Bond lengths and bond energies in conjugation and hyperconjugation

Abstract Dewar's arguments to the effect that all evidence for the existence of appreciable effects of π-electron resonance in the ground states of conjugated and hyperconjugated molecules is inconclusive, and that all observed effects are explainable as a result of differences in hybridization in carbon σ bond orbitals, are examined. It is shown that the CC bond-length shortenings predicted by up-to-date π-electron theory are not as great as usually supposed, and that the corresponding predicted lengthenings of CC and CC bonds are almost nil . It is concluded that Dewar is partly right in supposing that the lengths of conjugated and hyperconjugated CC single bonds are determined, to a greater extent than previously supposed, by the states of hybridization of the carbon σ bond orbitals. However, there seems to be no justification for Dewar's assumption that resonance shortenings are negligible. Tentative formulas for the contributions of resonance and hybridization are given, also a similar formula for the shortening of conjugated or hyperconjugated CC bonds in the cumulenes. Bond angles and hybridization are discussed, and the probable existence of marked deviations from ideal trigonal and digonal hybridization, often in opposite directions for CH and CC or CC carbon σ orbitals, is emphasized. These differences may help to explain why CH bonds are shortened less than CC bonds involving supposedly similar hybrids. Rather general bond energy formulas are given which assume additivity of bond energies but with a different bond energy for each hybrid type of CH or CC (or CC) bond. It is shown that these reduce to the usual simple formula which ignores differences in hybrid type under not unplausible assumptions which, however, are at fault in neglecting polar energies (i.e. ionic-covalent resonance energies), especially in CH bonds. Dewar's assumption that CH bond energies are essentially independent of hybridization, and his consequent conclusion that observed stabilization energies in conjugated systems are due entirely to strengthened hybrid CC bonds and not at all to π-electron resonance energy, is difficult to justify, especially in view of the expected importance of polar energies in CH bonds. A table of predicted polar energies for CH and CC bonds involving various hybrid CC σ orbitals is given. The estimation of π-electron energy in C 2 H 4 , and of π-electron conjugation energy in butadiene, based on some energy data for 90° twisting, are discussed. Diphenyl and s-cis butadiene are compared. The relative importance of π-electron resonance and hybridization in determining the dipole moments of conjugated and hyperconjugated molecules are discussed. While any conclusive theoretical calculation would in most cases be extremely complicated, there seems to be no good reason to doubt that π-electron resonance makes important contributions. Very strong evidence from nuclear magnetic resonance data is cited in support of usual current beliefs as to the role of π-electron resonance in charge distribution in conjugated and hyperconjugated molecules. Other evidence, including dipole moments in agreement with π-electron resonance theory predictions for molecules like fulvene and azulene where no differential hybridization effects should contribute, is cited.