Reanalysis of the microwave absorption spectrum of dimethyl methylphosphonate and its internal rotation problem

The Fourier transform microwave spectrum of dimethyl methylphosphonate studied by us previously is reanalyzed here to obtain more physically reasonable parameters describing the various Coriolis-like couplings between overall rotation and internal rotation of the two methoxy methyl tops. In particular, we use exactly the same frequencies and spectral assignments as in our previous study, but the least squares fit is started from a rather different set of initial molecular parameters and is carried out with a slightly smaller set of adjustable parameters. The standard deviation of the fit is not significantly changed, but convergence to a rather different minimum in parameter space is obtained. This new minimum does not change the three rotational constants significantly, but values for the 12 Coriolis coupling constants are dramatically rearranged, so that parameters arising from coupling between the two internal rotation motions are greatly reduced in magnitude. These new Coriolis constants bring the derived direction cosines for the methoxy methyl groups in the principal axis system into much better agreement with ab initio predictions. We have used our new parameters to derive internal rotation barrier heights for the two methyl groups of 280 and 188 cm−1.