Structures and hydrogen bonding in the 1 : 1 and 1 : 2 complexes of trimethylamine N-oxide with pentachlorophenol

The following crystalline complexes of trimethylamine N-oxide (TMAO) with p entachlorophenol (PCP): TMAO.PCP, TMAO.PCP . H2O and TMAO.2PCP have been pr epared and characterized by FTIR spectroscopy, quantum-mechanical calculati ons with DFT and semiempirical methods and X-ray diffraction (two complexes ). The crystals of TMAO.PCP are orthorombic, space group Peen with a = 27.6 21(6), b = 13.642(3), c = 7.191(1) Angstrom, V = 2709.6(9) Angstrom(3), Z = 8. The proton is transferred from PCP to TMAO and both residues are linked by an O-H ... O hydrogen bond of length 2.464(3) Angstrom and an angle of 168(4)degrees. The crystals of TMAO.PCP . H2O are monoclinic, space group C 2/c with a = 20.388(3), b = 10.314(2), c = 14.300(2) Angstrom, beta = 105.4 3(2)degrees, V = 2898.6(8) Angstrom(3), Z = 8. The proton is transferred fr om PCP to TMAO. Two pentachlorophenolates are bridged by two water molecule s to form an eight membered ring with four O-H . O hydrogen bonds of 2.663( 3) and 2.647(3) Angstrom, respectively. Each water molecule, in turn, addit ionally forms a hydrogen bond of 2.497(3) Angstrom to protonated TMAO.BLYP, SAM1 and PM3 calculations have been carried out of the most stable structu res. In the case of TMAO.PCP and TMAO.PCP . H2O a good agreement between th e calculated and X-ray data are obtained. FTIR spectra of the investigated complexes are consistent with the O ... O distances. TMAO.2PCP in acetonitr ile solution exists as a mixture of TMAO.PCP and solvated PCP. The observed shifts of the centre of gravity of the broad absorption in TMAO.PCP toward higher wavenumbers relative to those in complexes of pyridine N-oxides wit h comparable Delta pK(a) can be explained by the resonance interaction betw een the N-oxide group and aromatic ring. (C) 1999 Elsevier Science B.V. All rights reserved.