Crystal structure and molecular motion in pyridine N-oxide semiperchlorate

The crystal and molecular structure of (C5H5NO)(2). HClO4 has been determin ed at room temperature by X-ray diffraction. The compound crystallizes in t he monoclinic C2/c space group with a = 10.597(3), b = 8.919(2), c = 13.716 (2) Angstrom and beta = 103.99(2)degrees. The unit cell contains four ClO4 anions and four homoconjugated cations. The proton in the symmetric hydroge n-bond has been located on the two-fold axis and the O ... H+.... O bridge has the following,oeometry: O ... O = 2.429(4) Angstrom and < O ... H+... O = 176(1)degrees. The structure of the title complex has also been analyzed by the HF/6-31G(d,p), PM3 and SAM1 methods. In the calculations, the hydro gen-bonded proton is located asymmetrically in the bridge and the pyridine rings are not equivalent. The distances between the proton and the CI atom of the perchlorate ion are: 4.18 and 4.74 Angstrom (crystal), 3.98 Angstrom (PM3), 5.07 Angstrom (HF) and 9.23 Angstrom (SAM1). The results of H-1 NMR studies have revealed no pyridine ring reorientations, which means that in the time scale of this method the cation is rigid. The solid state FTIR sp ectrum of the complex shows a broad absorption extending from ca. 1700 down to 400 cm(-1), due to the H-bond vibration. This absorption is absent in R aman and inelastic neutron scattering spectra. The frequencies of the skele tal vibrations are also analyzed. (C) 1999 Elsevier Science B.V. All rights reserved.