Site-specific photochemistry of haloalkanes
Description
The production of abundant amounts of atomic hydrogen has been observed as a result of the ultraviolet photolysis of various haloalkanes. The inherent tendency for H-atom production to occur from chemically distinct carbon sites in the molecule is observed using deuterium labels in certain combinations. This site-specific photochemistry of H-atom production is studied at 193, 222, and 248 nm. Doppler spectroscopy and time-of-flight mass spectrometry techniques are used in conjunction with 1+1 (121.6 + 364.7 nm) photoionization through Lymann $\alpha$ for the detection of H atom fragments. Data may be treated to yield a quantitative value for H-atom production from each site: the site-specific branching ratios. Two- and three carbon haloalkanes are utilized as are compounds of the three common halogen type (Cl, Br, I). Trends in the site preference for C-H bond cleavage are readily observed, primarily correlated to photolysis wavelength. Definite carbon site preferences are invariably observed resulting from 248 nm photolysis, where as, 193 nm photolysis produces results which always reflect the number of H atoms on each carbon type. Analysis of data produced from 222 nm photolysis shows that the data are not self consistant and no definite conclusions can be drawn. Overall, the results indicate a possible trend with increased photolysis energy towards H-atom shifts. Further experimentation involving two photolysis lasers indicate that the observed photochemical trends are that of the alkyl radical intermediates. These are the first studies to reveal such information on the photochemistry of small alkyl free radicals