Triplet State of Benzaldehyde

The phosphorescence spectra of benzaldehyde‐h₆, −1d₁, −4d₁, −3, 5d₂ and −d₆ have been studied optically in close packed matrices (acetophenone and methyl benzoate) and relaxed matrices (methylcyclohexane and perfluoro‐n‐hexane) at 4.2°K. All the spectra were sharp and could be analyzed vibrationally. In the close packed matrices, of the 11 possible out‐of‐plane (a″) fundamentals, nine are observed in Herzberg—Teller borrowing of intensity. The strongest modes are the aldehyde‐H wag, the CHO torsion and CHO wag, and their intensities were found to be stronger than those of any totally symmetrical bands including the origin band. 2 and 3 quanta of aldehyde‐H‐wag and CHO‐torsional modes are found with intensity alternation. In addition to this intensity alternation, the lack of long sequences of any nontotally symmetric mode and the analysis of the Franck—Condon envelope show that the triplet benzaldehyde does not deviate far from the ground state geometry. We conclude it is coplanar or nearly so. Deuteration effects on the vibronic intensities are remarkable. The long lifetime, ${\text{∼4×10}}^{\text{−2}}\sec$ at 4.2°K, supports a planar geometry for the triplet. The large change of lifetime and spectral intensity on substitution of the aldehyde hydrogen by deuterium shows that the aldehyde H plays an important role in the vibronic interaction. In the relaxed matrices most of the out‐of‐plane fundamentals are too inactive to be identified in the phosphorescence. The only out‐of‐plane mode clearly identified is the fundamental CHO‐torsional mode and its overtone. Higher overtones are not found. Most of the intensity is in the 0–0 band and the $\mathrm{\nu \; (}\mathrm{C}=\mathrm{O})$ progression which is longer than in the close packed matrices. Also active are in‐plane modes such as δ(Φ‐CHO), ν(Φ‐CHO), ν₁, ν_9a, and ν_8a. The short lifetime in the relaxed matrices (fully 20 times shorter than in the close packed matrices), ${\text{∼2×10}}^{\text{−3}}\sec$ at 1.68°K, without appreciable deuteration effect establishes that the triplet benzaldehyde is highly nonplanar with the aldehyde group twisted out of plane. We conclude from the lifetime that the twist angle exceeds 60°. The vapor phosphorescence resembles the relaxed matrix phosphorescence, but with still reduced lifetime. We conclude that in the free molecule the aldehyde group in the benzaldehyde triplet is also strongly out of plane.