Allowed and Forbidden Character in the 3715-Å π* ←n System of Benzaldehyde

Abstract

The 3715‐Å system of benzaldehyde‐h 6, ‐d 1, and ‐d 6 has been observed in absorption in the vapor phase. In spite of some diffuseness two band types have been distinguished by their characteristic rotational contours. Bands showing one main peak in the contour belong to the electronically allowed part of the system, assigned as 1 A″(nπ*)← 1 A′ , and those showing two main peaks split by about 9 cm−1 belong to the electronically forbidden part of the system involving activity of nontotally symmetric (a″) vibrations. Assignment of the wavenumbers of the three most strongly active a″ vibrations to approximate normal coordinates is certain in the case of the CHO‐torsional vibration but less certain in the cases of the out‐of‐plane substituent vibration and an out‐of‐plane ring vibration. Qualitative estimates of intensity borrowed through activity of these three vibrations have been made using undistorted and distorted molecule wavefunctions of the Pople–Santry–Segal type. The calculated n‐electron density is highly delocalized (45% in the aromatic ring) and results show that an out‐of‐plane carbonring vibration should be the most strongly active of the three, supporting the tentative experimental assignment of the most strongly active vibration to such a mode. Further, the transition moment arising from the activity of this vibration is shown to be predominantly along the long in‐plane axis in agreement with experiment. The electronically allowed part of the system is stronger than the electronically forbidden part. Detailed consideration of the allowed intensity show that the largest part arises from delocalization of the n electrons. For this reason the conjugating benzene ring is effective in breaking down the local symmetry of the carbonyl group. Both the allowed and forbidden parts of the system are dominated by progressions involving the C = O stretching vibration. It was shown previously from the polarized excitation spectrum that the most intense progression in the system is polarized perpendicular to the molecular plane and a weaker one is polarized predominantly along the long in‐plane axis. These findings are confirmed through correlation with the vapor spectrum.