AROMATIC PROPELLENES .3. NMR, X-RAY CRYSTALLOGRAPHY AND SEMIEMPIRICALCALCULATIONS ON THE CONFORMATIONAL ISOMERISM OF AZOL-1'-YL)-3,6-BIS(3'',5''-DIMETHYLPYRAZOL-1'-YL) BENZENE

The molecular and crystal structures of two crystalline forms of 1,2,4 ,5-tetrakis(pyrazol-1'-yl)-3,6-bis(3 '',5 ''-dimethylpyrazol-1 ''-yl) benzene and one inclusion complex with two molecules of acetic acid we re determined by x-ray analysis. The acetic acid forms dimers through symmetry centers and the only interactions in the structures are mainl y due to weak C-H ... N interactions. All 14 possible conformations of the pyrazole with regard to the benzene ring were explored by means o f AM1 semi-empirical calculations, The observed conformation in the cr ystal structures agrees fairly well with the most stable conformation which presents the pyrazole rings with the N(2) alternating between bo th sides of the phenyl plane. These calculations allow one to identify the minor isomer present in solution together with the major isomer c orresponding to the crystal structure.