The Calculation of Bond Strengths from Photochemical Evidence

Abstract

When predissociation manifests itself in polyatomic molecules by a sudden broadening of the rotation lines of a discrete absorption spectrum, a close correspondence is to be expected between the beginning of the predissociation region and the strength of the bond involved. This principle is used to calculate the bond strengths of C–C in ethane, acetaldehyde, acetone and free acetyl (values: 72.1, 93.1, 96.5 and — 19.4 kcal., respectively) and of C–H in methane, acetaldehyde $\begin{array}{c}\mathrm{O}\\ ({\mathrm{CH}}_3\mathrm{C}-\mathrm{H})\end{array}$ , formaldehyde and free formyl (values: 94.8, 114.7, 103.3, and 0.1 kcal., respectively), where the bond strength is defined in reference to the state of the molecule at 0°K. Assuming the validity of the method, the bond strength values given are shown to be accurate within 1.0 kcal. The limitations of the method as well as some implications of the results are indicated.