A Raman and infrared study of 1-furoyl-3-monosubstituted and 3,3-disubstituted thioureas

Raman and IR spectra of two series of 1-furoylthiourea derivatives (19 compounds) were recorded and compared in order to identify the vibrations, which involve contributions from motions within the thioureido (NCSN) core. This procedure allowed an unequivocal identification of the ν(CS) vibration in these spectra. In 3-monosubstituted furoylthioureas (Series 2) the carbonyl group and the proton on N₃ are engaged in a strong hydrogen bond interaction. This leads to an "S"-shaped conformation of the CO and CS groups where these donor sites reach a maximum separation. In this conformation, the ν(CO) vibration is not influenced by the substituent. In the absence of that hydrogen bridge, in 3,3-disubstituted thiourea derivatives (Series 1), the CO and CS groups adopt an "U"-shaped conformation. In this conformation, the ν(CO) vibration shows a pronounced substituent dependence. These thiourea derivatives have been tested as ionophores for heavy-metal ion selective electrodes and their behavior in that sense correlates with the observed Raman and IR absorptions. The best performance in that application corresponds to compounds of Series 2, which showed the highest frequency values of the ν(CS) vibration. This fact was related to an appropriated nucleophilic character of the sulphur atom. From these data, Raman and IR spectra of these thiourea derivatives could be used as a predictor on their expected behavior in analytical applications as ionophores.