Structures and vibrational spectra of pinacol.: 1. Infrared and matrix infrared spectra of monomeric pinacol. Ab initio calculations on conformers and vibrational frequencies Original Research Article

The infrared spectra of monomeric pinacol molecules (2,3-dimethyl-2,3-butanediol; (CH3)2C(OH)C(OH)(CH3)2) have been recorded in the gas phase and dilute nonpolar solutions, and in an argon matrix. The vibrational data are consistent with the intramolecularly hydrogen-bonded G-type (gauche with respect to the central C–C bond) conformers and there is no evidence for the T-type (trans with respect to the central C–C bond) conformers, which have been observed in the condensed phases. This was confirmed by studying the infrared region 835–815 cm−1, which was found to be the most indicative to show spectral changes within the type of the conformers. In this region the band of the T-type conformers (assigned to the hybridized asymmetric vibration of the central CC and CO stretching modes) disappears when going from the condensed phases to phases, where pinacol molecules are monomeric. Ab initio HF/6-311G** (MP2/6-311G**) calculations support the experimental findings; the calculated relative energies for the tGg′, gGg′, g′Gg′, tTt, and gTg′ conformers are 0.0 (0.0), 3.4 (3.4), 5.1 (5.9), 7.9 (11.3), and 12.0 (14.0) kJ mol−1, respectively. Consequently, only the G-type conformers are sufficiently populated to give rise to observable spectral lines. Both experimental findings and theoretical calculations demonstrated that the bands in the argon matrix spectrum of pinacol are due to the most stable tGg′ conformer. Although the ab initio calculations predict that also the gGg′ and g′Gg′ conformers are present in the gas phase and in dilute nonpolar solutions their existence could not be confirmed experimentally. Hence, we conclude that the conformation sensitive bands may coincide in the spectra. The HF/6-311G** ab initio calculations for vibrational frequencies of pinacol are consistent with this conclusion, suggesting only small differences between the wavenumbers of the G-type conformers. Pinacol does not show infrared-induced photorotamerization in the low-temperature argon matrix. This is due to the high energy barrier to internal rotation around the central C–C bond as demonstrated by ab initio calculations. Assignments of the vibrational bands were made with the aid of computer animations of the ab initio calculated harmonic vibrations, common group frequencies, and analogy conclusions from related compounds. The deuterium derivatives [(CD3)2C(OH)C(OH)(CD3)2 and (CH3)2C(OD)C(OD)(CH3)2] of pinacol were also utilized even though their spectra were recorded only in the condensed phases.