Direct Probe of Vibrational Fingerprint and Combination Band Coupling.

Vibrational fingerprints and combination bands are a direct measure of couplings that control molecular properties. However, most combination bands possess small transition dipoles. Here we use multiple, ultrafast coherent infrared pulses to resolve vibrational coupling between CH3CN fingerprint modes at 918 and 1039 cm-1 and combination bands in the 2750-6100 cm-1 region via doubly vibrationally enhanced (DOVE) coherent multidimensional spectroscopy (CMDS). This approach provides a direct probe of vibrational coupling between fingerprint modes and near-infrared combination bands of large and small transition dipoles in a molecular system over a large frequency range.

Cyclohexane Vibrations: High-Resolution Spectra and Anharmonic Local Mode Calculations.

High-resolution infrared absorption spectra of cyclohexane have been recorded from 1100 to 4000 cm-1 at room temperature and 241 K. Cyclohexane is an oblate symmetric top with D3d symmetry. A rotational analysis was obtained for the ν27 (eu) and ν14 (a2u) CH2 scissor modes at 1452.9 and 1456.4 cm-1, respectively. Several combination modes were also assigned and rotationally analyzed. The C-H stretching modes are perturbed by overtone and combination modes of the CH2 scissor vibrations, and an anharmonic local mode calculation was needed to interpret the spectra. The four main strong allowed C-H stretching modes appear as two a2u eu pairs near at 2862 and 2933 cm-1. The Fermi-resonance local mode model coupling terms give physical insight into the effects that organize the cyclohexane vibrational energy levels. The unstrained cyclohexane molecule is a useful paradigm for six-membered rings in larger chemical and biological systems.

Self-thermophoresis at the nanoscale using light induced solvation dynamics.

Downsizing microswimmers to the nanoscale, and using light as an externally controlled fuel, are two important goals within the field of active matter. Here we demonstrate using all-atom molecular dynamics simulations that solvation relaxation, the solvent dynamics induced after visible light electronic excitation of a fluorophore, can be used to propel nanoparticles immersed in polar solvents. We show that fullerenes functionalized with fluorophore molecules in liquid water exhibit substantial enhanced mobility under external excitation, with a propulsion speed proportional to the power dissipated into the system. We show that the propulsion mechanism is quantitatively consistent with a molecular scale instance of self-thermophoresis. Strategies to direct the motion of functionalized fullerenes in a given direction using confined environments are also discussed.

Neopentane Vibrations: High Resolution Spectra and Anharmonic Calculations.

High resolution infrared absorption spectra of neopentane (2,2-dimethylpropane, C5H12) have been recorded in the mid-infrared region at room temperature and 232 K. Neopentane is a spherical top with Td symmetry. The high symmetry and low temperature allow for detailed comparison of theory and experiment for the analysis of the fundamental vibrations. Four strong bands with characteristic rotational structure of t2 modes were observed at 1257.6, 1369.4, 1472.5, and 1489.0 cm-1, and a fifth very weak band was found near 924.2 cm-1. Quantum chemical calculations (B3LYP/6-311++(d,p)/VPT2 and harmonic CCSD(T)-pVTZ) were carried out in both normal and local mode representations to help with the vibrational assignments and elucidate the couplings. The local mode representation combined with the experimental observations highlights the important potential couplings between neighboring methyl groups and their influence on observed transitions. We have analyzed spectral regions where the models failed to provide agreement and have identified those couplings responsible for the differences.

Isobutane Infrared Bands: Partial Rotational Assignments, ab Initio Calculations, and Local Mode Analysis.

High-resolution infrared spectra of the symmetric top isobutane CH(CH3)3 were assigned with the help of ab initio calculations. The strong parallel band ν5(a1) with an origin at 1396.54741(76) cm-1 and the ν4(a1) mode, the CH2 scissors, at 1478.20363(41) cm-1 were rotationally analyzed. The bands in the C-H stretching region were assigned with the help of an anharmonic calculation and a local mode analysis.

Identifying the first folded alkylbenzene via ultraviolet, infrared, and Raman spectroscopy of pentylbenzene through decylbenzene.

The conformational preferences of pentyl- through decylbenzene are studied under jet-cooled conditions in the gas phase. Laser-induced fluorescence excitation spectra, fluorescence-dip infrared spectra in the alkyl CH stretch region, and Raman spectra are combined to provide assignments for the observed conformers. Density functional theory calculations at the B3LYP-D3BJ/def2TZVP level of theory provide relative energies and normal mode vibrations that serve as inputs for an anharmonic local mode theory introduced in earlier work on alkylbenzenes with n = 2-4. This model explicitly includes anharmonic mixing of the CH stretch modes with the overtones of scissors/bend modes of the CH2 and CH3 groups in the alkyl chain, and is used to assign and interpret the single-conformation IR spectra. In octylbenzene, a pair of LIF transitions shifted -92 and -78 cm-1 from the all-trans electronic origin have unique alkyl CH stretch transitions that are fit by the local model to a g1g3g4 conformation in which the alkyl chain folds back over the aromatic ring π cloud. Its calculated energy is only 1.0 kJ mol-1 above the all-trans global minimum. This fold is at an alkyl chain length less than half that of the pure alkanes (n = 18), consistent with a smaller energy cost for the g1 dihedral and the increased dispersive interaction of the chain with the π cloud. Local site frequencies for the entire set of conformers from the local mode model show 'edge effects' that raise the site frequencies of CH2(1) and CH2(2) due to the phenyl ring and CH2(n - 1) due to the methyl group. The g1g3g4 conformer also shows local sites shifted up in frequency at CH2(3) and CH2(6) due to interaction with the π cloud.