The molecular and crystal structure of 3-nitropyrazole was determined
by X-ray analysis. The triclinic unit cell contains 12 molecules which
form four hydrogen-bonded (N-H ... N) trimers. Each trimer comprises
of pseudo-ring in a flattened envelope distorted towards a chair confo
rmation, The crystal packing consists of layers formed by centrosymmet
ric related trimers joined through C-H ... O interactions. Ab initio c
alculations were performed on 3(5)-nitro- and 4-nitropyrazole and thei
r corresponding protonated forms up to the MP2/6-31G* level of theory
. The origin of the difference in aqueous basicities between both nitr
opyrazoles is discussed. (C) 1997 by John Wiley & Sons, Ltd.