Saligenin (2-(hydroxymethyl)phenol) exhibits both strong and weak intramolecular electrostatic interactions. The bonds that result from these interactions compete with intermolecular hydrogen bonds once saligenin binds to one or more water molecules. Infrared (IR) ultraviolet (UV) ion-dip spectroscopy was used to study isolated saligenin–(H2O)n clusters (n = 1–3) in the far- and mid-IR regions of the spectrum. Both harmonic and anharmonic (coupled local modes and Born–Oppenheimer molecular dynamics) quantum chemical calculations were applied to assign cluster geometries to the measured spectra, and to assign vibrational modes to all spectral features measured for each cluster. The hydrated clusters with n = 1 and 2 have geometries that are quite similar to benzyl alcohol–water clusters, whereas the larger clusters with n = 3 show structures equivalent to the isolated water pentamer. Systematic shifts in the frequencies of three hydrogen bond (H-bond) deforming modes, namely OH stretching, OH torsion and H-bond stretching, were studied as a function of the hydrogen bond strength represented by either the OH bond length or the H-bond length. The shifts of the frequencies of these three modes correlate linearly to the OH length, despite both intra- and intermolecular H-bonds being included in this analysis. The OH torsion vibration displays the largest frequency shift when H-bonded, followed by the OH stretching vibrations and finally the H-bond stretching frequency. The frequency shifts of these H-bond deforming modes behave non-linearly as a function of the H-bond length, asymptotically approaching the frequency expected for the non H-bonded modes. The nonlinear behavior was quantified using exponential functions.

A theoretical calculation of the laser-induced fluorescence excitation spectrum from X∼2E→A∼2A1 is carried out for CH3O and CD3O using a transition dipole moment surface expanded up to second order. The vibronic form of these operators is obtained using symmetry arguments. The A∼2A1 vibrational levels are calculated using Van Vleck perturbation theory, and the latter is used to adjust harmonic constants of the potential to match experimental fundamentals. The CH3O fit force field is tested on CD3O. For both molecules the transition energies are well reproduced, but there are systematic differences between experimental and theoretical intensities. The origins of these differences are discussed.Highlight► Theoretical calculation of electronic excitation spectrum of CH3O is carried out. ► The vibronic Hamiltonian and transition dipole operators are determined using 3-fold symmetry. ► Van Vleck perturbation theory is used to adjust excited state harmonic constants. ► Theoretical and experimental transition frequencies of CH3O and CD3O spectra are in good agreement. ► We discuss origins behind theory and experimental relative intensity differences.

The infrared (IR) spectroscopy of the alkyl CH stretch region (2750–3000 cm–1) of a series of bicyclic hydrocarbons and free radicals has been studied under supersonic expansion cooling in the gas phase, and compared with a theoretical model that describes the local mode stretch–bend Fermi resonance interactions. The double resonance method of fluorescence-dip infrared (FDIR) spectroscopy was used on the stable molecules 1,2-dihydronaphthalene, 1,4-dihydronaphthalene, tetralin, indene, and indane using the S0–S1 origin transition as a monitor of transitions. Resonant ion-dip infrared (RIDIR) spectra were recorded for the trihydronaphthyl (THN) and inden-2-yl methyl (I2M) radicals. The previously developed model Hamiltonian (J. Chem. Phys. 2013, 138, 064308) incorporates cubic stretch–bend coupling with parameters obtained from density functional theory methods. Full dimensional calculations are compared to reduced dimensional Hamiltonian results in which anharmonic CH stretches and CH2 scissor modes are Fermi coupled. Excellent agreement between theoretical results is found. Scale factors of select terms in the reduced dimensional Hamiltonian, obtained by fitting the theoretical Hamiltonian predictions to the experimental spectra, are found to be similar to previous work. The resulting Hamiltonian predicts successfully all the major spectral features considered in this study. A simplified model is introduced in which the CH2 groups are decoupled. This model enables the assignment of many of the spectral features. The model results are extended to describe the CH stretch spectrum of the chair and twist-boat conformers of cyclohexane. The chair conformer is used to illustrate the shortcomings of the CH2 decoupling model.

A theoretical model Hamiltonian [J. Chem. Phys. 2013, 138, 064308] for describing vibrational spectra associated with the CH stretch of CH2 groups is extended to molecules containing methyl and methoxy groups. Results are compared to the infrared (IR) spectroscopy of four molecules studied under supersonic expansion cooling in gas phase conditions. The molecules include 1,1-diphenylethane (DPE), 1,1-diphenylpropane (DPP), 2-methoxyphenol (guaiacol), and 1,3-dimethoxy-2-hydroxybenzene (syringol). Transforming the bending normal mode vibrations of CH3 groups to local scissor vibrations leads to model Hamiltonians which share many features present in our model Hamiltonian for the stretching vibrations of CH2 Fermi coupled to scissor modes. The central difference arises from the greater scissor–scissor coupling present in the CH3 case. Comparing anharmonic couplings between these modes and the stretch–bend Fermi coupling for a variety of systems, it is observed that the anharmonic couplings are robust; their values are similar for the four molecules studied as well as for ethane and methanol. Similar results are obtained with both density functional theory and coupled-cluster calculations. This robustness suggests a new parametrization of the model Hamiltonian that reduces the number of fitting parameters. In contrast, the harmonic contributions to the Hamiltonian vary substantially between the molecules leading to important changes in the spectra. The resulting Hamiltonian predicts most of the major spectral features considered in this study and provides insights into mode mixing and the consequences of the mixing on dynamical processes that follow ultrafast CH stretch excitation.

The J = 0 infrared spectrum of methoxy is theoretically calculated for the ground X̃2E state using a quartic potential energy force field, and the quadratic dipole moment expansion is calculated ab initio at the CCSD(T) level of theory and cc-pVTZ basis. Writing these expansions with vibronic operators whose symmetry properties are defined with respect to C3v rotation greatly simplifies these calculations. With minor adjustments to the force field, excellent agreement with experiment is found for both the transition energies of CH3O and those of CD3O. The role of Jahn–Teller and Fermi coupling is illustrated by scaling these terms by a parameter δ that varies from 0 to 1. Plotting the eigenvalues as a function of δ yields a correlation diagram connecting the harmonic eigenvalues to those of the fully coupled problem. The spectrum for CH3O is determined using a combination of Davidson and Lanczos iteration schemes. The spectral features are found to be dominated by Jahn–Teller effects, but direct Fermi coupling and indirect potential couplings have important roles. The origin of the complexities in the CH stretch region are discussed.

We present a general approach for modeling multidimensional infrared spectra based on a combination of phenomenological fitting and ab initio electronic structure calculations. The vibrational Hamiltonian is written in terms of bilinearly coupled Morse oscillators that represent local carbonyl stretches. This should be contrasted with the previous approach, where the anharmonic Hamiltonian was given in terms of normal-mode coordinates (Baiz et al. J. Phys. Chem. A 2009, 113, 9617). The bilinearly coupled Morse oscillator Hamiltonian is parametrized such that the frequencies and couplings are consistent with experiment, and the anharmonicities are computed by density functional theory. The advantages of the local-mode versus normal-mode approaches are discussed, as well as the ability of different density functionals to provide accurate estimates of the model parameters. The applicability and usefulness of the new approach are demonstrated in the context of the recently measured multidimensional infrared spectra of dimanganese decacarbonyl. The shifts in local site frequencies, couplings, and anharmonicities due to hydrogen bonding to the individual carbonyls are explored. It is found that, even though the effect of hydrogen bonding is nonlocal, it is additive.

The large-amplitude vibrational dynamics of methoxy (CH3O), resulting from the conical intersection at the C3v geometry, are investigated using variational methods. A two-dimensional model Hamiltonian consisting of two vibrational degrees of freedom and the coupling between them is presented in a diabatic representation of the electronic degrees of freedom. The observed complex dynamics are understood in terms of the multiple timescales that arise as the initial wave packet passes through the conical intersection. This model Hamiltonian is extended to include a full-dimensional treatment of methoxy. A quartic potential is calculated using both single and multiple configuration methods. The vibronic eigenstates are calculated using a series of basis set contraction schemes taking a primitive basis of harmonic oscillator wave functions as the starting point. Our final results, including spin-orbit coupling, are compared to experiment.

The recent reaction surface Hamiltonian model for the double proton tunneling in formic acid dimer of Barnes et al. [G.L. Barnes, S.M. Squires, E.L. Sibert, J. Phys. Chem. B. 112 (2008) 595.] has been applied to the calculation of the symmetric OH stretching Raman spectra. We interpret the full Raman spectra obtained through use of a simplified, single minimum spectrum. Extensive state mixing is found, leading to broad spectral features. Results compare well with the experimental measurements of Bertie et al. [J.E. Bertie, K.H. Michaelian, H.H. Eysel, D. Hager, J. Chem. Phys. 85 (9) (1986) 4779]. We also report improvements upon our previous approach and present ground state and fundamental frequencies as well as tunneling splittings obtained with our new method.

Studies of vibrational energy flow in various polar and nonpolar molecules that follows the ultrafast excitation of the CH and OH stretch fundamentals, modeled using semiclassical methods, are reviewed. Relaxation rates are calculated using Landau−Teller theory and a time-dependent method, both of which consider a quantum mechanical solute molecule coupled to a classical bath of solvent molecules. A wide range of decay rates are observed, ranging from 1 ps for neat methanol to 50 ps for neat bromoform. In order to understand the flow rates, it is argued that an understanding of the subtle mixing between the solute eigenstates is needed and that solute anharmonicities are critical to facilitating condensed phase vibrational relaxation. The solvent-assisted shifts of the solute vibrational energy levels are seen to play a critical role of enhancing or decreasing lifetimes.

The large-amplitude vibrational dynamics of methoxy (CH3O), resulting from the conical intersection at the C3v geometry, are investigated using variational methods. A two-dimensional model Hamiltonian consisting of two vibrational degrees of freedom and the coupling between them is presented in a diabatic representation of the electronic degrees of freedom. The observed complex dynamics are understood in terms of the multiple timescales that arise as the initial wave packet passes through the conical intersection. This model Hamiltonian is extended to include a full-dimensional treatment of methoxy. A quartic potential is calculated using both single and multiple configuration methods. The vibronic eigenstates are calculated using a series of basis set contraction schemes taking a primitive basis of harmonic oscillator wave functions as the starting point. Our final results, including spin-orbit coupling, are compared to experiment.