Conformation of 1-benzyloxy-4-methoxypyridinium perchlorate: X-ray, AM1 and PM3 studies

Abstract

The crystal structure of 1 -benzyloxy-4-methoxypyridinium perchlorate has been deduced from single-crystal X-ray diffraction data. The crystals are monoclinic, space group P2₁/a, with a= 9.254 0(7), b= 21.532(2), c= 7.335 2(8)Å, β= 102.69(1)° and Z= 4. The final R value is 0.084 for 1593 observed reflections. The molecule is extended with a trans conformation around the central C(1)–O(1) bond linking the phenyl and pyridine residues. The aromatic ring planes are almost perpendicular to the linking C(1)–O(1) bond and the rings are twisted 9.1 (2)° to each other. The N–O distance [1.401 (7)Å] is much longer than that found in pyridine N-oxides and is typical for an N–O single bond. Both the PM3 and AM1 methods predict that the trans conformer is less stable (by ca. 3 kcal mol^–1) than the gauche conformer in the gas phase. The problem of the conformation of the phenyl group (D) and pyridinium group (A) in compounds of the type DCH(R¹)CHR²A (R¹= R²= H or alkyl) is reviewed.