Rotational spectra of the diastereomers of Soman

The pure rotational spectrum of the nerve agent Soman has been recorded using a pulsed-molecular-beam Fourier-transform microwave spectrometer. The spectrum consists of transitions from two different isomers. The two distinguishable isomers are likely the SS] (or RR]) and the RS] (or SR]) diastereomers that result from the two chiral centers in the molecule. The rotational constants determined for the A internal rotor states of the SS, RR] and RS, SR] isomers are A=1645.39765(9) MHz, B=591.97752(3) MHz, and C=547.58168(3) MHz, and A=1635.0580(1), B=600.14889(6), and C=556.45840(6) MHz, respectively, where type A, k=1 or 1σ standard uncertainties are given. Structural assignments of the diastereomers were made based on comparisons of the calculated electric dipole moments at the MP2/6-311G level of theory with the observed selection rules. Many of the rotational transitions are split into doublets and correspond to the A- and E-state transitions arising from internal rotation of the methyl top attached to the phosphorus atom. These splittings have been used to obtain the V₃ barriers of the methyl groups for both diastereomers. This work is part of an ongoing project aimed at generating a spectral database of chemical agents and related families of compounds.