Photodissociation of glyoxal: Resolution of a paradox

Photofragmentation of glyoxal, C2H2O2, under collision free conditions proc eeds by internal conversion from S-1 to vibrationally excited S-0, which is observed to dissociate into H-2+CO+CO (28%), H2CO+CO (65%), and HCOH+CO (7 %). Early molecular orbital calculations placed the barrier for the formald ehyde channel 12-20 kcal/mol above the three body fragmentation channel, co ntrary to what would have been expected from the branching ratios. The best calculational estimate of the barrier for the three body fragmentation was approximate to8 kcal/mol higher than the reported activation energy for th e thermal decomposition of glyoxal. These problems have been resolved by th e more accurate ab initio molecular orbital calculations reported in the pr esent note. With the complete basis set extrapolation method of G. Petersso n and co-workers using an atomic pair natural orbital basis set (CBS-APNO), the calculated heats of reaction that are within 0.4-0.8 kcal/mol of the e xperimental values for glyoxal-->H-2+2CO, H2CO+CO, and 2 HCO. The barrier c omputed for H2CO+CO is 54.4 kcal/mol, in excellent agreement with the high pressure limit of the activation energy for thermal decomposition of glyoxa l. The computed barrier for the three body fragmentation is 4.8 kcal/mol hi gher than the H2CO+CO channel, in agreement with the observed lower yield f or this channel. (C) 2001 American Institute of Physics.