Abstract
The mechanism of cycloreversion of cyclobutane radical anion (cโC~4~H) has been investigated at the UB3LYP/6โ31++G(d,p) level, and compared with those of neutral cโC~4~H~8~ and cโC~4~H radical cation. Although both cโC~4~H and C~2~H~4~ are shown to be Rydberg states unstable with respect to electron ejection, the activation barrier for the โrotatingโ cycloreversion of cโC~4~H (37.3 kcal/mol) is lower by about 25.2 kcal/mol than that of cโC~4~H~8~, and even the intervention of tetramethylene radical anion intermediate may reduce the activation barrier for the cycloreversion of cโC~4~H~8~ by about 8.4 kcal/mol, mainly due to stronger electronโdeficiency of intermediate biradical species than closeโshell cyclobutanes. For the cycloreversion for cโC~4~H, side isomerization reaction may be efficiently prevented by the low kinetic stability of tetramethylene radical anion intermediate towards dissociation, just different from the radical cation case. Our theoretical results have suggested the possibility of electronโattachment catalysis of the cycloreversion of some electronโdeficient substituted cyclobutanes. ยฉ 2003 Wiley Periodicals, Inc. J Comput Chem 24: 340โ344, 2003