Ab Initio Study of the Torsional Spectrum of Glycolaldehyde

The torsional spectrum of glycolaldehyde (COH−CH2OH) is predicted from MP4(SDTQ)/cc-pVQZ ab initio calculations. This monosaccharide of astrophysical interest shows two large amplitude vibrations that confer nonrigid properties: the central bond torsion and the OH torsion. It presents five stable conformers, three of Cs symmetry (I-cis, II-trans, and IV-cis) and a double minimum of C1 symmetry (III-trans and III‘-trans). The favorite geometry, I-cis, stabilizes by the formation of a hydrogen bond connecting the eclipsed oxygen atoms. The relative energies of the secondary minima are 1278.2 (II-trans), 1297.2 (III and III‘-trans) and 1865.2 cm-1 (IV-cis). A barrier of 1895 cm-1 hinders the cis → trans interconversion. The barriers for the OH torsion are 2040 (cis) and 191 and 713 cm-1 (trans). The torsional energy levels have been calculated variationally up to J = 6 using a flexible model in two dimensions. Far-infrared frequencies and intensities have been determined at room temperature. In addition, the rotational parameters of the I-cis conformer have been computed to be A = 18 583.85 MHz, B = 6563.83 MHz, C = 5009.34 MHz, DJ = −20.18 kHz, DJK = −82.36 kHz, and DK = 138.38 kHz at the zero vibrational level. The parameters corresponding to the trans form are A = 38 019.19 MHz, B = 4406.79 MHz, C = 4076.73 MHz, and DJ = −109.28 kHz.