THE CIS FORM OF N-METHYLTHIOUREA IN THE SOLID-STATE - RAMAN-SPECTROSCOPY, X-RAY-DIFFRACTION AND AB-INITIO MOLECULAR-ORBITAL CALCULATIONS

The Raman spectra of N-methylthiourea have been studied and the confor mation in solutions and in the solid state has been investigated. Prom inent Raman bands are observed at about 700 and 775 cm(-1) in the aque ous solution but only the latter band persists in the solid state. The normal coordinate analysis based on ab initio molecular orbital calcu lations has shown that the 700 cm(-1) band is due to the C=S stretchin g mode of the trans form and the 775 cm(-1) band is due to the same mo de of the cis form, This spectral interpretation leads to a conformati onal form of N-methylthiourea in the solid state to be the cis form bu t 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 temper ature dependence of the relative intensities of the two C=S stretching bands, in combination with the previous results of methyl proton nucl ear magnetic resonance data, the cia form has been found to be more st able in solution, in agreement with the results of ab initio molecular orbital calculations. (C) 1997 Elsevier Science B.V.