Concerning the Conformation of Isolated Benzylideneaniline

From PE.‐spectroscopical studies the torsional angle φ of the N‐phenyl ring in isolated benzylideneaniline 1 has been found to be definitely smaller than φ= 90°. An approximate valueφ= 36°has been estimated which is even smaller than the one observed in the crystal (φ= 55°) and suggested to prevail also in solutions of 1 . A reevaluation of the gas phase optical spectrum of isolated 1 supports a torsional angle similar to that found in the other phases. Calculations of the most stable conformation of 1 as well as of stilbene and azobenzene by the MINDO/3‐technique lead to torsional angles φ= 90° for both phenyl rings in all cases. These results are at variance with the experimental results and suggest that MINDO/3‐like its less advanced precursor MINDO/2 or like CNDO/2–is unreliable for low energy processes involving rotation of π‐systems connected by essential single bonds. It is concluded that the π‐energy of benzylideneaniline, like that of stilbene or azobenzene, would favor a planar conformation. The increased torsional angle in 1 as compared to the other two iso ‐conjugate systems arises from a larger steric interaction between phenyl‐ and bridgeprotons.