INTRAMOLECULAR HOMOLYTIC TRANSLOCATION CHEMISTRY - AN AB-INITIO STUDYOF 1,N-HALOGEN ATOM-TRANSFER REACTIONS IN SOME OMEGA-HALOALKYL RADICALS

Ab initio calculations using all-electron (3-21G(), 6-311G**) and pse udopotential (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 UH F/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 the ory used in this study. Energy barriers for these translocation reacti ons of between 120.0 (1,7-iodine transfer) and 191.0 kJ mol(-1) (1,5-c hlorine 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 associa ted with ring size and long carbon-halogen separations in transition s tates (9-11) which lead to significant deviations from the collinear a rrangement of attacking and leaving radicals preferred in transition s tates involved in homolytic substitution reactions at halogen. The dep endence of transition state energy on attack angle at halogen has been explored for the attack of methyl radical at chloromethane. At the MP B/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 atta cking and leaving groups. The mechanistic implications of these predic tions are discussed.