Abstract:The crystal structures of 5-benzyl-2-thiohydantoin (5-BTH) and 1-acetyl-5-benzyl-2-thiohydantoin (1-Ac-5-BTH) have been determined by X-ray diffraction. In the 5-BTH crystals, the enantiomeric (R)-and (S)-5-BTH molecules are connected to form cyclic dimers via the hydrogen bonds of the thioamide and the amide moieties. On the other hand, the intermolecular hydrogen bonds in 1-Ac-5-BTH crystals form an infinite chain. These differences 1 SpectroscLett_kunimoto_revised in the hydrogen bond pattern are also discussed in the IR and Raman spectra. The ab initio molecular orbital calculations (Gaussian 03) with 6-31G(d,p) basis set were carried out for 5-BTH and 1-Ac-5-BTH to get the preferred conformation.