A b i n i t i o studies of structural features not easily amenable to experiment. 12. The molecular structure of bicyclo(2.1.0)pentane and the usefulness of a b i n i t i o geometries in interpreting microwave r s-structure model uncertainties

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Abstract
The molecular structure of bicyclo(2.1.0)pentane was refined by standard single determinant ab initio procedures on the 4‐21G level to interpret the relatively large model uncertainties of the microwave rs structure by S.N. Mathur, M.D. Harmony, and R.D. Suenram [J. Chem. Phys. 64, 4340 (1976)]. In order to provide estimates for rs−re (ab initio) corrections for C–C bonds, the structures of cyclopropane, n‐propane, n‐butane, and n‐pentane were also refined in the same way. The calculated differences Δre between the C–C bond distances in the cyclopropane ring (0.026 Å) and in the cyclobutane ring (0.036 Å) of bicyclo(2.1.0)pentane were found in close agreement with the differences observed (Δrs; 0.029 and 0.037 Å, respectively). When estimates for the differences between rs and re (ab initio), derived from analogous sytems, were applied to the calculated C–C bond distances in the cyclopropane and in the cyclobutane ring of bicyclo(2.1.0)pentane, respectively, the calculated parameters were C1–C4 = 1.535 Å, C1–C5 = 1.509 Å, C1–C2 = 1.528 Å, and C2–C3 = 1.564 Å. The corresponding experimental values are 1.536(3), 1.507(4), 1.528(6), and 1.565(3) Å, respctively. Hence, the preferred conclusion of this study is that the trends in the experimental C–C bond distances of bicyclo(2.1.0)pentane very likely describe true structural features of this system, even though some of the individual parameters are unresolved due to experimental error limits. It is not unreasonable to assume that high qualtiy ab initio geometries may be generally of value in interpreting substitution structures with model dependent uncertainties which are not known exactly.