Site-specific photochemistry of haloalkanes and their associated alkyl radical intermediates
Description
Large amounts of neutral hydrogen atoms are easily produced by photolysis of various haloalkanes. The natural propensity for a species to produce H atoms from chemically distinct sites within a molecule is investigated. The primary targets are alkyl radical intermediates, and a variety of haloalkane systems are used as precursors. Deuterium atoms are selectively substituted for H atoms to mark a specific site in the parent molecule. This labelling allows one to determine with relative certainty the site from which the H atoms are being produced. The experiments are conducted using different photolysis wavelengths, specifically 193, 222, and 248 nm. Combinations of photolysis wavelengths, in conjunction with a 1 + 1 (121.6 + 364.7 nm) resonance-enhanced multiphoton ionization probe through the Lyman-$\alpha$ transition in atomic hydrogen, allow positive identification of the reactive site. Once the H atoms are ionized they are accelerated into a time-of-flight mass spectrometer, detected by a microchannel plate detector, and converted into an intensity signal. Doppler spectroscopy is used to analyze and quantify the data. Normal and branched propyl systems of three common halogens (Cl, Br, and I) are used as precursors. Previous experiments have shown that photolysis of these systems at 248 nm yields definite site-specific production of H atoms whereas similar photolysis at 193 nm gives probabilistic results. This investigation takes an important step by adding a second photolysis laser to the system to show conclusively that the H atoms are being produced from alkyl radical intermediates