Rotational spectrum of Sarin

As part of an effort to examine the possibility of using molecular-beam Fourier-transform microwave spectroscopy to unambiguously detect and monitor chemical warfare agents, we report the first observation and assignment of the rotational spectrum of the nerve agent Sarin (GB) (Methylphosphonofluoridic acid 1-methyl-ethyl ester, CAS #107-44-8) at frequencies between 10 and 22 GHz. Only one of the two low-energy conformers of this oganophosphorus compound (C4H10FO2P) was observed in the rotationally cold (T-rot < 2 K) molecular beam. The experimental asymmetric-rotor ground-state rotational constants of this conformer are A = 2874.0710(9) MHz, B = 1168.5776(4) MHz, C = 1056.3363(4) MHz (Spe A standard uncertainties are given, i.e., 1 <sigma>), as obtained from a least-squares analysis of 74 n-, L7-, and c type rotational transitions. Several of the transitions are split into doublets due to the internal rotation of the methyl group attached to the phosphorus. The three-fold-symmetry barrier to internal rotation estimated from these splittings is 677.0(4) cm(-1). Ab initio electronic structure calculations using Hartree-Fock, density functional, and Moller-Plesset perturbation theories have also been made. The structure of the lowest-energy conformer determined from a structural optimization at the MP2/6-311G'* level of theory is consistent with our experimental findings. (C) 2001 Academic Press.