Trends in the Torsional Potentials of Methoxy and Trifluoromethoxy Groups: An ab Initio and Density Functional Study on the Structure of para-Substituted Pyridines and Pyridinium Cations

Abstract

The structures and the torsional potentials around the aryl-O bond of 4-methoxypyridine, 4-trifluoromethoxypyridine, 4-methoxypyridinium cation, and 4-trifluoromethoxypyridinium cation were studied systematically with the aid of large-scale ab initio calculations performed at the Møller−Plesset second-order level and with a density functional method. In all compounds, the most stable conformer corresponds to the structure in which the methoxy or the trifluoromethoxy group is in an eclipsed conformation, oxygen and carbon atoms coplanar with the pyridine ring. The highest barrier to rotation, about 9.0 kcal mol-1, is predicted for 4-methoxypyridinium cation at an orthogonal conformation with a torsional angle of 90°. The lowest torsional barrier, less than 1 kcal mol-1, is obtained for 4-trifluoromethoxypyridine. The systematic variations of the calculated torsional potentials of these compounds are compared to those recently computed for methoxy- and trifluoromethoxybenzene and for their para-deprotonated anions. The trends in the energy differences between eclipsed and orthogonal conformations are correlated with concomitant changes of some of the optimized geometrical parameters in this series. Particularly suited are the two carbon−oxygen distances forming the ether linkage, and even more so their difference. The simple and astonishingly regular structural and energetic trends in this series can be rationalized by the assumption of a systematic decrease of the conjugation of the arene with the methoxy and trifluoromethoxy groups upon going from the 4-methoxypyridinium cation to the para-deprotonated trifluoroanisole anion.