Intramolecular vibrational energy redistribution in nucleobases: Excitation of NH stretching vibrations in adenine-uracil + HO.

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 HO(2ν) 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 HO(2ν) to the uracil-NH stretching mode is efficient and rapidly followed by intramolecular vibrational energy redistribution (IVR) resulting from coupling between vibrational modes. An important pathway is IVR carrying energy to the NH-stretching mode of the adenine moiety in a subpicosecond scale, the energy build-up being sigmoidal, when HO interacts with the uracil-NH bond. The majority of intermolecular hydrogen bonds between the base pair and HO are 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 coupled CC and CN vibrations. The resonance also exists between the frequencies of HO bend and NH stretch, thus facilitating VET. When HO 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 unidirectionality discussed in terms of the effects of uracil CH stretches. The energy distributed in the CH bonds is found to be significant. The IVR process is found to be nearly temperature independent between 200 and 400 K.