Intramolecular Homolytic Translocation Chemistry: An ab Initio Study of 1,n-Halogen Atom Transfer Reactions in Some omega-Haloalkyl Radicals.

Ab initio calculations using all-electron (3-21G(()()), 6-311G) and pseudopotential (DZP) basis sets, with (MP2, QCISD) and without (UHF) the inclusion of electron correlation, predict that 1,n-halogen transfer reactions in the 5-halo-1-pentyl (6), 6-halo-1-hexyl (7), and 7-halo-1-heptyl radicals (8) proceed via C(s)- and/or C(2)-symmetric transition states (9-11), except for the 5-bromo-1-pentyl (6, X = Br) radical for which a C(s)-symmetric transition state (9) was located only at the UHF/3-21G(()()) level of theory and the 5-iodo-1-pentyl radical (6, X = I) for which no transition state (9) could be located at any level of theory used in this study. Energy barriers for these translocation reactions of between 120.0 (1,7-iodine transfer) and 191.0 kJ mol(-)(1) (1,5-chlorine transfer) are predicted at the MP2/DZP level of theory; QCISD/DZP (single-point) calculations predict similar energy barriers. These high energy barriers are a consequence of unfavorable factors associated with ring size and long carbon-halogen separations in transition states (9-11) which lead to significant deviations from the collinear arrangement of attacking and leaving radicals preferred in transition states involved in homolytic substitution reactions at halogen. The dependence of transition state energy on attack angle at halogen has been explored for the attack of methyl radical at chloromethane. At the MP2/DZP level of theory, attack angles of between 80 and 90 degrees are calculated to lead to increases in energy barrier of about 100 kJ mol(-)(1) when compared with the collinear (180 degrees ) arrangement of attacking and leaving groups. The mechanistic implications of these predictions are discussed.