Normal coordinates for the planar vibrations of urea using 15N and 18O frequency shift data

Microscopic origin of pressure-induced phase-transitions in urea: a detailed investigation through first principles calculations

The potential crystal structures and properties of urea as a function of pressure were studied using ab initio based electronic structure calculations. The enthalpy-pressure behavior shows that urea undergoes a pressure induced structural phase transition from P4[combining macron]21m (phase I) → P212121 (phase III) at 0.66 GPa with a volume collapse of 4.83%, driven by softening of the acoustic mode along the Γ-X direction. Another phase transition from the P212121 → P21212 structure was identified at 3.09 GPa. The violation of Born stability criteria in the P212121 structure along with softening of the acoustic mode in the U-R direction was responsible for the pressure induced phase transition. Furthermore, the application of pressure led to the breaking and formation of N-HO bonds in the crystal structure of urea during the phase transition, i.e., the H-acceptor capacitance of the oxygen atoms was varied between phases I/IV and -III. Band structure calculations were performed using a hybrid functional (Heyd, Scuseria and Ernzerhof, HSE) which includes a part of exact Fock-exchange. The computed electronic band structure showed that the urea polymorphs are insulators with a direct band gap of 6.21, 6.85 and 6.99 eV for phase-I, -III and -IV, respectively, at selected pressures. We have also presented the dielectric functions (real (ε1(ω)) and imaginary (ε2(ω)) parts), refractive index and absorption coefficients to explore the optical characteristics of the urea phases. The geometric interpretation of intermolecular interactions were quantitatively visualized using Hirshfeld surface analysis. Our results provide a complete picture of various properties of urea polymorphs that lay the foundation for further understanding of structures and their applications.

Kinetics of Formation and Dissociation of [Cr(3)O(O(2)CCH(3))(6)(urea)(3)](+): An Example of Statistically Controlled Kinetics and Equilibrium

Kinetics of the overall reaction [Cr(3)O(O(2)CCH(3))(6)(H(2)O)(3)](+) + 3 urea right harpoon over left harpoon [Cr(3)O(O(2)CCH(3))(6)(urea)(3)](+) + 3H(2)O have been studied spectrophotometrically. Monophasic kinetics were observed in both directions. The reverse steps, of urea dissociation, were monitored using an analytical technique which permits direct determination of the concentration of liberated urea and does not require knowledge of extinction coefficients of intermediate species. Results imply that consecutive steps occur with rate constants in close to the statistical ratios of k(1):k(2):k(3) = 3:2:1 and k(-)(1):k(-)(2):k(-)(3) = 1:2:3. Rates indicate strong labilization of urea, compared to the case of mononuclear complex [Cr(urea)(6)](3+).