The growing realization that photoinduced bending of slender photoreactive single crystals is surprisingly common has inspired researchers to control crystal motility for actuation. However, new mechanically responsive crystals are reported at a greater rate than their quantitative photophysical characterization; a quantitative identification of measurable parameters and molecular-scale factors that determine the mechanical response has yet to be established. Herein, a simple mathematical description of the quasi-static and time-dependent photoinduced bending of macroscopic single crystals is provided. This kinetic model goes beyond the approximate treatment of a bending crystal as a simple composite bilayer. It includes alternative pathways for excited-state decay and provides a more accurate description of the bending by accounting for the spatial gradient in the product/reactant ratio. A new crystal form (space group P21/n) of the photoresponsive azo-dye Disperse Red 1 (DR1) is analyzed within the constraints of the aforementioned model. The crystal bending kinetics depends on intrinsic factors (crystal size) and external factors (excitation time, direction, and intensity).

Hydrogen-bond assisted enormous broadening of infrared spectra of phenol-water cationic cluster: An ab initio mixed quantum-classical study Potential energy hypersurfaces ͑PESs͒ for four fluoroform complexes ͑with acetonitrile, ethyleneoxide, formaldehyde, and water͒ were explored at the HF, MP2, and B3LYP/6-311 ϩϩG(d,p) levels of theory. Anharmonic C-H stretching vibrational frequency shifts are reported for all minima located on the studied PESs. In all cases, the lowest-energy minimum occurs for a C-H¯O(N) hydrogen-bonded arrangement and is characterized by a significant C-H frequency blueshift ͑upshift͒, while additional minima ͓for ''reversed'' orientations, in which there is no direct C-H¯O(N) contact͔ show only small C-H frequency upshifts. The large blueshifts found for the hydrogen-bonded arrangements are predominantly caused by the electronic exchange interaction, as revealed by Kitaura-Morokuma ͑KM͒ analysis, while the purely electrostaticϩpolarization interaction leads to C-H frequency redshifts, which was proven both by the KM analysis and the charge field perturbational ͑CFP͒ approach. The large net blueshifting effect of the exchange contribution is only possible thanks to the smallness of the redshifting electrostaticϩpolarization contribution, which, in turn, is a consequence of the fact that d (0) /dr CH is negative for the fluoroform molecule. In all cases the charge transfer as well as the dispersion energy contributions lead to frequency redshifts. The small C-H blueshifts for the reversed orientations are almost completely governed by the electrostatic interaction, as shown by the KM and CFP analyses. All these characteristics of blueshifting H bonds are in line with a previously outlined general model concerning ''standard'' and ''blueshifting'' H-bonding interactions ͓K. Hermansson, J. Phys. Chem. A 106, 4695 ͑2002͔͒.

Molecular dynamics and electric field strength simulations are performed in order to quantify the structural, dynamic, and vibrational properties of non-H-bonded (dangling) OH groups in the hydration shell of neopentane, as well as in bulk water. The results are found to be in good agreement with the experimentally observed high-frequency (∼3660 cm(-1)) OH band arising from the hydration shell of neopentanol dissolved in HOD/D(2)O, obtained by analyzing variable concentration Raman spectra using multivariate curve resolution (Raman-MCR). The simulation results further indicate that hydration shell dangling OH groups preferentially point toward the central carbon atom of neopentane to a degree that increases with the lifetime of the dangling OH.

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The average OH stretching vibrational frequency for the water molecules in the first hydration shell around a Li(+) ion in a dilute aqueous solution was calculated by a hybrid molecular dynamics + quantum-mechanical ("MD + QM") approach. Using geometry configurations from a series of snapshots from an MD simulation, the anharmonic, uncoupled OH stretching frequencies were calculated for 100 first-shell OH oscillators at the B3LYP and HF/6-31G(d,p) levels of theory, explicitly including the first shell and the relevant second shell water molecules into charge-embedded supermolecular QM calculations. Infrared intensity-weighting of the density-of-states (DOS) distributions by means of the squared dipole moment derivatives (which vary by a factor of 20 over the OH stretching frequency band at the B3LYP level), changes the downshift from approximately -205 to -275 cm(-1) at the B3LYP level. Explicit inclusion of relevant third-shell water molecules in the supermolecular cluster leads to a further downshift by approximately -30 cm(-1). Our final estimated average downshift is approximately -305 cm(-1). The experimental value lies somewhere in the range between -290 and -420 cm(-1). Also, the absolute nu(OH) frequency is well reproduced in our calculations. "In-liquid" instantaneous correlation curves between nu(OH) and various typical H-bond strength parameters such as R(O...O), R(H...O), the intramolecular OH bond length, and the IR intensity are presented. Some of these correlations are robust and persist also for the rather distorted instantaneous geometries in the liquid; others are less so.