Redistribution of vibrational energy in the adenine-uracil base pair is studied when the base pair undergoes an intermolecular interaction with an overtone-bending vibration excited H2O molecule. Energy transfer is calculated using the structural information obtained from density functional theory in the solution of the equations of motion. Intermolecular vibrational energy transfer (VET) from H2O overtone-bending to the uracil-NH is efficient and rapidly followed by i ntramolecular vibrational energy redistribution (IVR) resulting from coupling between vibrational modes. An important pathway is IVR carrying energy to the NH-stretching mode of adenine moiety in a subpicosend scale, the energy build-up being sigmoidal. The majority of intermolecular hydrogen bonds between the base pair and H2O is weakened but unbroken, during the ultrafast energy redistribution period. Lifetimes of intermolecular HB are on the order of 0.5 ps. The efficiency of IVR in the base pair is due to near-resonance between CC and CN vibrations. The condition also facilitates VET between H2O and NH. When H2O interacts with the NH bond at the adenine end of the base pair, energy flow in the reverse direction to the uracil-NH stretch is negligible. The energy distributed in the CH bonds is found to be significant, whereas the third HB CH...O in the base pair does not significantly affect the overall redistribution. The IVR process is found to be nearly temperature independent between 200-400 K.

Energy transfer and bond dissociation of C-Hmethyl and C-Hring in excited toluene in the collision with H2 and D2 have been studied by use of classical trajectory procedures at 300 K. Energy lost by the vibrationally excited toluene to the ground-state H2/D2 is not large, but the amount increases with increasing vibrational excitation from 5000 and 40,000 cm −1. The principal energy transfer pathway is vibration to translation (V-T) in both systems. The vibration to vibration (V-V) step is important in toluene + D2, but plays a minor role in toluene + H2. When the incident molecule is also vibrationally excited, toluene loses energy to D2, whereas it gains energy from H2 instead. The overall extent of energy loss is greater in toluene + D2 than that in toluene + H2. The different efficiency of the energy transfer pathways in two collisions is mainly due to the near-resonant condition between D2 and C-H vibrations. Collision-induced dissociation of C-Hmethyl and C-Hring bonds occurs when highly excited toluene (55,000-70,400 cm) interacts with the ground-state H2/D2. Dissociation probabilities are low (10) but increase exponentially with rising vibrational excitation. Intramolecular energy flow between the excited C-H bonds occurring on a subpicosecond timescale is responsible for the bond dissociation.

The kinetics of the radical-polar molecule reaction CH 3 + HBr → CH 4 + Br has been studied at temperatures between 150 and 1000 K using classical dynamics procedures. Potential energy surfaces constructed using analytical forms of inter-and intramolecular interaction energies show a shallow well and barrier in the entrance channel, which affect the collision dynamics at low temperatures. Different collision models are used to distinguish the reaction occurring at low-and high-temperature regions. The reaction proceeds rapidly via a complex-mode mechanism below room temperature showing strong negative temperature dependence, where the effects of molecular attraction, H-atom tunneling and recrossing of collision complexes are found to be important. The temperature dependence of the rate constant between 400 and 1000 K is positive, the values increasing in accordance with the increase of the mean speed of collision. The rate constant varies from 7.6 × 10 −12 at 150 K to 3.7 × 10 −12 at 1000 K via a minimum value of 2.5 × 10 −12 cm 3 molecule −1 s −1 at 400 K.

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In the uracil−H 2 O complex, the vibrational energy initially stored in the OH(v = 1) stretch efficiently transfers to the first overtone-bending mode under a near-resonant condition. The relaxation of the overtone vibration redistributes its energy to uracil and the two hydrogen bonds in the intermolecular zone, which consists of the OH bond and the bonds between nearby C, N, O, and H atoms of uracil. The uracil NH bond and the hydrogen bond it formed with the H 2 O molecule, N−H•••O, store the major portion of the energy released by the relaxing bending mode, thus forming a localized hot band in the intermolecular zone. Energy transfer to the bonds beyond the zone is found to be not significant. The excited uracil NH is found to transfer its energy to the bending mode, thus indicating that the hydrogen bond of N− H•••O is the principal energy pathway in both directions. In the presence of efficient near-resonant energy transfer pathways, the time evolution of the centers of mass distance shows the phenomenon of beats. One global and two different local minima energy structures are considered. The results of energy transfer do not differ significantly, suggesting that the two hydrogen bonds in all three structures have similar contributions to the energy transfer.