Photolysis of Methanol at 185 nm. Quantum Mechanical Calculations and Product Study

Article abstract of DOI:10.1021/ja00328a011

A series of ab initio MRD-CI calculations is carried out in conjunction with new experimental investigations in order to study the photolysis of methanol in its (n-3s) excited state at 185 nm.Potential surfaces for the key primary processes of O-H, C-O, and C-H scission are obtained as well as for the H2 elimination reaction which leads to formaldehyde production.Comparison with known transition and dissociation energy data indicates that the present calculations are accurate to within 0.1 eV over the entire potential surface of interest in this study.In agreement with the recent work of Kassab et al. it is found that O-H scission proceeds along a purely repulsive pathway corresponding to the conversion of the 3s Rydberg orbital into a 1s AO of the hydrogen atom, but when the C-O bond is broken a new feature is found, namely a slight barrier for this process caused by a sharply avoided crossing between the respective Rydberg and valence electronic configurations in this case.The fact that the mass of the hydrogen atom fragment is much smaller than that of either the methyl or hydroxyl radicals tends to further promote O-H scission over the corresponding C-O bond-breaking process, in agreement with known quantum yield results for the photolysis of saturated alcohols; this mass effect is confirmed by new experimental evidence also obtained in the present study which shows that the quantum yields of C-O scission products are enhanced by deuteration at the oxygen atom of methanol.Finally, the calculations indicate that H2 elimination proceeds with a slight barrier in the excited state potential curve, quite possibly involving a nonconcerted mechanism in which the O-H bond is first lengthened substantially before a second hydrogen atom is abstracted from the carbon atom of the same molecule.