Microwave spectrum of glycerol: Observation of a tunneling chiral isomer V.V. Ilyushin a , R.A. Motiyenko a , F.J. Lovas b, * , D.F. Plusquellic b a Institute of Radio Astronomy of NASU, Chervonopraporna 4, 61002 Kharkov, Ukraine b Optical Technology Division, National Institute of Standards and Technology, 100 Bureau Drive Mail Stop 8441, Gaithersburg, MD 20899-8441, USA Received 22 January 2008; in revised form 12 February 2008 Available online 10 March 2008 Abstract The microwave spectrum of the sugar alcohol glycerol (CH 2 OHCHOHCH 2 OH) has been measured over the frequency range 8.2– 26.5 GHz with a pulsed-beam Fourier-transform microwave spectrometer. Five conformers of glycerol have been assigned and identified with the aid of ab initio electronic structure calculations. One of the higher energy conformers exhibits tunneling of the three hydroxyl groups along a chiral inversion pathway similar to the tunneling of the C2 polyol, ethylene glycol. Stark effect measurements were carried out on several of the lowest energy conformers to provide accurate determinations of the dipole moment components. Ó 2008 Elsevier Inc. All rights reserved. Keywords: Ab initio calculation; Dipole moment; Glycerol; Microwave spectrum; Rotational spectrum; Tunneling 1. Introduction Glycerol (CH 2 OHCHOHCH 2 OH) is the simplest alco- hol with three hydroxyl groups. It is widely distributed in nature in the form of its esters, namely glycerides, which are the principal constituents of fats and oils. It has many commercial applications from pharmaceutics and cosmet- ics to the manufacture of dynamite. Also glycerol is a mol- ecule of potential astrophysical interest since it is a derivative of the three carbon sugar, glyceraldehyde. Recently, the simplest sugar, glycolaldehyde (CH 2 OH- CHO) [1] and its polyol derivative, ethylene glycol (CH 2 OHCH 2 OH) [2], were detected towards the interstel- lar molecular cloud, Sgr B2 (N-LMH). The identification of sugar alcohols and sugar acids in the Murchinson and Murray meteorites [3] has increased the interest in investi- gating interstellar sugars and their derivatives. Also this molecule presents information about the energies involved in intramolecular hydrogen bonding of vicinal hydroxyl groups. These properties were the motivations for the cur- rent study of the rotational spectra of glycerol in a pulsed- nozzle Fourier-transform microwave (FTMW) spectro- meter. Glycerol is an asymmetric top molecule with a number of low energy conformers. The 12 conformers determined from previous [4] and current ab initio calculations are shown in the Figs. 1 and 2. We use the conformer labelling scheme employed in Ref. [4] which was first introduced by Radom et al. [5]. This labelling scheme provides a five char- acter label, XXxxx, where X (capital for the oxygen and lower case for the hydroxyl hydrogens) can be G, G 0 or T, depending on the gauche or trans (anti) position of the atom considered with respect to a reference bond. G and G 0 indicate an anticlockwise or a clockwise rotation from the cis (syn) position of O4–C1–C2–O5 or O6–C3–C2–O5 dihedral angles for the X values and similarly for the HOCC dihedral angles represented by x. The first X repre- sents the position of O4 with respect to O5, and the second X the position of O6 with respect to O5. The first x repre- sents the position of the hydroxyl hydrogen H7 with respect to C2, the second x the position of H8 with respect to C1 and, finally, the third x the position of H9 with respect to C2. 0022-2852/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2008.02.005 * Corresponding author. Fax: +1 301 869 5700. E-mail address: Francis.Lovas@nist.gov (F.J. Lovas). www.elsevier.com/locate/jms Available online at www.sciencedirect.com Journal of Molecular Spectroscopy 251 (2008) 129–137