Accurate heats of atomization and accurate bond lengths.: II. Polyenes and polyphenyls

Accurate heats of atomization and bond lengths are calculated for several polyenes and polyphenyls, using the s.c.f.–l.c.a.o.–m.o. method described in Part I (1). Localization of π and σ bonds is found in the linear polyenes and in the diphenyl-substituted polyenes. The estimated stabilization energy per CC bond (SECC) gives a direct measure of the π electron conjugation and σ bond compression. Conjugation stabilization and steric hindrance are analyzed in 1,3-butadiene and biphenyl; the predicted geometries are in good agreement with experimental results. The minimum energy for any planar structure is found to be associated with the s.c.f. calculated bond lengths. The validity of pi bond order as a measure of molecular stability in the polyphenyls is discussed.