The cis form of N-methylthiourea in the solid state: Raman spectroscopy, X-ray diffraction and ab initio molecular orbital calculations

Abstract

The Raman spectra of N-methylthiourea have been studied and the conformation in solutions and in the solid state has been investigated. Prominent Raman bands are observed at about 700 and 775 cm −1 in the aqueous solution but only the latter band persists in the solid state. The normal coordinate analysis based on ab initio molecular orbital calculations has shown that the 700 cm −1 band is due to the CS stretching mode of the trans form and the 775 cm −1 band is due to the same mode of the cis form. This spectral interpretation leads to a conformational form of N-methylthiourea in the solid state to be the cis form but not the trans form as has been thought for a long time. This result was confirmed by the present X-ray structure analysis. From the temperature dependence of the relative intensities of the two CS stretching bands, in combination with the previous results of methyl proton nuclear magnetic resonance data, the cis form has been found to be more stable in solution, in agreement with the results of ab initio molecular orbital calculations.