IONIZED METHYL FORMATE (CH3OCHO.+) AND ITS DISTONIC ISOMER (.CH2OC+HOH)

Ab initio molecular orbital calculations have been carried out for ionized methyl formate (CH3OCHO.+) and its distonic isomer (.CH2OC+HOH). The ground state of ionized methyl formate is 2A' (sigma) and the lowest energy structure has COCO syn and a staggered methyl group (2a). The 2A'' (pi) state (3) lies 74 kJ mol-1 above the 2A' state (2a); its preferred conformation also has COCO syn but in this case the methyl group is eclipsed. The two structures of the distonic isomer (4) of lowest energy are both asymmetric but may be described approximately as COCO syn and OCOH anti (4a) and COCO anti and OCOH syn (4b); 4a lies 62 kJ mol-1 lower in energy than 2a. Rearrangement of 2a to 4 requires 41 kJ mol-1. Vertical ionization of methyl formate produces an ion lying 35 kJ mol-1 above 2a; consequently, rearrangement to the distonic ion 4 only requires a further 6 kJ mol-1. The calculations thus predict that ionization of methyl formate initially produces the sigma-state of the methyl formate radical cation (2a) which may rearrange with a small barrier to the distonic isomer 4.