Dimerization of pyrazole in slit jet expansions

Pyrazole dimer is observed for the first time in a free jet expansion. Its IR-active N-H stretching vibration is shifted by -269cm(-1) relative to the monomer. Along the 600 mm slit jet expansion, the average number density of pyrazole dimers is approximate to 10(11) cm(-3). Exploratory quantum chemical calculations including electron correlation are in good agreement with the observed frequency shift and confirm reciprocal hydrogen bonding with bent hydrogen bonds in a planar C-2h structure, as postulated by W. Hockel 65 years ago in this journal. Nanomatrix isolation spectra can be obtained by using Ar as the carrier gas. The more strongly coupled vibrational dynamics in pyrazole trimer is illustrated.