Conformations of solid 2-methyl-4-(p-X-phenylazo)imidazoles by C-13 CP MASNMR spectroscopy and PM3 semi-empirical calculations

Solid 2-methyl-4-(p-X-phenylazo)imidazoles form hydrogen bonded chains with N-H ... N bonds and C-H ... O or C-H ... N interaction. Depending on the n ature and orientation of the substituents X it was possible to identify one tautomer if X=H (2), Br (3), NO2 (4) and the two, a- and b-tautomers in th e crystal unit if X=OCH3 (1). The C-13 CP MAS NMR spectra of (4) indicate t he presence of phenyl ring dynamics. A preferred structure present in the s olid state is that with different lengths of C1'-N and C4-N bonds and with higher dipole moment. (C) 1999 Elsevier Science B.V. All rights reserved.