Daniel P. Tabor

Isomer-Specific Spectroscopy of Benzene–(H2O)n, n = 6,7: Benzene’s Role in Reshaping Water’s Three-Dimensional Networks

The water hexamer and heptamer are the smallest sized water clusters that support three-dimensional hydrogen-bonded networks, with several competing structures that could be altered by interactions with a solute. Using infrared-ultraviolet double resonance spectroscopy, we record isomer-specific OH stretch infrared spectra of gas-phase benzene-(H2O)(6,7) clusters that demonstrate benzenes surprising role in reshaping (H2O)(6,7). The single observed isomer of benzene-(H2O)6 incorporates an inverted book structure rather than the cage or prism. The main conformer of benzene-(H2O)7 is an inserted-cubic structure in which benzene replaces one water molecule in the S4-symmetry cube of the water octamer, inserting itself into the water cluster by engaging as a π H-bond acceptor with one water and via C-H···O donor interactions with two others. The corresponding D(2d)-symmetry inserted-cube structure is not observed, consistent with the calculated energetic preference for the S4 over the D(2d) inserted cube. A reduced-dimension model that incorporates stretch-bend Fermi resonance accounts for the spectra in detail and sheds light on the hydrogen-bonding networks themselves and on the perturbations imposed on them by benzene.

Fermi resonance effects in the vibrational spectroscopy of methyl and methoxy groups.

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.

Anharmonic modeling of the conformation-specific IR spectra of ethyl, n-propyl, and n-butylbenzene

Conformation-specific UV-IR double resonance spectra are presented for ethyl, n-propyl, and n-butylbenzene. With the aid of a local mode Hamiltonian that includes the effects of stretch-scissor Fermi resonance, the spectra can be accurately modeled for specific conformers. These molecules allow for further development of a first principles method for calculating alkyl stretch spectra. Across all chain lengths, certain dihedral patterns impart particular spectral motifs at the quadratic level. However, the anharmonic contributions are consistent from molecule to molecule and conformer to conformer. This transferability of anharmonicities allows for the Hamiltonian to be constructed from only a harmonic frequency calculation, reducing the cost of the model. The phenyl ring alters the frequencies of the CH2 stretches by about 15 cm(-1) compared to their n-alkane counterparts in trans configurations. Conformational changes in the chain can lead to shifts in frequency of up to 30 cm(-1).

Local Mode Approach to OH Stretch Spectra of Benzene-(H2O)n Clusters, n = 2-7.

Isomer-specific resonant ion-dip infrared spectra are presented for benzene-water (Bz-(H2O)n) clusters with two to seven water molecules. Local mode Hamiltonians based on scaled M06-2X/6-311++G(2d,p) density functional calculations are presented that accurately model the spectra across the entire OH stretch region (3000-3750 cm(-1)). The model Hamiltonians include the contribution from the water bend overtone and an empirical parameter for the local OH stretch-bend Fermi coupling. The inclusion of this coupling is necessary for accurate modeling of the infrared spectra of clusters with more than three water molecules. For the cyclic water clusters (n = 3-5), the benzene molecule perturbs the system in a characteristic way, distorting the cycle, splitting degeneracies, and turning on previously forbidden transitions. The local OH stretch site frequencies and H···OH hydrogen bond lengths follow a pattern based on the each water monomers proximity to benzene. The patterns observed for these cyclic water clusters provide insight into benzenes effects on the three-dimensional hydrogen-bonded networks present in water hexamer and heptamer structures, which also have their spectra dramatically altered from their pure water counterparts.

High-accuracy extrapolated ab initio thermochemistry of the vinyl, allyl, and vinoxy radicals

Enthalpies of formation at both 0 and 298 K were calculated according to the HEAT (High-accuracy Extrapolated Ab initio Thermochemistry) protocol for the title molecules, all of which play important roles in combustion chemistry. At the HEAT345-(Q) level of theory, recommended enthalpies of formation at 0 K are 301.5 ± 1.3, 180.3 ± 1.8, and 23.4 ± 1.5 kJ mol(-1) for vinyl, allyl, and vinoxy, respectively. At 298 K, the corresponding values are 297.3, 168.6, and 16.1 kJ mol(-1), with the same uncertainties. The calculated values for the three radicals are in excellent agreement with the corresponding experimental values, but the uncertainties associated with the HEAT values for vinoxy are considerably smaller than those based on experimental studies.

Modeling the CH Stretch Vibrational Spectroscopy of M+[Cyclohexane] (M = Li, Na, and K) Ions

The CH stretch vibrations of M(+)[cyclohexane][Ar] (M = Li, Na, and K) cluster ions were theoretically modeled. Results were compared to the corresponding infrared photodissociation spectra of Patwari and Lisy [ J. Chem. Phys A 2007 , 111 , 7585 ]. The experimental spectra feature a substantial spread in CH stretch vibration frequencies due to the alkali metal cation binding to select hydrogens of cyclohexane. This spread was observed to increase with decreasing metal ion size. Exploring the potential energy landscape revealed the presence of three conformers whose energy minima lie within ∼1 kcal of each other. It was determined that in all conformers the metal ion interacts with three hydrogen atoms; these hydrogen atoms can be either equatorial or axial. The corresponding spectra for these conformers were obtained with a theoretical model Hamiltonian [ J. Chem. Phys. 2013 , 138 , 064308 ] that consists of local mode CH stretches bilinearly coupled to each other and Fermi coupled to lower frequency modes. Frequencies and coupling parameters were obtained from electronic structure calculations that were subsequently scaled on the basis of previous studies. Theoretical spectra of a single low energy conformer were found to match well with the experimental spectra. The relative frequency shifts with changing metal ion size were accurately modeled with parameters generated by using ωB97X-D/6-311++(2d,p) calculations.

Infrared-Enhanced Fluorescence-Gain Spectroscopy: Conformation-Specific Excited-State Infrared Spectra of Alkylbenzenes

An ultraviolet-infrared (UV-IR) double-resonance method for recording conformation-specific excited-state infrared spectra is described. The method takes advantage of an increase in fluorescence signal in phenylalkanes produced by infrared excitation of the S1 origin levels of different conformational isomers. The shorter lifetimes of these IR-excited molecules, combined with their red-shifted emission, provides a way to discriminate the fluorescence due to the infrared-excited molecules from the S1 origin fluorescence, resulting in spectra with high signal-to-noise ratios. Spectra for a series of phenylalkanes and a capped phenylalanine derivative (Ac-Phe-NHMe) demonstrate the potential of the method. The excited-state spectrum in the alkyl CH stretch region of ethylbenzene is well-fit by an anharmonic model developed for the ground electronic state, which explicitly takes into account stretch-bend Fermi resonance.

A perturbative description of non-adiabatic effects in methoxy vibrations

Non-adiabatic couplings between the quasi-diabatic ground states, used to describe the vibrations of the methoxy (CH3O) radical, are investigated. The vibrations of the state are known to be strongly influenced by Jahn–Teller couplings. These couplings are often modelled in a quasi-diabatic electronic state representation. There is an additional non-adiabatic coupling between these states. This latter coupling is elucidated and its effects are quantified using the Van Vleck perturbation theory. The origins of the non-adiabatic coupling are assumed to arise via interactions of the degenerate ground state with the lowest lying excited electronic state. The resulting three-state electronic Hamiltonian is developed to approximately describe the rocking vibrations of methoxy. It is shown that minor changes to the implementation of the Van Vleck transformations enable one to highlight the role of the non-adiabatic coupling in a straightforward manner. These couplings are found to lead to shifts of the order of a few wavenumbers for the lowest several vibrational states.