Through-Bond and Through-Space Interactions of Substituents and the Nonlinearity of X-C Y Linkages II. Comparative Theoretical Study of Molecules Involving Triply Bonded Atoms of the Type CH3C CX (X = H, F, Cl, CN), YCZ(2) (Y = H-2, O, F-2, Cl-2 and Z = CN), and CX3Y (X = H, F and Y = CN, NC)

Ab initio SCF calculations using the split valence basis sets 3-21G, 4-21G, 4-31 G, 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 C C triple bond may be highly sensitive to effects of substituents, particularly when these effects are associated with sigma-bonding interaction. This is reflected, for instance, by the appreciable contraction of the C C bond length in 1-fluoropropyne (1-FP) of 1.173 angstrom in comparison to the same bond in cyanopropyne, 1.183 angstrom. One of the interesting results of the present work is the markedly short C-F bond distance of 1.275 angstrom 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 angstrom and 1.306 angstrom, respectively, indicate the presence of fluorine negative hyperconjugation (anomeric effect). The characteristic nonlinearity of the C-C N 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-C N moieties and the electronegativity of Y.