The two competitive photodissociation channels in cyano carbonyls (NCC(O)X, X = CH3, CH(CH3)(2), C(CH3)(3), OCH3) at 193 nm. A study by photofragmenttranslational energy spectroscopy

The photodissociation of a series of four cyano carbonyl compounds NCC(O)X with X = methyl, isopropyl, tert-butyl, and methoxy was studied after excit ation at 193 nm using photofragment translational energy spectroscopy. For all the fragments generated (OCCN, XCO, CO, CN, X) the kinetic energy distr ibutions were measured and the two radical decay channels, NCC(O)X --> CN OCX and NCC(O)X --> OCCN + X, were identified. Dissociation leading to CN + OCX is the main decay path (similar to 85%) for acetyl cyanide (X methyl) , but is the minor pathway for X = isopropyl (30%), X tert-butyl (17%), and X = methoxy (<5%). The primary fragments CN + OCX were found to be stable with respect to secondary dissociation in all cases, except for acetyl cyan ide which exhibits spontaneous fragmentation of the acetyl fragments to CH3 + CO with a yield of similar to 9%. The stability of the remaining acetyl + CN fragment pairs is probably due to electronic excitation of one of the fragments. Elimination of X is the major decay path for X = methoxy, and th e anisotropic recoil distribution of the fragments suggests the decay to be fast on the time scale of a parent rotation. Within the homologous series X = methyl, isopropyl, and tert-butyl the propensity for X elimination, and thus OCCN production, increases with the size of the alkyl moiety. The obs erved trend toward increasing fragment internal energy with increasing size of the alkyl fragment indicates a considerable amount of randomization of the excess energy prior to bond scission. The investigation of the four com pounds proved methyl cyanoformate to be the most favorable species for an e fficient photolytical production of stable OCCN radicals, whereas acetyl cy anide is the most efficient source of CN radicals within this series.