The Compositions of Hetero solvates Formed in the Formamide-Water System at 298 K According to Proton Magnetic Relaxation Data

The formamide-water system was studied by the proton spin-spin relaxation method at 298 K (5 MHz, the Carr-Purcell-Meiboom-GiII method). The concentration dependences of the proton spin-spin relaxation rate and the amplitude of the 1H NMR signal of the formamide - water system at 298 K were investigated. The non-monotone character of the above dependences and the presence of the extreme points were revealed. It appears to be connected with the change of the life time τb of the formamide and water molecules in solvation spheres. The intensive destruction of the liquid water structure by the formamide molecules up to 0.5 mole fraction of formamide was supposed. The maximum of the structure disorder of both components was within 0.25 to 0.40 mole fraction of formamide. The calculations of the solvate compositions were performed by the mathematical modelling method according to the CPESSP program (Complex formation Parameters of Equilibrium in Solution with Solid Phases, Y. Salnikov, Kazan University, Russia). The compositions of the hetero solvates were determined: (HCONH2)1(H2O)20, (HCONH2)1(H2O)5, (HCONH2)1(H2O)2, (HCONH2)1(H2O)1, (HCONH2)3(H2O). The stability constants within the mole-fraction scale, the molar coefficients of the relaxation effectiveness K20 and the molar amplitudes A20 of 1H NMR signal of the hetero solvates were calculated. The results obtained can serve as evidence in support of our contention, that the structure of the formamide water system in the concentration range under study is mainly determined by monomeric hetero solvates. (author)