Microwave spectrum and structure of methyl phosphonic difluoride R.D. Suenram a, * , F.J. Lovas a , D.F. Plusquellic a , M.W. Ellzy b , J.M. Lochner b , J.O. Jensen b , A.C. Samuels b a Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-8441, USA b Edgewood Chemical Biological Center, Edgewood Area, Aberdeen Proving Ground, MD 21010-5424, USA Received 21 June 2005; in revised form 27 September 2005 Available online 16 November 2005 Abstract The rotational spectrum of methyl phosphonic difluoride has been reinvestigated using a pulsed-molecular-beam Fabry–Perot cavity microwave spectrometer. The enhanced resolution of the Fourier transform microwave (FTMW) spectrometer (compared to the original work done in a conventional Stark spectrometer) has allowed the measurement of small A–E splittings of many of the rotational tran- sitions caused by the internal rotation of the methyl top. The barrier to internal rotation, V 3 = 676 (25) cm 1 , has been determined exper- imentally from the A–E splittings of the rotational transitions in the ground vibrational state. This barrier height is substantially lower than the previously determined value for the barrier, which was 1252 (14) cm 1 . High-level ab initio calculations at the MP2/aug-cc- pVTZ level predict a barrier to internal rotation of 638 cm 1 , in agreement with the experimentally determined value found here. The high sensitivity of the FTMW spectrometer has also permitted the measurement of the 13 C and 18 O isotopomers in natural abun- dance. The addition of these two isotopomers has allowed an improved structural determination. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Methyl phosphonic difluoride; Rotational spectrum; Fourier transform microwave spectrum; Molecular structure; Molecular beam spectrum; Ab initio calculations; Methyl barrier to internal rotation 1. Introduction Methyl phosphonic difluoride (also known as DF) is a synthetic precursor used to produce the G-series of nerve agents such as sarin [1] and soman [2]. Typically methyl phosphonic dichloride and methyl phosphonic difluoride are reacted with an alcohol to form a condensate in which the G-agent is produced with the elimination of hydrochlo- ric acid and hydrogen fluoride. In the reaction, one of the fluorine atoms on methyl phosphonic difluoride is removed along with the hydroxyl hydrogen of the alcohol and a new bond is formed between the phosphorus atom and the oxy- gen of the hydroxyl group of the alcohol [3]. The rotational spectrum of methyl phosphonic difluo- ride was first investigated by Durig and co-workers in 1976 [4]. In that work, they reported the microwave, Raman, and infrared spectrum of DF. In 1982, Durig and co-workers [5] published a second paper in which they reinvestigated the microwave spectrum and corrected a number of errors in their first work. In 1986, Gillies and co-workers [6] reported yet another microwave study in which they reassigned a number of vibrational satellite bands observed by Durig et al in their second paper [5]. From these combined studies, the rotational spectrum of the ground state and several vibrational satellite bands have been assigned for the normal species from which a rough estimate of the structure was obtained. The molecule is a near oblate top and this led to a misassignment of the electric dipole moment in the original paper, where a-type and c-type selection rules were reported [4]. In Durig, et al.Õs second paper, [5] a number of b-type transitions were observed and measured. They also re-eval- www.elsevier.com/locate/jms Journal of Molecular Spectroscopy 235 (2006) 18–26 0022-2852/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2005.10.001 * Corresponding author. Present address: Department of Chemistry, University of Virginia, McCormick Rd., Charlottesville, VA 22904-4319. Fax: +1 434 924 3710. E-mail address: suenramr@earthlink.net (R.D. Suenram).