UNIMOLECULAR PHOTOCHEMISTRY OF ALKANETHIOLS STUDIED BY PHOTODISSOCIATION PHOTOIONIZATION MASS-SPECTROMETRY

Unimolecular photodissociation of isomeric alkanethiols in the C3 to C -8 size range was studied by using photodissociation-photoionization m ass spectrometry. Photodissociation was performed with 193- and 248-nm radiation. The primary products were photoionized with coherent vacuu m ultraviolet radiation and detected in a time-of-flight mass spectrom eter. Three basic reaction channels were observed: C-S, C-C, and S-H b ond dissociation. The branching ratios for fragmentation through these channels were found to be strongly dependent upon molecular structure and photodissociation wavelength. All compounds gave intense products of C-S bond dissociation. Photoionization and secondary fragmentation characteristics of the primary products suggested that complete stati stical partitioning of the excess internal energy did not occur. Small molecules gave C-C bond dissociation at the alpha position, while lar ger molecules showed increasing probability for cleavage at the beta a nd gamma positions. Cleavage of the alpha C-C bond was strongly suppre ssed relative to other reaction channels when the expected product was a methyl radical. Complex rearrangements were observed for molecules exhibiting suppression of C-C bond dissociation. Cleavage of the S-H b ond was unambiguously observed only for small molecules (less-than-or- equal-to C5). The branching ratio for cleavage of the S-H bond was gre ater at 248 nm than 193 nm. This behavior is consistent with the excit ed-state electronic configurations, which show a greater contribution from sigmaSH at 248 nm than at 193 nm. Both the S-H and C-S bond diss ociation characteristics suggest that the excited electronic state pla ys an important role in the photodissociation process.