Spectroscopic Analysis and Geometry Assignment of the Minimum Energy Conformations of 2-Phenoxypyridines and Diphenyl Ethers

Abstract

Abstract The conformational preferences of 2-phenoxypyridines and diphenyl ethers were determined by minimum-energy optimization using the method of ab initio molecular orbital calculations with STO-3G basis sets, and by spectral measurements and their analyses based on CNDO/S-CI calculations. The o,o-disubstituents of the methyl groups to diphenyl ether and 2-phenoxypyridine, i.e., 1,3-dimethyl-2-phenoxybenzene and 2-(2,6-xylyloxy)pyridine, respectively, prefer a symmetric skew conformation. The conformation of unsubstituted diphenyl ether (1a) is nonrigid with a dihedral angle, about 90°, of two phenyl rings at room temperature. Spectral observations at 77 K have provided evidence for this conclusion. On the other hand, 2-phenoxypyridine (2a) has been found to possess sufficient internal barriers to stabilize the two aromatic rings in a skew form. The weak nuclear repulsion, compared with that in 1a, would mainly contribute to stability in the skew form. The characteristic 1La band observed for 2a is attributed to the configuration interaction of the n–π* excited state to the 1La states in the skew form.