The electron spin resonance spectra of 1-adamantylcyclobutadiene radical cations

Abstract

By photolysing solutions of the appropriate alkynes and aluminium chloride in dichloromethane, the following cyclobutadiene radical cations have been generated, and their e.s.r. spectra recorded: 1-Ad₄C₄^+˙, ([3,5,8-²H₃]-1-Ad)₄C₄^+˙, ([²H₁₅]-1-Ad)₄C₄^+˙, cis- and trans-Me₂-1-Ad₂C₄^+˙, and cis-Et₂-1-Ad₂C₄^+˙(1-Ad = 1-adamantyl). It is concluded that in 1-Ad₄C₄^+˙, the bulky adamantyl groups cause the central ring to distort from planar, but that there is no restriction of rotation about the adamantyl–ring bond, and that proton hyperfine coupling to the δ-CH group is larger than to the γ-CH₂ groups. In the e.s.r. spectra of the deuteriated compounds, a variety of ¹³C satellites can be distinguished, and they have been assigned using probability theory. The interpretation of the spectra is supported by UMINDO/3-INDO calculation. In cis-Me₂-1-Ad₂C₄^+˙, a(Me) is lower, and in trans-Me₂-1-Ad₂C₄^+˙ it is higher, than in Me₄C₄^+˙. The various steric and electronic effects, and solvent and counterion interactions, which may contribute to this ordering, are discussed.