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

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 ...