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.
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