Experimental and theoretical study of the sulfamic acid-urea deep eutectic solvent

A sulfamic acid-urea mixture deep eutectic solvent has been intensively investigated in the last few years as an agent for sulfating natural substances alternative to toxic complexes of sulfur trioxide with organic bases. We first report on the examination of the sulfamic acid-urea deep eutectic solvents with different ratios of active components. This complex has been studied by Fourier-transform infrared spectroscopy, X-ray diffraction, and thermal analysis. The melting points of the synthesized deep eutectic solvents have been determined and, using the data obtained, the thermodynamic characteristics, including the enthalpy, entropy, chemical potential, and effective interaction parameter, have been calculated. The calculations have been made by the density functional theory, quantum theory of atoms in molecules, reduced density gradient, electron localization function, highest occupied molecular orbital-lowest unoccupied molecular orbital, and molecular electrostatic potential methods. The structure of the sulfamic acid-urea complex has been optimized within the density functional theory. The Laplacian and interaction energy of the complexes have been calculated using the quantum theory of atoms in molecules. The reduced density gradient and electron localization function have been used to explore the intermolecular interaction of sulfamic acid with urea taken in different ratios. It has been found using the reduced density gradient that, with an increase in the number of urea molecules mixed with sulfamic acid in a deep eutectic solvent, the blue and green regions of the reduced density gradient functions significantly increase, which is indicative of an increase in the number of hydrogen bonds and van der Waals interaction regions. It has been demonstrated by the molecular electrostatic potential mapping that the nucleophilic and electrophilic attack areas increase with the urea content in the mixture with sulfamic acid. It has been determined by analyzing the thermodynamic characteristics of the mixtures with different urea contents that an increase in the number of urea molecules leads to the almost linear growth in the heat capacity, entropy, and E (thermal). According to the obtained linear dependences, the R2 value is high (0.9996–1.0000), which is indicative of the maximum heat capacity and E (thermal).