Theoretical aspects of the photochemistry of methanol, methylamine, and related materials

The photochemistries of methanol and methylamine are computationally rationalized using ab initio methods. It is shown that the lowest excited singlet states of these and related materials are n,3s Rydberg in character. These states are computationally shown to evolve adiabatically to the valence ground states of the various radical products along the NH, CN, CO, and OH bond rupture pathways in methylamine and methanol, respectively. The NH and CN n,3s bond rupture surfaces display minima in the region of the Franck-Condon excitation geometry. The NH bond ruptures in n,3s singlet ammonia and methylamine are shown to be identical in having small activation energies. The CN excited state bond rupture shows a much larger activation energy, indicating that trialkylamines should display some photostability in the region of the 0-0 transition. In methanol, neither CO nor OH excited-state bond rupture coordinates show minima. The observed preference for OH bond rupture in the UV photochemistry of methanol is rationalized as resulting from the lighter mass of the H atom as well as the computed more repulsive nature of the OH bond rupture. In methanol, both 1,2- and 1,1-H/sub 2/ molecular elimination excited-state pathways are examined. 1,2-H/sub 2/ elimination is found to have a small activation energy while the 1,1-elimination is difficult. The concept of de-Rydbergization is fully developed in order to rationalize the change in electronic character occurring along these various excited state pathways.