Spectroscopic and theoretical investigation of the conformational space of a pyrazolo-thiazole precursor of extended dipole diazafulvenium methide intermediates

The structure, preferred conformers and vibrational spectra of the pyrazolo-thiazole precursor of extended dipole diazafulvenium methide intermediates, dimethyl 2,2-dioxo-1H,3H-pyrazolo[1,5-c][1,3]thiazole-6,7-dicarboxylate (DPTD) were investigated in low-temperature noble gas matrices (Ar, Xe), low temperature neat amorphous and crystalline phases and in KBr pellet (crystal and melted phases) by infrared spectroscopy, supported by quantum chemical calculations. Two types of conformers were observed spectroscopically in the matrices and in the neat amorphous solid resulting from fast condensation of the vapour of the compound onto the cold (20 K) substrate of the cryostat. These conformers correspond to the two pairs of nearly degenerated structures exhibiting skew/cis (conformers S′C and SC′) and gauche/trans (conformers G′T and GT′) arrangements of the NC–CO/CC–CO moieties. In the crystalline phase, the vibrational signature of the compound indicates that it exists in a skew/cis arrangement. After melting of the crystal, a conformational mixture is formed, where both skew/cis and gauche/trans forms exist in equilibrium and, with all probability, also conformer G′C′. This latter conformer cannot exist in the low temperature matrices and amorphous state, in view of the very low energy barrier that separates this form from the lower energy SC′ conformer, which can be easily surpassed during deposition of the compound (conformational cooling).