogenerated by visible light from silver ions in a thin polymer film within a diffraction-limited focal area. The emission is resolved both spectrally and temporally to demonstrate that the source of the signal is surface-enhanced Raman scattering (SERS) from multiple silver nanoparticles generated in the diffraction-limited spot. The time evolution of the SERS signal is sigmoidal in shape and well described by Avrami phase transformation kinetics. The rate constant for the Avrami transformation depends linearly on illumination intensity, consistent with single photon photoreduction of the silver percholorate starting material as the limiting step to form silver nanoparticles. The asymptotic kinetic growth SERS signal exhibits a linear dependence on illumination intensity. Avrami analysis of the kinetics indicates that transformation is constrained to two dimensions, consistent with the ∼10 nm thin film nature of the sample. The technique presented provides a novel route to large-scale periodic molecular sensor arrays with long-term stability, diffraction-limited resolution (<1 μm), and laser-based spatial/temporal control of the formation kinetics.

We present results on state-resolved scattering studies for seeded CO2 supersonically cooled molecular beams (Einc = 61.9(40) kJ/mol) from a series of room-temperature ionic liquids (RTILs). These RTILs are composed of Cn-methylimidazolium cations with BF4– or Tf2N– counteranions. The final rovibrational quantum state distributions from these nonequilibrium surface scattering collisions are monitored by high-resolution diode laser absorption spectroscopy as a function of (i) cation alkyl chain length and (ii) anion size, and analyzed to yield the propensity for thermal desorption (TD) versus impulsive scattering (IS) dynamics. For a fixed BF4– or Tf2N– counteranion, the distributions reveal an increase in the TD fraction (α) with the C atom number (n) in the alkyl side chain, which provides evidence for selective preference of nonpolar groups at the gas–liquid interface with increasing chain length. Conversely, for short carbon chains (n = 4), the thermal fraction decreases when the anion is changed from a compact and less polarizable BF4– to the bulkier and more polarizable Tf2N–, whereas any sensitivity to anion identity essentially vanishes for longer alkyl chains (n = 8, 12). These combined data illustrate a number of interesting trends in anion versus cation competition for interfacial sites, specifically (i) the presence of interfacial anions at the surface layer for sufficiently short alkyl headgroups, (ii) inertial “stiffening” due to increasing average surface mass, as well as (iii) a propensity for larger anion sizes in the interfacial region. Finally, the TD probabilities follow the exact opposite trend in “bulk” Henry’s Law solubility constants with respect to anion size, which further highlights the intrinsically nonequilibrium dynamics sampled by hyperthermal collisions at the gas–liquid interface.

Full three dimensional (3D) translational distributions for quantum state-resolved scattering dynamics at the gas–liquid interface are presented for experimental and theoretical studies of CO2 + perfluorinated surfaces. Experimentally, high resolution absorption profiles are measured as a function of incident (θinc) and scattering (θscat) angles for CO2 that has been scattered from a 300 K perfluorinated polyether surface (PFPE) with an incident energy of Einc = 10.6(8) kcal mol−1. Line shape analysis of the absorption profiles reveals non-equilibrium dynamics that are characterized by trapping–desorption (TD) and impulsive scattering (IS) components, with each channel simply characterized by an effective “temperature” that compares very well with previous results from rotational state analysis [Perkins and Nesbitt, J. Phys. Chem. A, 2008, 112, 9324]. From a theoretical perspective, molecular dynamics (MD) simulations of CO2 + fluorinated self-assembled monolayer surface (F-SAMs) yield translational probability distributions that are also compared with experimental results. Trajectories are parsed by θscat and J, with the results rigorously corrected by flux-to-density transformation and providing comparisons in near quantitative agreement with experiment. 3D flux and velocity distributions obtained from MD simulations are also presented to illustrate the role of in- and out-of-plane scattering.

Rotational orientation/alignment dynamics of CO2 scattered from a perfluorinated polyether (PFPE) liquid surface has been investigated via direct absorption experimental studies and theoretical molecular dynamics (MD) simulations. Experimentally, polarization modulation of a single mode diode laser is combined with lock-in detection to measure circular/linear IR polarizance due to CO2 scattering from the surface at θinc = 60° and Einc = 10.6(8) kcal/mol and probed over a series of final scattering angles. The differential absorption intensities are related through Fano−Macek theory to the three lowest multipole moments (A0, A2+, and O1−) which describe collisionally induced orientation and alignment at the liquid surface. The total scattering population reflects both trapping-desorption (TD) and impulsive scattering (IS) components, with a strong positive anisotropy in the MJ distribution that indicates preferential CO2 scattering from the surface with a forward (i.e., “topspin”) sense of end-over-end tumbling. Theoretical trajectory simulations provide 3D CO2 flux and J state distributions scattering from fluorinated self-assembled monolayers (F-SAMs) and are compared with experimental results as a function of final rotational state. Specifically, trends in the theoretical orientation/alignment moments are in remarkable agreement over the full range of J states but with values consistently overpredicted by nearly 2-fold, which may reflect a higher level of local ordering for F-SAMS vs a gas−PFPE liquid interface.

Supported room-temperature ionic liquid (RTIL) membranes are of industrial interest as possible carbon-sequestration and CO2-scrubbing agents, specifically in natural gas reforming processes. We report state-resolved scattering studies of CO2 from alkylimidazolium-based RTIL surfaces, containing either BF4− or Tf2N− anions. The scattered CO2 exhibits a two-temperature rotational distribution, well described in the context of a trapping−desorption/impulsive scattering (TD/IS) model. We find that the scattering dynamics are highly dependent upon the anion identity, implying that the anions are present at the interface and play a role in the gas−liquid collisions. Trends in the trapping−desorption fractions are in stark opposition with trends in the bulk Henry’s Law solubility constants, clearly highlighting the critical role of nonequilibrium dynamics over equilibrium bulk solubility properties in these high-energy collisions. These results are discussed in the context of composition, geometry, and local corrugation at the interface.Keywords: CO2; ionic liquid interfaces; room-temperature ionic liquids; state-resolved scattering;

RNA folding thermodynamics are crucial for structure prediction, which requires characterization of both enthalpic and entropic contributions of tertiary motifs to conformational stability. We explore the temperature dependence of RNA folding due to the ubiquitous GAAA tetraloop−receptor docking interaction, exploiting immobilized and freely diffusing single-molecule fluorescence resonance energy transfer (smFRET) methods. The equilibrium constant for intramolecular docking is obtained as a function of temperature (T = 21−47 °C), from which a van’t Hoff analysis yields the enthalpy (ΔH°) and entropy (ΔS°) of docking. Tetraloop−receptor docking is significantly exothermic and entropically unfavorable in 1 mM MgCl2 and 100 mM NaCl, with excellent agreement between immobilized (ΔH° = −17.4 ± 1.6 kcal/mol, and ΔS° = −56.2 ± 5.4 cal mol−1 K−1) and freely diffusing (ΔH° = −17.2 ± 1.6 kcal/mol, and ΔS° = −55.9 ± 5.2 cal mol−1 K−1) species. Kinetic heterogeneity in the tetraloop−receptor construct is unaffected over the temperature range investigated, indicating a large energy barrier for interconversion between the actively docking and nondocking subpopulations. Formation of the tetraloop−receptor interaction can account for ∼60% of the ΔH° and ΔS° of P4−P6 domain folding in the Tetrahymena ribozyme, suggesting that it may act as a thermodynamic clamp for the domain. Comparison of the isolated tetraloop−receptor and other tertiary folding thermodynamics supports a theme that enthalpy- versus entropy-driven folding is determined by the number of hydrogen bonding and base stacking interactions.

Quantum state-resolved energy transfer dynamics at the gas−liquid interface are explored through a comparison of classical molecular dynamics (MD) simulations and previously reported experimental studies (Perkins, B. G.; et al. J. Phys. Chem. A 2008, 112, 9234). Theoretically, large scale MD trajectory calculations have been performed for collisions of CO2 with a model fluorinated self-assembled monolayer surface (F-SAMs), based on an explicit atom-atom interaction potential obtained from earlier theoretical studies (Martínez-Núñez, E.; et al. J. Phys. Chem. C 2007, 111, 354). Initial conditions for the simulations match those in the experimental studies where high-energy jet-cooled CO2 molecules (Einc = 10.6(8) kcal/mol, <Erot> ≈ 10 cm−1) are scattered from a 300 K perfluorinated liquid surface (PFPE) from a range of incident angles (θinc = 0−60°). Nascent CO2 rotational distributions prove to be remarkably well characterized by a simple two-temperature trapping-desorption (TD) and impulsive scattering (IS) model with nearly quantitative agreement between experimental and theoretical column integrated densities. Furthermore, three-dimensional (3D) quantum state resolved flux maps for glancing incident angles (θinc ≈ 60°) reveal broad, lobular distributions peaking strongly in the forward subspecular direction as cosn(θscat − θ′) with n ≈ 5.6(1.2) and θ′ ≈ 49(2)°.

The first high-resolution IR spectra of a jet-cooled phenyl radical are reported, obtained via direct absorption laser spectroscopy in a slit-jet discharge supersonic expansion. The observed A-type band arises from fundamental excitation of the out-of-phase symmetric CH stretch mode (ν19) of b2 symmetry. Unambiguous spectral assignment of the rotational structure to the phenyl radical is facilitated by comparison with precision 2-line combination differences from Fourier transform microwave and direct absorption mm-wave measurements on the ground state [R. J. McMahon et al., Astrophys. J., 2003, 590, L61]. Least-squares fits to an asymmetric top Hamiltonian permit the upper-state rotational constants to be obtained. The corresponding gas-phase vibrational band origin at 3071.8904 (10) cm−1 is in remarkably good agreement with previous matrix isolation studies [A. V. Friderichsen et al., J. Am. Chem. Soc., 2001, 123, 1977], and indicates only a relatively minor red shift (≈0.9 cm−1) between the gas and Ar matrix phase environment. Such studies offer considerable promise for further high resolution IR study of other aromatic radical species of particular relevance to combustion phenomena and interstellar chemistry.

Large-amplitude tunneling in vinyl radical over a C2v planar transition state involves CCH bending excitation coupled to all other internal coordinates, resulting in a significant dependence of barrier height and shape on vibrational degrees of freedom at the zero-point level. An ab initio potential surface for vinyl radical has been calculated at the CCSD(T) level (AVnZ; n = 2, 3, 4, 5) for vibrationally adiabatic 1D motion along the planar CCH bending tunneling coordinate, extrapolated to the complete basis set (CBS) limit and corrected for anharmonic zero-point effects. The polyatomic reduced moment of inertia is calculated explicitly as a function of tunneling coordinate, with eigenvalues and tunneling splittings obtained from numerical solution of the resulting 1D Schrödinger equation. Linear scaling of the CBS potential to match predicted and observed tunneling splittings empirically yields an adiabatic barrier height of ΔEadiab = 1696(20) cm−1 which, when corrected for zero-point energy contributions, translates into an effective barrier of ΔEeff = 1602(20) cm−1 consistent with estimates (ΔE = 1580(100) cm−1) by Tanaka and coworkers [J. Chem. Phys., 2004, 120, 3604–3618]. These zero-point-corrected potential surfaces are used to predict tunneling dynamics in vibrationally excited states of vinyl radical, providing strong support for previous jet-cooled high-resolution infrared studies [Dong et al., J. Phys. Chem. A, 2006, 110, 3059–3070] in the symmetric CH2 stretch mode.

Molecular beam scattering dynamics at the gas−liquid interface are investigated for CO2 (Einc = 10.6(8) kcal/mol) impinging on liquid perfluoropolyether (PFPE), with quantum state (v, J) populations measured as a function of incident (θinc) and final (θscat) scattering angles. The internal state distributions are well-characterized for both normal and grazing incident angles by a two-component Boltzmann model for trapping desorption (TD) and impulsive scattering (IS) at rotational temperatures Trot(TD/IS), where the fractional TD probability for CO2 on the perfluorinated surface is denoted by TD and IS densities (ρ) as α = ρTD/(ρTD + ρIS). On the basis of an assumed cos(θscat) scattering behavior for the TD flux component, the angular dependence of the IS flux at normal incidence (θinc = 0°) is surprisingly well-modeled by a simple cosn(θscat) distribution with n = 1.0 ± 0.2, while glancing incident angles (θinc = 30°, 45°, and 60°) result in lobular angular IS distributions scattered preferentially in the forward direction. This trend is also corroborated in the TD fraction α, which decreases rapidly under non-normal incident conditions as a function of backward versus forward scattering direction. Furthermore, the extent of rotational excitation in the IS channel increases dramatically with increasing angle of incidence, consistent with an increasing rotational torque due to surface roughness at the gas−liquid interface.

Stereodynamics at the gas–liquid interface provides insight into the important physical interactions that directly influence heterogeneous chemistry at the surface and within the bulk liquid. We investigate molecular beam scattering of CO₂ from a liquid perfluoropolyether (PFPE) surface in vacuum [incident energy E_inc = 10.6(8) kcal/mol, incident angle θ_inc = 60°] to specifically reveal rotational angular-momentum directions for scattered molecules. Experimentally, internal quantum state populations and M_J distributions are probed by high-resolution polarization-modulated infrared laser spectroscopy. Analysis of J-state populations reveals dual-channel scattering dynamics characterized by a two-temperature Boltzmann distribution for trapping–desorption and impulsive scattering. In addition, molecular dynamics simulations of CO₂ + fluorinated self-assembled monolayers have been used to model CO₂ + PFPE dynamics. Experimental results and molecular dynamics simulations reveal highly oriented CO₂ distributions that preferentially scatter with “top spin” as a strongly increasing function of J state.

Abstract

We present two quantum calculations of the infrared spectrum of protonated methane (CH₅⁺) using full-dimensional, ab initio–based potential energy and dipole moment surfaces. The calculated spectra compare well with a low-resolution experimental spectrum except below 1000 cm^–1, where the experimental spectrum shows no absorption. The present calculations find substantial absorption features below 1000 cm^–1, in qualitative agreement with earlier classical calculations of the spectrum. The major spectral bands are analyzed in terms of the molecular motions. Of particular interest is an intense feature at 200 cm^–1, which is due to an isomerization mode that connects two equivalent minima. Very recent high-resolution jet-cooled spectra in the CH stretch region (2825 to 3050 cm^–1) are also reported, and assignments of the band origins are made, based on the present quantum calculations.

Docking kinetics and equilibrium of fluorescently labeled RNA molecules are studied with single-molecule FRET methods. Time-resolved FRET is used to monitor docking/undocking transitions for RNAs containing a single GAAA tetraloop-receptor tertiary interaction connected by a flexible single-stranded linker. The rate constants for docking and undocking are measured as a function of Mg2+, revealing a complex dependence on metal ion concentration. Despite the simplicity of this model system, conformational heterogeneity similar to that noted in more complex RNA systems is observed; relatively rapid docking/undocking transitions are detected for approximately two-thirds of the RNA molecules, with significant subpopulations exhibiting few or no transitions on the 10- to 30-s time scale for photobleaching. The rate constants are determined from analysis of probability densities, which allows a much wider range of time scales to be analyzed than standard histogram procedures. The data for the GAAA tetraloop receptor are compared with kinetic and equilibrium data for other RNA tertiary interactions.

The fluorescence of single ZnS overcoated CdSe quantum dots (QDs) embedded in the evanescent optical field above a prism surface has been studied using an apertureless near-field scanning optical microscope (ANSOM). We demonstrate that the fluorescence intensity of an individual QD can be enhanced by as much as fivefold for a silicon (Si) probe located over the QD. Furthermore, the ratio of fluorescence intensity with the probe directly over versus adjacent to the QD is substantially greater (up to 90-fold). This higher contrast in the ANSOM images results from interference between direct and probe-scattered fluorescence for the adjacent-probe geometry. These optical enhancements are very sensitive to probe tip cleaning, while this has negligible effect on the topological images. The typical full width at half maximum (FWHM) of the ANSOM images is 30–40 nm, while the FWHM of the topographic image is ∼20 nm. While this optical FWHM is 10–20-fold below the optical diffraction limit, conversion from non-contact to intermittent contact mode is predicted to increase the enhancements and decrease the ANSOM FWHM by major factors.

In this work, the kinetics of short, fully complementary oligonucleotides are investigated at the single-molecule level. Constructs 6–9 bp in length exhibit single exponential kinetics over 2 orders of magnitude time for both forward (kon, association) and reverse (koff, dissociation) processes. Bimolecular rate constants for association are weakly sensitive to the number of basepairs in the duplex, with a 2.5-fold increase between 9 bp (k′on = 2.1(1) × 106 M−1 s−1) and 6 bp (k′on = 5.0(1) × 106 M−1 s−1) sequences. In sharp contrast, however, dissociation rate constants prove to be exponentially sensitive to sequence length, varying by nearly 600-fold over the same 9 bp (koff = 0.024 s−1) to 6 bp (koff = 14 s−1) range. The 8 bp sequence is explored in more detail, and the NaCl dependence of kon and koff is measured. Interestingly, kon increases by >40-fold (kon = 0.10(1) s−1 to 4.0(4) s−1 between [NaCl] = 25 mM and 1 M), whereas in contrast, koff decreases by fourfold (0.72(3) s−1 to 0.17(7) s−1) over the same range of conditions. Thus, the equilibrium constant (Keq) increases by ≈160, largely due to changes in the association rate, kon. Finally, temperature-dependent measurements reveal that increased [NaCl] reduces the overall exothermicity (ΔΔH° > 0) of duplex formation, albeit by an amount smaller than the reduction in entropic penalty (−TΔΔS° < 0). This reduced entropic cost is attributed to a cation-facilitated preordering of the two single-stranded species, which lowers the association free-energy barrier and in turn accelerates the rate of duplex formation.

For RNA to fold into compact, ordered structures, it must overcome electrostatic repulsion between negatively charged phosphate groups by counterion recruitment. A physical understanding of the counterion-assisted folding process requires addressing how cations kinetically and thermodynamically control the folding equilibrium for each tertiary interaction in a full‐length RNA. In this work, single-molecule FRET (fluorescence resonance energy transfer) techniques are exploited to isolate and explore the cation-concentration‐dependent kinetics for formation of a ubiquitous RNA tertiary interaction, that is, the docking/undocking of a GAAA tetraloop with its 11‐nt receptor. Rate constants for docking (kdock) and undocking (kundock) are obtained as a function of cation concentration, size, and valence, specifically for the series Na+, K+, Mg2 +, Ca2 +, Co(NH3)63 +, and spermidine3 +. Increasing cation concentration accelerates kdock dramatically but achieves only a slight decrease in kundock. These results can be kinetically modeled using parallel cation-dependent and cation‐independent docking pathways, which allows for isolation of the folding kinetics from the interaction energetics of the cations with the undocked and docked states, respectively. This analysis reveals a preferential interaction of the cations with the transition state and docked state as compared to the undocked RNA, with the ion–RNA interaction strength growing with cation valence. However, the corresponding number of cations that are taken up by the RNA upon folding decreases with charge density of the cation. The only exception to these behaviors is spermidine3 +, whose weaker influence on the docking equilibria with respect to Co(NH3)63 + can be ascribed to steric effects preventing complete neutralization of the RNA phosphate groups.Download high-res image (87KB)Download full-size imageHighlights► Single-molecule FRET study of cation-induced RNA folding. ► Kinetics of tetraloop–receptor docking/undocking versus cation valence and size. ► Increasing cation concentration accelerates docking and decelerates undocking. ► Cation valence is the primary determinant of efficacy in promoting RNA folding. ► Critical kinetic and thermodynamic benchmarks for cation-mediated RNA folding.

Proper assembly of RNA into catalytically active three-dimensional structures requires multiple tertiary binding interactions, individual characterization of which is crucial to a detailed understanding of global RNA folding. This work focuses on single-molecule fluorescence studies of freely diffusing RNA constructs that isolate the GAAA tetraloop-receptor tertiary interaction. Freely diffusing conformational dynamics are explored as a function of Mg2+ and Na+ concentration, both of which promote facile docking, but with 500-fold different affinities. Systematic shifts in mean fluorescence resonance energy transfer efficiency values and line widths with increasing [Na+] are observed for the undocked species and can be interpreted with a Debye model in terms of electrostatic relaxation and increased flexibility in the RNA. Furthermore, we identify a 34 ± 2% fraction of freely diffusing RNA constructs remaining undocked even at saturating [Mg2+] levels, which agrees quantitatively with the 32 ± 1% fraction previously reported for immobilized constructs. This verifies that the kinetic heterogeneity observed in the docking rates is not the result of surface tethering. Finally, the KD value and Hill coefficient for [Mg2+]-dependent docking decrease significantly for [Na+] = 25 mM vs. 125 mM, indicating Mg2+ and Na+ synergy in the RNA folding process.

The first high-resolution IR spectra of a jet-cooled phenyl radical are reported, obtained via direct absorption laser spectroscopy in a slit-jet discharge supersonic expansion. The observed A-type band arises from fundamental excitation of the out-of-phase symmetric CH stretch mode (ν19) of b2 symmetry. Unambiguous spectral assignment of the rotational structure to the phenyl radical is facilitated by comparison with precision 2-line combination differences from Fourier transform microwave and direct absorption mm-wave measurements on the ground state [R. J. McMahon et al., Astrophys. J., 2003, 590, L61]. Least-squares fits to an asymmetric top Hamiltonian permit the upper-state rotational constants to be obtained. The corresponding gas-phase vibrational band origin at 3071.8904 (10) cm−1 is in remarkably good agreement with previous matrix isolation studies [A. V. Friderichsen et al., J. Am. Chem. Soc., 2001, 123, 1977], and indicates only a relatively minor red shift (≈0.9 cm−1) between the gas and Ar matrix phase environment. Such studies offer considerable promise for further high resolution IR study of other aromatic radical species of particular relevance to combustion phenomena and interstellar chemistry.

Full three dimensional (3D) translational distributions for quantum state-resolved scattering dynamics at the gas–liquid interface are presented for experimental and theoretical studies of CO2 + perfluorinated surfaces. Experimentally, high resolution absorption profiles are measured as a function of incident (θinc) and scattering (θscat) angles for CO2 that has been scattered from a 300 K perfluorinated polyether surface (PFPE) with an incident energy of Einc = 10.6(8) kcal mol−1. Line shape analysis of the absorption profiles reveals non-equilibrium dynamics that are characterized by trapping–desorption (TD) and impulsive scattering (IS) components, with each channel simply characterized by an effective “temperature” that compares very well with previous results from rotational state analysis [Perkins and Nesbitt, J. Phys. Chem. A, 2008, 112, 9324]. From a theoretical perspective, molecular dynamics (MD) simulations of CO2 + fluorinated self-assembled monolayer surface (F-SAMs) yield translational probability distributions that are also compared with experimental results. Trajectories are parsed by θscat and J, with the results rigorously corrected by flux-to-density transformation and providing comparisons in near quantitative agreement with experiment. 3D flux and velocity distributions obtained from MD simulations are also presented to illustrate the role of in- and out-of-plane scattering.

This work describes a novel surface-scattering technique which combines resonance enhanced multiphoton ionization (REMPI) with velocity-map imaging (VMI) to yield quantum-state and 2D velocity component resolved distributions in the scattered molecular flux. As an initial test system, we explore hyperthermal scattering (Einc = 21(5) kcal mol−1) of jet cooled HCl from Au(111) on atomically flat mica surfaces at 500 K. The resulting images reveal 2D (vin-plane and vout-of-plane) velocity distributions dominated by two primary features: trapping/thermal-desorption (TD) and a hyperthermal, impulsively scattering (IS) distribution. In particular, the IS component is strongly forward scattered and largely resolved in the velocity map images, which allows us to probe correlations between rotational and translational degrees of freedom in the IS flux without any model dependent deconvolution from the TD fraction. These correlations reveal that HCl molecules which have undergone a large decrease in velocity parallel to scattering plane have actually gained the most rotational energy, reminiscent of a dynamical energy constraint between these two degrees of freedom. The data are reduced to a rotational energy map that correlates 〈Erot〉 with velocity along and normal to the scattering plane, revealing that exchange occurs primarily between rotation and the in-plane kinetic energy component, with vout-of-plane playing a relatively minor role.

Multiphoton photoelectron emission from individual Au nanorods deposited on indium tin oxide (ITO) substrates is studied via scanning photoionization microscopy, based on femtosecond laser excitation at frequencies near the rod longitudinal surface plasmon resonance (LSPR). The observed resonances in photoemission correlate strongly with plasmon resonances measured in dark field microscopy (DFM), thus establishing a novel scheme for wavelength-resolved study of plasmons in isolated metallic nanoparticles based on highly sensitive electron counting methods. In this work, we explore experimental and theoretical effects of (i) morphology and (ii) aspect ratio (AR) for longitudinal plasmon resonance behavior in Au nanorods. A quasilinear dependence between LSPR and aspect ratio (AR) is experimentally determined [Δλ ≈ +100(10) nm/AR unit] for Au nanorods on ITO, in excellent agreement with the first principles value from finite element computer modeling [Δλ = +108(5) nm/AR unit]. Interestingly, however, LSPR values for larger vs. smaller diameter rods (w ≈ 20 nm and 10 nm) are systematically red-shifted [ΔE ≈ −0.03(1) eV; Δλ ≈ +15(5) nm at λ ≈ 800 nm], indicating that electromagnetic retardation effects must also be considered for highest accuracy in LSPR position. To augment these results, the influence of the dielectric environment on the rod LSPR has been explored both experimentally and numerically. In particular, detailed finite-element simulations for ITO supported Au nanorods are found to yield plasmon resonances in near quantitative agreement (ΔE ≈ ±0.04 eV) with experiment, with residual differences arising from uncertainty in the refractive index of the ITO thin film. Furthermore, the results indicate that plasmon resonance predictions based on infinitely thick ITO substrates are reliable to a few meV for film thicknesses larger than approximately twice the rod width.

Large-amplitude tunneling in vinyl radical over a C2v planar transition state involves CCH bending excitation coupled to all other internal coordinates, resulting in a significant dependence of barrier height and shape on vibrational degrees of freedom at the zero-point level. An ab initio potential surface for vinyl radical has been calculated at the CCSD(T) level (AVnZ; n = 2, 3, 4, 5) for vibrationally adiabatic 1D motion along the planar CCH bending tunneling coordinate, extrapolated to the complete basis set (CBS) limit and corrected for anharmonic zero-point effects. The polyatomic reduced moment of inertia is calculated explicitly as a function of tunneling coordinate, with eigenvalues and tunneling splittings obtained from numerical solution of the resulting 1D Schrödinger equation. Linear scaling of the CBS potential to match predicted and observed tunneling splittings empirically yields an adiabatic barrier height of ΔEadiab = 1696(20) cm−1 which, when corrected for zero-point energy contributions, translates into an effective barrier of ΔEeff = 1602(20) cm−1 consistent with estimates (ΔE = 1580(100) cm−1) by Tanaka and coworkers [J. Chem. Phys., 2004, 120, 3604–3618]. These zero-point-corrected potential surfaces are used to predict tunneling dynamics in vibrationally excited states of vinyl radical, providing strong support for previous jet-cooled high-resolution infrared studies [Dong et al., J. Phys. Chem. A, 2006, 110, 3059–3070] in the symmetric CH2 stretch mode.