DIFFERENCES BETWEEN THE N-CENTER-DOT-H-CENTER-DOT-O AND O-CENTER-DOT-H-CENTER-DOT-O HYDROGEN-BONDS IN COMPLEXES OF 2,6-DICHLORO-4-NITROPHENOL WITH PYRIDINES AND PYRIDINE N-OXIDES

Complexes of five pyridines and nine pyridine N-oxides with 2,6-dichlo ro-4-nitrophenol (DCNP) in solution and the solid state were studied b y Fourier transform IR and UV spectroscopy, by quantum-mechanical calc ulations with the semiempirical parametric method 3 (PM3) and by X-ray analysis. The crystals of the 1 : 1 complex of 4-methoxy-2,6-dimethyl pyridine N-oxide with DCNP are monoclinic, space group P2(1)/n, a = 4. 5936(5) Angstrom, b = 21.953(3) Angstrom, c = 15.664(2) Angstrom, beta = 92.87(1)degrees, V = 1577.6(8) Angstrom(3), Z = 4. The molecules of the complex are joined together by an N+O-H ... O- hydrogen bond with an O ... O distance of 2.425(3) Angstrom, a C-O- distance of 1.286(3) Angstrom and a (N+O)-H ... O- angle of 152.9 degrees. The PM3 method predicts for all the investigated complexes two minima, the deeper one for B ... HA complexes and the shallower one for the B+-H ... A(-) fo rms. For the 4-methylpyridine complex the N+-H ... O- distance is repr oduced correctly but for the 4-methoxy-2,6-dimethylpyridine N-oxide co mplex the N+-H ... O- distance is too long, The predicted hydrogen-bon d angles differ from the experimental values by more than 10 degrees. In solid state complexes of pyridines the N ... O distances and the br oad absorption due to a protic vibration are not directly related to D elta pK(a). This is due to the crystal packing forces. In solution the broad absorption varies with Delta pK(a). A band in the 3500 cm(-1) r egion due to the solvated phenol is present in all investigated comple xes in solution. Absorption in the 3000-2000 cm(-1) region of pyridine complexes is more intense than that of the pyridine N-oxides, in agre ement with the difference in N ... O and N-O ... O distances. The broa d absorption in the spectra of pyridine complexes is more influenced b y solvent effects than in the pyridine N-oxide complexes. The UV spect ra of the pyridine complexes show two bands due to B ... H-A (305-315 nm) and B+-H ... A(-) (382-395 nm) forms. The UV spectra of complexes of pyridine N-oxides of intermediate strengths in CH2Cl2 are not combi nations of the spectra of phenol and phenolate, The band in the interm ediate position denotes that neither species close to phenol nor to ph enoxide ion is present. In these complexes the proton is probably loca lized in a single minimum and the minimum moves from the donor to the acceptor or, what is more probable, reorganization of the solvent mole cules around the complex is faster than the time range of UV spectrosc opy. In acetonitrile the situation is quite different as two bands are present, in agreement with a prototropic equilibrium. Effects of solv ent, concentration and stoichiometry on interactions of DCNP with pyri dines and pyridine N-oxides are compared and discussed. An extended me chanism of the proton-transfer reaction is proposed.