UV-INDUCED ROTAMERIZATION AND VIBRATIONAL-SPECTRA OF THE CONFORMERS OF CYANOMETHYL FORMATE - MATRIX-ISOLATION INFRARED AND AB-INITIO STUDIES

Citation
M. Dahlqvist et al., UV-INDUCED ROTAMERIZATION AND VIBRATIONAL-SPECTRA OF THE CONFORMERS OF CYANOMETHYL FORMATE - MATRIX-ISOLATION INFRARED AND AB-INITIO STUDIES, The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, 101(7), 1997, pp. 1260-1266
Citations number
25
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Chemistry Physical
ISSN journal
1089-5639
Volume
101
Issue
7
Year of publication
1997
Pages
1260 - 1266
Database
ISI
SICI code
1089-5639(1997)101:7<1260:URAVOT>2.0.ZU;2-Y
Abstract
Ab initio RHF and MP2 calculations with the standard 6-31G and 6-311G * basis sets were carried out on cyanomethyl formate (HCOOCH2CN). The calculations indicated two stable conformers (Z,ap and Z,sc) for this molecule. However, the energy difference between the conformers varie d at different levels of theory. The calculations with the 6-311G* ba sis set and correlated wave functions indicated equal energy for these conformers, whereas all the other calculations slightly preferred the Zap conformer (by 0.2-2.1 kJ mol(-1)). The existence of the Z,ap and Z,sc conformers of cyanomethyl formate was confirmed by matrix isolati on infrared studies in argon. Immediately after deposition the spectru m showed two peaks both in carbonyl and asymmetric C-O-C stretching re gions. The behavior of these pairs of infrared peaks upon UV irradiati on and the comparison with the results obtained for related esters str ongly indicate that these doublets are caused by two different conform ers. It was observed that the Z,ap and Z,sc conformers can be surmount ed thermally in an argon matrix. This is consistent with the calculate d values of 2.6 kJ mol(-1) (MP2/6-31G) or 3.7 kJ mol(-1) (RHF/6-31G*) for the potential energy barrier of this process. A conformer interco nversion process, proceeding from both the Z,ap and Z,sc to the E,sc c onformer, was induced by UV irradiation at 248 nm The E,se conformer i s the main product of isomerization. This was established by ab initio calculations and by analysis of the spectral changes in the (C=O) and (C-O-C) stretching legions. Upon prolonged irradiation at the same wa velength, the E,se conformer decomposed to stable products including a hydrogen-bonded (1:1) complex of CO and cyanomethanol. (CO2 productio n was also observed. However, its counterpart could not be identified, presumably due to further decomposition.)