The value of the π‐bond order–bond length relationship in geometry prediction and chemical bonding interpretation

Abstract

AbstractThe π‐bond order–bond length relationship is reintroduced to the literature and extended to heteronuclear bonds by presenting graphs derived solely by theoretical methods. π‐bond order and overlap population results for carbon–carbon, carbon–nitrogen, and carbon–oxygen bonds obtained from ab initio STO‐3G calculations using theoretically‐optimized geometries are reported for a series of pteridines and for a wide range of small organic molecules. The order–length correlation graphs are used in predicting the “intrinsic” single bond lengths for sp2 – sp2 and sp – sp hybridized CC, CN, and CO bonds, and in evaluating the relative importance of hybridization, π‐electron delocalization and bond polarization effects in causing bond shortening in conjugated and hyperconjugated molecules. The calculated value of the π‐bond order for a given bond in a molecule is shown to be relatively insensitive to moderate geometry changes: Hence, a use for the correlation graphs in geometry prediction is suggested. Some results for the extended 4‐21G basis set are also presented.