The Two Competitive Photodissociation Channels in Cyano Carbonyls (NCC(O)X, X = CH_3, CH(CH_3)_2, C(CH_3)_3, OCH_3) at 193 nm. A Study by Photofragment Translational Energy Spectroscopy

摘要

The photodissociation of a series of four cyano carbonyl compounds NCC(O)X with X = methyl, isopropyl, tert-butyl, and methoxy was studied after excitation at 193 nm using photofragment translational energy spectroscopy. For all the fragments generated (OCCN, XCO, CO, CN, X) the kinetic energy distributions 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 (～85%) for acetyl cranide (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 cyanide which exhibits spontaneous fragmentation of the acetyl fragments to CH_3 + CO with a yield of ～9%. The stability of the remaining acetyl + CN fragmet pairs is robably due to electronic excitation of one of the fragments. Elimination of X is the major decay path for X = methoxy, and the anisotropic recoil distribution of the fragments suggests the decay to be fast on the tiem 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 observed trend toward increasing fragment internal energy iwth 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 compounds proved methyl cyanoformate to be the most favorable species for an efficient photolytical production of stable OCCN radicals, whereas acetyl cyanide is the most efficient source of CN radicals within this series.