The cyclopropane ring (A) has many properties which differ from higher cycloalkanes and are more (a) analogous to those of the vinyl group (C=C). In particular, the C(A) hybrid orbitals used in substituentbond formation are closer to sp 2 than sp 3, while a conformation-dependent conjugative ability is exhibited with zr-acceptor substituents. The Cambridge Crystallographic Database (CCD) has been used to obtain the (b) mean X-ray-determined geometries of A-A fragments (A = C-C, C=C, C=O, C-N), which are compared with the geometries of analogous fragments where/x is replaced by C-C and C=C. Changes in the D-A bond length (D = C-C, A, C=C) have been analysed in terms of hybridization and conjugative effects. The X-ray data indicate that the C(A) hybrid involved in the substituent bond is approximately sp 22, while the (c) relative conjugative ability A:vinyl is ca 71%. Problems involved in precise retrieval and geometric analysis of high-incidence organic fragments from CCD are discussed in terms of (i) restrictions on substitution and environment, (ii) acceptance criteria for coordinate sets, (iii) geometric screening of derived geometry to obtain unbiased mean values.
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Frank H. Allen (1981) studied this question.