MOLECULAR-STRUCTURE OF PYRIDINE N-OXIDE COMPLEX WITH 2,6-DICHLORO-4-NITROPHENOL

A complex of pyridine N-oxide (PyO) with 2,6-dichloro-4-nitrophenol (D CNP) was studied by X-ray diffraction, FT-IR spectroscopy and quantum- mechanical calculations with the DFT and semiempirical methods. The cr ystals of the PyO . DCNP are triclinic, space group P (1) over bar, a = 6.833(1) Angstrom, b = 8.717(2) Angstrom, c = 11.482(2) Angstrom, al pha = 98.93(2)degrees, beta = 93.63 (1)degrees, gamma = 109.12(2)degre es, V = 633.6(3) Angstrom(3), Z = 2. The molecules of the complex are joined by the N-O ... H-O hydrogen bond with the O ... O distance of 2 .476(2) Angstrom, and the O(4)... H(1)-O(1) angle of 165.1 degrees. Th e dihedral angle between the planes of the bridged pyridine and phenyl rings is 71.8 degrees. The weak C-H ... O, C-H ... Cl interactions an d stacking forces stabilize three dimensional packing pattern. The SAM 1 and DFT methods predict one minimum for B ... H-A form, while the PM 3 method predicts two minima, the deeper one for B ... H-A complex and the shallower one for B+-H ... A(-) form. For the most stable complex es the predicted O ... O distances are longer than the experimental va lue by 0.141, 0.067 and 0.231 Angstrom, respectively for the DFT, SAM1 and PM3 methods. The calculated bond lengths, except N(1)-O(4), are l onger than those fi om the X-ray as results of intermolecular interact ions in the crystal. The SAM1 geometry of PyO . DCNP is slightly bette r than this obtained by the PM3 method and it is recommended as input in nb initio calculations. The protonic broad absorption in the 1500-2 50 cm(-1) region is typical for such a short hydrogen bond and the pro ton motion may be described by a potential curve with an asymmetric do uble minimum. Proton motion in the bridge is faster than the time rang e of IR spectroscopy.