Microwave spectrum and structure of methyl phosphonic difluoride

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 transitions caused by the internal rotation of the methyl top. The barrier to internal rotation, V-3 = 676 (25) cm(-1), has been determined experimentally 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-ccpVTZ 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 C-13 and O-18 isotopomers in natural abundance. The addition of these two isotopomers has allowed an improved structural determination. (c) 2005 Elsevier Inc. All rights reserved.