Cycloreversion of Formylcyclobutane Radical Anion:  Two-Step Rotating Mechanism

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2004-06, Vol.108 (24), p.5253-5257
• Main Authors: Qu, Zheng-wang, Zhu, Hui, Zhang, Ru-bo, Zhang, Xiao-dong, Ai, Xi-cheng, Zhang, Xing-kang, Zhang, Qi-yuan
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp031307y

The [2+2] cycloreversion reaction of formylcyclobutane radical anion (c-C4H7-CHO•-) has been investigated at the UB3LYP level with the augmented Dunning's correlation-consistent polarized valence double-ζ basis set supplied with four even-tempered sp shells. Very diffuse p−π*-like singly occupied orbitals (SOMO) are found for the c-C4H7-CHO•- and product CH2CHCHO•- radical anions, necessitating the use of a rather diffuse basis set for mechanistic study. The respective electron affinities of c-C4H7-CHO and CH2CHCHO are calculated to be 5.4 and 16.1 kcal/mol, showing the ability to bind an extra electron. The intermediate structure •(CH2)3CHCHO- is found to be a valence-bound distonic anion apt to the elimination of C2H4. The present two-step “rotating” cycloreversion mechanism for c-C4H7-CHO•- is formally similar to the biradical one for neutral cyclobutane structures, but with evidently lower potential barrier. For efficient electron-attachment catalysis, the extra electron should be trapped by suitable functional groups in some orbitals with substantial overlap with the σ*-orbitals of the cyclobutane structure.