Through-bond and through-space interactions of substituents and the nonlinearity of X-C≡Y Linkages

Ab initio SCF calculations using the split valence basis sets 3–21G, 4–21G, 4–31G, 5–31G, and 3–321G, which were augmented by a set of polarization functions on the nonhydrogen atoms, were performed on a series of propynes, nitriles, isonitriles, and geminal dinitriles. Substituent effects on the structures of these molecules have been interpreted via appropriate concepts. In contrast to widespread opinion, the CC triple bond may be highly sensitive to effects of substituents, particularly when these effects are associated with -bonding interaction. This is reflected, for instance, by the appreciable contraction of the CC bond length in 1 -fluoropropyne (1 -FP) of 1.173 Å in comparison to the same bond in cyanopropyne, 1.183 Å. One of the interesting results of the present work is the markedly short C-F bond distance of 1.275 Å in 1-FP. The most acceptable rationale for this bonding behavior is provided by electrostatic arguments. The geometric results of both F₂C(CN)₂ and CF₃NC, particularly the short C-F bond distance of 1.323 Å and 1.306 Å, respectively, indicate the presence of fluorine negative hyperconjugation (anomeric effect). The characteristic nonlinearity of the C-CN chain in several geminally substituted dinitriles, YC(CN)₂, has been discussed. It appears that a relationship exists between the direction of bending of the C-CN moieties and the electronegativity of Y.