The theoretical study of torsion-vibrational dynamics in methanol and the improvement of CW-CRDS experimental apparatus

Research using both theory and experimental avenues is integral to understanding intramolecular vibrational redistribution energy (IVR). Earlier experimental results indicated that the large amplitude internal rotation coordinate in methanol accelerates IVR especially when coupled to other small-amplitude vibrations. Theoretical methods such as ab initio calculations, adiabatic approximation and full-dimensional models are used to understand how the coupling of the torsional motion to small-amplitude vibrations influences IVR.;Experimentally observed spectra of the 3nu1-6nu1 OH stretch overtone region show that the stretch of the anti CH bond and the torsional motion around the CO bond are coupled to the OH stretch. In this work, we performed ab initio calculations on the (CH3OH→CH 2O+2H) reactive channel that takes both static and dynamic correlation effects into account. Our calculations show that a partial double bond between the C-O is formed early in the reaction coordinate. This partial double bond leads to an increase of the torsional barrier height that explains the experimental observations.;A four-dimensional model developed by Wang and Perry was used to compare the approximate adiabatic separation of the torsion and the CH stretches in methanol to an exact solution of the same Hamiltonian. The adiabatic approximation accounts for the correct energy level splittings at low torsional energies including the inverted torsional tunneling splittings, but does not account for the correct 2-fold and 4-fold systematic near degeneracies at high torsional excitation. However, the adiabatic approximation was able to show the origin of the strong IVR coupling and the scaling of the IVR coupling matrix elements.;Finally, a high-resolution experimental apparatus was developed to record the spectrum spanning the range of the polyad vCH=2 region. This experimental spectrum will be used to test and challenge the findings of both Perry's and Halonen-Hannien's theoretical models.