HEAT-CAPACITIES OF THIOUREA AND SOME N-ALKYLTHIOUREAS

The molar heat capacities at constant pressure of thiourea (TU), 1-met hyl-2-thiourea (MMTU), 1-ethyl-2-thiourea (METU), 1,3-diethyl-thiourea (DETU) and 1,1,3,3-tetramethyl-2-thiourea (TMTU) were determined by d ifferential scanning calorimetry (DSC) from 310 K to about (10 to 30) K below their melting points. Values at 298.15 K were obtained by extr apolating the smoothed fitting equations as a function of temperature. For 1,3-dimethyl-2-thiourea (DMTU), measurements at around room tempe rature were used for direct determination of the C-p,C-m experimental value at 298.15 K. C-p,C-m's at 298.15 K of all compounds increased li nearly with the number of carbon atoms in the alkyl substituents. The contribution of (25.5 +/- 1.5) J K-1 mol(-1) per-CH2-group was in good accordance with the values previously obtained for some series of sma ll peptides. The molar heat capacities of isomeric METU and DMTU were almost identical, whereas those of isomeric DETU and TMTU displayed an appreciable divergence. (C) 1997 Elsevier Science B.V.