For some time we have been interested in the reaction kinetics of sulfur addition to trivalent organophosphorus compounds and have used IR spectroscopy to follow the progress of these reactions. Many spectral changes occur upon sulfur addition which are not specifically related to the addition of sulfur, but rather to a change in valence of phosphorus from three to five. These changes involve shifts of C-H stretches and skeletal vibrations to higher frequencies. Similar changes occur upon the addition of oxygen to trivalent phosphorus compounds. These many spectral changes sometimes make it difficult to decide which band(s), if any, can be assigned to the P=S (or P=O) vibration in these compounds. There was considerable disagreement in the early literature about the proper assignment of the P=S frequency. Hooge and Christen assigned bands in the region between 800 and 535 cm<sup>-1</sup> to that vibration, but Nyquist and Muelder assigned bands in the region around 800 cm<sup>-1</sup> to P-O-C vibrations with predominantly P-O character. A study of the literature suggests that bands due to P=S vibrations generally appear below 800 cm<sup>-1</sup>, although assignments as high as 865 cm<sup>-1</sup> still appear in fairly recent literature? These bands are often doublets due, presumably, to the existence of rotational isomers. The presence of such isomers has been established by Nyquist and Muelder and by Mortimer, among others, through studies of the temperature dependence of the spectra of a number of compounds. The above references include compounds with both P=O and P=S bonds and studies of dilute solutions as well as neat liquids and the solid state. Examples of two plausible structures of methyl thionophosphate isomers are shown in Fig. 1.

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