Several years ago, Nyquist and Potts published a paper on the characteristic frequencies of thiol esters and related compounds, in which they concluded that the C—S stretching frequency of the [Equation] group lies in the 880 to 1030 cm−1 region. Their assignment of the C—S stretch in thiolacetic acid to the band at 993 cm−1 is in conflict with the assignments of Sheppard and Mecke and Spiesecke of 626 cm−1 for this vibration. Other authors have adopted the Nyquist and Potts assignments, but comparisons of the spectra of several compounds have led the present writer to agree with the original assignment.
G. A. Crowder, "The C-S Stretching Frequency in Thiol Acids and Esters," Appl. Spectrosc. 26, 486-487 (1972)