[OHO](-) and (+)[NHO](-) hydrogen bonds in the 2 : 1 adduct of pentachlorophenol with 7-methyl-1,5,7-triazabicyclo[4.4.0] dec-5-ene [(PCP)(2)center dot MTBD]

The results of X-ray diffraction and IR spectroscopic studies for 2:1 penta chlorophenol-7-methyl-1,5,7-triazabicyclo[4.3.0]dec-5-ene [(PCP)(2). MTBD] adduct are reported. The geometry of MTBD cations reflects the equal distri bution of the positive charge among three nitrogen atoms. Short asymmetric [OHO](-) hydrogen bonds with (OO)-O-... distance of 2.508(2) A and NH+... O - hydrogen bonds with N ... O distance of 2.802(2) Angstrom are formed show ing broad IR absorption with two maxima located at similar to 1200 and simi lar to 2400 cm(-1). The second maximum is interpreted as due to the 0 --> 2 transition between split levels in an asymmetric double minimum potential. One of the oxygen atoms forms an additional O ... H-N+ hydrogen bonding wi th an MTBD cation. The situation is somewhat different in acetonitrile solu tion whose IR spectrum shows continuous absorption extended over whole the IR region. In acetonitrile, dissociation to free OHO- and +NH ions takes pl ace and the OHO- bridges become dynamically symmetric, The broadening is in terpreted in terms of a stochastic distribution of the geometry and the Zun del polarizability theory. (C) 2000 Elsevier Science B.V. All rights reserv ed.