STRUCTURES AND VIBRATIONAL-SPECTRA OF PINACOL - 1 - INFRARED AND MATRIX INFRARED-SPECTRA OF MONOMERIC PINACOL - AB-INITIO CALCULATIONS ON CONFORMERS AND VIBRATIONAL FREQUENCIES

Citation
M. Dahlqvist et al., STRUCTURES AND VIBRATIONAL-SPECTRA OF PINACOL - 1 - INFRARED AND MATRIX INFRARED-SPECTRA OF MONOMERIC PINACOL - AB-INITIO CALCULATIONS ON CONFORMERS AND VIBRATIONAL FREQUENCIES, Chemical physics, 229(2-3), 1998, pp. 137-147
Citations number
17
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Physics, Atomic, Molecular & Chemical
Journal title
ISSN journal
0301-0104
Volume
229
Issue
2-3
Year of publication
1998
Pages
137 - 147
Database
ISI
SICI code
0301-0104(1998)229:2-3<137:SAVOP->2.0.ZU;2-4
Abstract
The infrared spectra of monomeric pinacol molecules (2.3-dimethyl-2,3- butanediol; (CH3)(2)C(OH)C(OH)(CH3)(2)) have been recorded in the gas phase and dilute nonpolar solutions, and in an argon matrix. The vibra tional data are consistent with the intramolecularly hydrogen-bonded G -type (gauche with respect to the central C-C bond) conformers and the re 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), wh ich was found to be the most indicative to show spectral changes withi n the type of the conformers. In this region the band of the T-type co nformers (assigned to the hybridized asymmetric vibration of the centr al CC and CO stretching modes) disappears when going from the condense d phases to phases, where pinacol molecules are monomeric. Ab initio H F/6-311G* (MP2/6-311G**) calculations support the experimental findin gs; the calculated relative energies for the tGg', gGg', g'Gg', tTt, a nd gTg' conformers are 0.0 (0.0), 3.4 (3.4), 5.1 (5.9), 7.9 (11.3), an d 12.0 (14.0) kJ mol(-1), respectively. Consequently, only the G-type conformers are sufficiently populated to give rise to observable spect ral lines. Both experimental findings and theoretical calculations dem onstrated that the bands in the argon matrix spectrum of pinacol are d ue to the most stable tGg' conformer. Although the ab initio calculati ons 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 wi th this conclusion, suggesting only small differences between the wave numbers of the G-type conformers. Pinacol does not show infrared-induc ed photorotamerization in the low-temperature argon matrix, This is du e 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, a nd analogy conclusions from related compounds. The deuterium derivativ es [(CD3)(2)C(OH)C(OH)(CD3)(2) and (CH3)(2)C(OD)C(OD)(CH3)(2)] of pina col were also utilized even though their spectra were recorded only in the condensed phases. (C) 1998 Elsevier Science B.V.