Gas-phase cyclobutyl radical (•C4H7) is produced via pyrolysis of cyclobutylmethyl nitrite (C4H7(CH2)ONO). Other •C4H7 radicals, such as 1-methylallyl and allylcarbinyl, are similarly produced from nitrite precursors. Nascent radicals are promptly solvated in liquid He droplets, allowing for the acquisition of infrared spectra in the CH stretching region. For the cyclobutyl and 1-methylallyl radicals, anharmonic frequencies are predicted by VPT2+K simulations based upon a hybrid CCSD(T) force field with quadratic (cubic and quartic) force constants computed using the ANO1 (ANO0) basis set. A density functional theoretical method is used to compute the force field for the allylcarbinyl radical. For all •C4H7 radicals, resonance polyads in the 2800–3000 cm–1 region appear as a result of anharmonic coupling between the CH stretching fundamentals and CH2 bend overtones and combinations. Upon pyrolysis of the cyclobutylmethyl nitrite precursor to produce the cyclobutyl radical, an approximately 2-fold increase in the source temperature leads to the appearance of spectral signatures that can be assigned to 1-methylallyl and 1,3-butadiene. On the basis of a previously reported •C4H7 potential energy surface, this result is interpreted as evidence for the unimolecular decomposition of the cyclobutyl radical via ring opening, prior to it being captured by helium droplets. On the •C4H7 potential surface, 1,3-butadiene is formed from cyclobutyl ring opening and H atom loss, and the 1-methylallyl radical is the most energetically stable intermediate along the decomposition pathway. The allylcarbinyl radical is a higher-energy •C4H7 intermediate along the ring-opening path, and the spectral signatures of this radical are not observed under the same conditions that produce 1-methylallyl and 1,3-butadiene from the unimolecular decomposition of cyclobutyl.

The entrance channel complex in the exothermic OH + CH4 → H2O + CH3 reaction has been isolated in helium nanodroplets following the sequential pick-up of the hydroxyl radical and methane. The a-type OH stretching band was probed with infrared depletion spectroscopy, revealing a spectrum qualitatively similar to that previously reported in the gas phase, but with additional substructure that is due to the different internal rotation states of methane (jCH4 = 0, 1, or 2) in the complex. We fit the spectra by assuming the rotational constants of the complex are the same for all internal rotation states; however, subband origins are found to decrease with increasing jCH4. Measurements of deuterated complexes have also been made (OD–CH4, OH–CD4, and OD–CD4), the relative linewidths of which provide information about the flow of vibrational energy in the complexes; vibrational lifetime broadening is prominent for OH–CH4 and OD–CD4, for which the excited OX stretching state has a nearby CY4 stretching fundamental (X, Y = H or D).

Water clusters are formed in helium droplets via the sequential capture of monomers. One or two neon atoms are added to each droplet prior to the addition of water. The infrared spectrum of the droplet ensemble reveals several signatures of polar, water tetramer clusters having dipole moments between 2D and 3D. Comparison with ab initio computations supports the assignment of the cluster networks to noncyclic “3 + 1” clusters, which are ∼5.3 kcal/mol less stable than the global minimum nonpolar cyclic tetramer. The (H2O)3Ne + H2O ring insertion barrier is sufficiently large, such that evaporative helium cooling is capable of kinetically quenching the nonequilibrium tetramer system prior to its rearrangement to the lower energy cyclic species. To this end, the reported process results in the formation of exotic water cluster networks that are either higher in energy than the most stable gas-phase analogs or not even stable in the gas phase.

Catalytic thermal cracking of O2 is employed to dope helium droplets with O(3P) atoms. Mass spectrometry of the doped droplet beam reveals an O2 dissociation efficiency larger than 60%; approximately 26% of the droplet ensemble is doped with single oxygen atoms. Sequential capture of O(3P) and HCN leads to the production of a hydrogen-bound O–HCN complex in a 3Σ electronic state, as determined via comparisons of experimental and theoretical rovibrational Stark spectroscopy. Ab initio computations of the three lowest lying intermolecular potential energy surfaces reveal two isomers, the hydrogen-bound (3Σ) O–HCN complex and a nitrogen-bound (3Π) HCN–O complex, lying 323 cm–1 higher in energy. The HCN–O to O–HCN interconversion barrier is predicted to be 42 cm–1. Consistent with this relatively small interconversion barrier, there is no experimental evidence for the production of the nitrogen-bound species upon sequential capture of O(3P) and HCN.

The infrared spectrum of the X̃2E2″ tropyl radical has been recorded in the range of the CH-stretch vibrational modes using the helium droplet isolation technique. Two bands are observed at 3053 and 3058 cm–1. The electronic degeneracy of the ground state results in a Jahn–Teller interaction for two of the CH-stretch modes, i.e., first-order interaction for E3′ symmetry modes and second-order interaction for E2′ symmetry modes. The experimentally observed bands are assigned to the E1′ and E3′ CH-stretch modes. The E1′ mode is infrared-active, whereas the E3′ mode is inactive in the absence of the Jahn–Teller interaction. The transition to the upper component of the Jahn–Teller split E3′ mode gains intensity via vibronic coupling, giving rise to the second experimentally observed band.

Chlorine atoms, generated through the thermal decomposition of Cl2, are solvated in superfluid helium nanodroplets and clustered with HCl molecules. The H–Cl stretching modes of these clusters are probed via infrared laser spectroscopy. A band centered at ∼2880.8 cm–1 is assigned to the binary Cl–HCl complex on the basis of HCl pressure dependence and difference mass spectra. The band lies in the “free” HCl stretching region, implying that the complex is not hydrogen bound. Furthermore, the breadth of the band (∼2 cm–1 fwhm) is consistent with an assignment to a predominantly b-type component of the H–Cl stretch, as the dominant b-type selection rules and A rotational constant allow for high energy rotational excitations that efficiently couple to droplet excitations, resulting in fast rotational deactivation. Despite the lack of rotational fine structure, which would verify the assignment, the observed band is consistent with the stabilization of a weakly bound complex having an approximately L-shaped geometry. Frequency computations for a rigid, L-shaped complex reveal that the transition dipole moment vector points almost entirely along the b inertial axis; indeed, the signal-to-noise ratio in our experiment precluded the observation of an a-type component of the HCl stretching band for the complex. No bands were observed that could be assigned to a linear H-bonded Cl–HCl complex. Additionally, we located bands that are consistent with the formation of Cl2–HCl, Cl2–(HCl)2, and Cl–(HCl)2. Two vibrations of the Cl–(HCl)2 complex were found, and harmonic frequencies and intensities computed for a cyclic structure are consistent with the observations.

Substantial non-Arrhenius behavior has been previously observed in the low temperature reaction between the hydroxyl radical and methanol. This behavior can be rationalized assuming the stabilization of an association adduct in the entrance channel of the reaction, from which barrier penetration via quantum mechanical tunneling produces the CH3O radical and H2O. Helium nanodroplet isolation and a serial pick-up technique are used to stabilize the hydrogen bonded prereactive OH··CH3OH complex. Mass spectrometry and infrared spectroscopy are used to confirm its production and probe the OH stretch vibrations. Stark spectroscopy reveals the magnitude of the permanent electric dipole moment, which is compared to ab initio calculations that account for wide-amplitude motion in the complex. The vibrationally averaged structure has Cs symmetry with the OH moiety hydrogen bonded to the hydroxyl group of methanol. Nevertheless, the zero-point level of the complex exhibits a wave function significantly delocalized over a bending coordinate leading to the transition state of the CH3O producing reaction.

Vibrational spectroscopy and helium nanodroplet isolation are used to determine the gas-phase thermochemistry for isomerization between conformations of the model dipeptide, N-acetylglycine methylamide (NAGMA). A two-stage oven source is implemented to produce a gas-phase equilibrium distribution of NAGMA conformers, which is preserved when individual molecules are captured and cooled to 0.4 K by He nanodroplets. With polarization spectroscopy, the IR spectrum in the NH stretch region is assigned to a mixture of two conformers having intramolecular hydrogen bonds composed of either five- or seven-membered rings, C5 and C7, respectively. The C5 to C7 interconversion enthalpy and entropy, obtained from a van’t Hoff analysis, are −4.52 ± 0.12 kJ/mol and −12.4 ± 0.2 J/(mol·K), respectively. The experimental thermochemistry is compared to high-level electronic structure theory computations.

•CH2O has been probed in He droplets in the region of the ν1 and ν5 stretch bands.•IHe contributions about the a, b and c inertial axes are significantly different.•The IHe about the a-axis indicates near complete breakdown of adiabatic following.Formaldehyde has been characterized in superfluid helium nanodroplets in the region of the symmetric (ν1) and antisymmetric (ν5) stretching bands. The band origins are blue shifted, consistent with a decrease in the dipole moment and polarizability in going from the ground to excited vibrational states, and the lines within each band are homogeneously broadened, with vibrational lifetimes of 26 and 6.2 ps for assigned transitions in the v1 and v5 bands, respectively. The A, B, and C constants are renormalized to 79%, 65%, and 49% of their gas phase values, corresponding to moments of inertia of helium, IHe, of 0.488, 6.88, and 15.3 amu Å2, about the inertial a-, b-, and c-axes, respectively. While partial breakdown of the adiabatic approximation is expected for rotations about the b- and c-axes, the small value of IHe about the a-axis is indicative of near complete breakdown. The larger value of IHe about the c-axis in comparison to the b-axis is attributed to the greater anisotropy of the interaction potential in the plane of H2CO [M.D. Wheeler, A.M. Ellis, Chem. Phys. Lett. 374 (2003) 392].Graphical abstract

A global six-dimensional potential surface for the hydridotrioxygen radical (HOOO) is needed for an accurate assessment of its atmospheric abundance. We report inertial dipole moment components obtained from Stark spectra of the trans-HOOO system solvated in superfluid helium, and these are shown to be stringent benchmarks for theoretical computations of the potential surface. Computed dipole moment components at the CCSD(T)/CBS equilibrium geometry disagree qualitatively with the experimental values. The role of large-amplitude motion and vibrational averaging is assessed by computing the ground-state wave function on a relaxed, two-dimensional potential surface for the HO1O2O3 torsional and O1O2 bond-stretching coordinates. The experimental and computed vibrationally averaged dipole moments agree only after shifting the potential along the O1O2 bond coordinate, indicating that single-reference CCSD(T)/CBS computations underestimate re(O1O2) by ∼0.08 Å. An optimized trans-HOOO geometry at the composite all-electron CCSDT(Q)/CBS level reveals that the inclusion of full triples and a perturbative treatment of quadruple excitations leads to an increase in re(O1O2) by 0.07 Å.Keywords: ab initio; dipole moments; helium droplets; hydridotrioxygen; hydrotrioxy radical; potential energy surfaces; Stark spectroscopy;

A combination of liquid He droplet experiments and multireference electronic structure calculations is used to probe the potential energy surface for the reaction between the propargyl radical and O2. Infrared laser spectroscopy is used to probe the outcome of the low temperature, liquid He-mediated reaction. Bands in the spectrum are assigned to the acetylenic CH stretch (ν1), the symmetric CH2 stretch (ν2), and the antisymmetric CH2 stretch (ν13) of the trans-acetylenic propargyl peroxy radical (•OO—CH2—C≡CH). The observed band origins are in excellent agreement with previously reported anharmonic frequency computations for this species [Jochnowitz, E. B.; Zhang, X.; Nimlos, M. R.; Flowers, B. A.; Stanton, J. F.; Ellison, G. B. J. Phys. Chem. A 2010, 114, 1498]. The Stark spectrum of the ν1 band provides further evidence that the reaction leads only to the trans-acetylenic species. There are no other bands in the CH2 stretching region that can be attributed to any of the other three propargyl peroxy isomers/conformers that are predicted to be minimum energy structures (gauche-acetylenic, cis-allenic, and trans-allenic). There is also no evidence for the kinetic stabilization of a van der Waals complex between propargyl and O2. A combination of multireference and coupled-cluster electronic structure calculations is used to probe the potential energy surface in the neighborhood of the transition state connecting reactants with the acetylenic adduct. The multireference based evaluation of the doublet-quartet splitting added to the coupled-cluster calculated quartet state energies yields what are likely the most accurate predictions for the doublet potential curve. This calculation suggests that there is no saddle point for the addition process, in agreement with the experimental observations. Other calculations suggest the possible presence of a small submerged barrier.

Ethane and ethane clusters (N ≈ 102–104) were studied inside helium droplets with infrared laser spectroscopy. The spectra were measured in the 2880–3000 cm–1 range, which covers the ν5, ν8+11, and ν7 vibrational bands of ethane. Partially resolved rotational fine structure in the spectrum of the monomer reveals solvent-induced band origin blue shifts that are each approximately 1 cm–1. The effective BHe and AHe rotational constants were found to be reduced by 52% and 16% in comparison to their gas phase values, respectively. Spectra of the clusters show the same three bands shifted toward low frequency by approximately 10 cm–1 because of intermolecular interactions in the clusters. The spectra of the ethane clusters are dominated by the ν7 band, whereas the relative intensities of the ν5 and ν8+11 bands are about a factor of 5 weaker than for single molecules or for solid ethane, the spectrum of which is also reported here.

We report the ro-vibrational spectrum of the ν3(e′) band of the methyl radical (CH3) solvated in superfluid 4He nanodroplets. Five allowed transitions produce population in the NK = 00, 11, 10, 22 and 20 rotational levels. The observed transitions exhibit variable Lorentzian line shapes, consistent with state specific homogeneous broadening effects. Population relaxation of the 00 and 11 levels is only allowed through vibrationally inelastic decay channels, and the PP1(1) and RR0(0) transitions accessing these levels have 4.12(1) and 4.66(1) GHz full-width at half-maximum line widths, respectively. The line widths of the PR1(1) and RR1(1) transitions are comparatively broader (8.6(1) and 57.0(6) GHz, respectively), consistent with rotational relaxation of the 20 and 22 levels within the vibrationally excited manifold. The nuclear spin symmetry allowed rotational relaxation channel for the excited 10 level has an energy difference similar to those associated with the 20 and 22 levels. However, the PQ1(1) transition that accesses the 10 level is 2.3 and 15.1 times narrower than the PR1(1) and RR1(1) lines, respectively. The relative line widths of these transitions are rationalized in terms of the anisotropy in the He-CH3 potential energy surface, which couples the molecule rotation to the collective modes of the droplet.

The ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide was vaporized at 420 K, and the ion-pair constituents were entrained in a beam of liquid He nanodroplets and cooled to 0.4 K. The vapor pressure was optimized such that each He droplet picked up a single ion-pair from the gas phase. Infrared spectroscopy in the CH stretch region reveals bands that are assigned to intact ion-pairs on the basis of comparisons to ab initio harmonic frequency computations of 23 low energy isomers. The He droplet spectrum is consistent with a weighted sum of the computed harmonic spectra, in which the weights are determined from ab initio computations of the relative free energies at 420 K. Anharmonic resonance polyads in the CH stretch region are treated explicitly, which improves the agreement between the experiment and computed spectra for ion-pairs. For isomers having a strong cation···anion hydrogen bonding interaction, the imidazolium C(2)-H stretch fundamental is shifted to lower energy and into resonance with the overtones and combination bands of the imidazolium ring stretching modes, resulting in a spectral complexity in the CH stretch region that is fully resolved in the He droplet spectrum. The assignment of the infrared spectrum to ion-pairs is confirmed through polarization spectroscopy measurements that reveal the permanent electric dipole moment of the He-solvated species to be 11 ± 2 D. The computed permanent electric dipole moments for the low energy isomers of the [emim+][Tf2N–] ion-pairs fall in the range 9–13 D, whereas the computed dipole moments of decomposition products of the ionic liquid are less than 4.3 D.

The X2Π3/2 hydroxyl (OH) radical has been isolated in superfluid 4He nanodroplets and probed with infrared laser depletion spectroscopy. From an analysis of the Stark spectrum of the Q(3/2) transition, the Λ-doublet splittings are determined to be 0.198(3) and 0.369(2) cm–1 in the ground and first excited vibrational states, respectively. These splittings are 3.6 and 7.2 times larger than their respective gas phase values. A factor of 1.6 increase in the Q(1/2) Λ-doublet splitting was previously reported for the He solvated X2Π1/2 NO radical [von Haeften, K.; Metzelthin, A.; Rudolph, S.; Staemmler, V.; Havenith, M. Phys. Rev. Lett. 2005, 95, 215301]. A simple model is presented that reproduces the observed Λ-doublet splittings in He-solvated OH and NO. The model assumes a realistic parity dependence of the rotor’s effective moment of inertia and predicts a factor of 3.6 increase in the OH ground state (J = 3/2) Λ-doubling when the B0e and B0f rotational constants differ by less than one percent.

The sequential addition of HCN and Al to a helium nanodroplet leads barrierlessly to a bent, planar, 2A′, HCNAl species, which is computed to be 20.3 kcal/mol below the separated reactants. A ro-vibrational band near 2690 cm−1 is assigned to the ν1 (a′) CH stretch. A Natural Bond Orbital analysis of this reaction product reveals that both ligand-to-metal σ-donation and metal-to-ligand π-donation are responsible for the strong bonding between HCN and Al. A significant donor–acceptor interaction between the C–N and C–H anti-bonding orbitals is responsible for the bent geometry and the 619 cm−1 red shift of the CH stretch band upon complexation.Graphical abstractHighlights► The Al + HCN reaction is carried out at low temperature in a helium nanodroplet. ► Barrierless association leads to a bent HCNAl species with a 130° HCN bond angle. ► A Rotationally resolved CH stretch band for bent HCNAl is found red shifted 619 cm−1 from free HCN. ► Natural Bond Orbital analysis reveals substantial Al p-electron delocalization within HCNAl.

Helium nanodroplet isolation and infrared laser spectroscopy are used to investigate the CH3 + O2 reaction. Helium nanodroplets are doped with methyl radicals that are generated in an effusive pyrolysis source. Downstream from the introduction of CH3, the droplets are doped with O2 from a gas pick-up cell. The CH3 + O2 reaction therefore occurs between sequentially picked-up and presumably cold CH3 and O2 reactants. The reaction is known to lead barrierlessly to the methyl peroxy radical, CH3OO. The ∼30 kcal/mol bond energy is dissipated by helium atom evaporation, and the infrared spectrum in the CH stretch region reveals a large abundance of droplets containing the cold, helium solvated CH3OO radical. The CH3OO infrared spectrum is assigned on the basis of comparisons to high-level ab initio calculations and to the gas phase band origins and rotational constants.

Helium nanodroplet isolation and a tunable quantum cascade laser are used to probe the fundamental CO stretch bands of aluminum carbonyl complexes, Al–(CO)n (n ≤ 5). The droplets are doped with single aluminum atoms via the resistive heating of an aluminum wetted tantalum wire. The downstream sequential pick-up of CO molecules leads to the rapid formation and cooling of Al–(CO)n clusters within the droplets. Near 1900 cm–1, rotational fine structure is resolved in bands that are assigned to the CO stretch of a linear 2Π1/2 Al–CO species and the asymmetric and symmetric CO stretch vibrations of a planar C2v Al–(CO)2 complex in a 2B1 electronic state. Bands corresponding to clusters with n ≥ 3 lack resolved rotational fine structure; nevertheless, the small frequency shifts from the n = 2 bands indicate that these clusters consist of an Al–(CO)2 core with additional CO molecules attached via van der Waals interactions. A second n = 2 band is observed near the CO stretch of Al–CO, indicating a local minimum on the n = 2 potential consisting of an “unreacted” (Al–CO)–CO cluster. The line width of this band is ∼0.3 cm–1, which is about 30 times broader than the transitions within the Al–CO band. The additional broadening is consistent with a homogeneous mechanism corresponding to a rapid vibrational excitation induced reaction within the (Al–CO)–CO cluster to form the covalently bonded Al–(CO)2 complex. Ab initio CCSD(T) calculations and natural bond orbital (NBO) analyses are carried out to investigate the nature of the bonding in the n = 1, 2 complexes. The NBO calculations show that both π-donation (from the occupied aluminum p orbital into a π* antibonding CO orbital) and σ-donation (from CO into the empty aluminum p orbitals) play a significant role in the bonding, analogous to transition-metal carbonyl complexes. The large red shift observed for the CO stretch vibrations is consistent with this bonding analysis.

Infrared (IR) laser spectroscopy is used to probe the rotational and vibrational dynamics of the (HCN)m−Mn (M = K, Ca, Sr) complexes, either solvated within or bound to the surface of helium nanodroplets. The IR spectra of the (HCN)m−K (m = 1−3), HCN−Sr, and HCN−Ca complexes have the signature of a surface species, similar to the previously reported spectra of HCN−M (M = Na, K, Rb, Cs) [Douberly, G. E.; Miller, R. E. J. Phys. Chem. A 2007, 111, 7292.]. A second band in the HCN−Ca spectrum is assigned to a solvated complex. The relative intesities of the two HCN−Ca bands are droplet size dependent, with the solvated species being favored in larger droplets. IR−IR double resonance spectroscopy is used to probe the interconversion of the two distinct HCN−Ca populations. While only a surface-bound HCN−Sr species is initially produced, CH stretch vibrational excitation results in a population transfer to a solvated state. Complexes containing multiple HCN molecules and one Sr atom are surface-bound, while the ν1 (HCN)2Ca spectrum has both the solvated and surface-bound signatures. All HCN−(Ca,Sr)n (n ≥ 2) complexes are solvated following cluster formation in the droplet. Density-functional calculations of helium nanodroplets interacting with the HCN−M show surface binding for M = Na with a binding energy of 95 cm−1. The calculations predict a fully solvated complex for M = Ca. For M = Sr, a 2.2 cm−1 barrier is predicted between nearly isoenergetic surface binding and solvated states.