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Exploring the microscopic reaction mechanism of dicyandiamide (DCD) synthesis using calcium cyanamide (CaCN2) is highly desirable because of the low conversion of reactants and selectivity of DCD products. DCD synthesis consists of a two-step sequential hydrolysis of CaCN2, followed by dimerization of cyanamide to DCD in an alkaline environment. Density functional theory (DFT) results revealed that the rate-limiting step (RLS) was the formation of a C-N bond between the cyanamide and cyanamide anion in the dimerization of the DCD reaction. Secondary reactions of cyanamide with water, hydrogen sulfide, and DCD were also analyzed. The effects of solvation on the principal and secondary reactions were systematically explored. A single explicit water molecule can significantly lower the free energy barrier of the RLS. Water molecules facilitate the C-N bonding of the reactants in DCD reactions, resulting in a reduction in the free energy barrier of the RLS. The facilitation of double explicit water for the reaction is weaker than that of single explicit water and even yields negative catalysis. The effect of the [OH(H2O)3]− cluster lowering the reaction barrier with the hydrogen-bonding network is the most remarkable, which can alter the reaction path by the direct and indirect involvement of OH− ions. Furthermore, the reaction rate constants were computed by canonical variational theory with the Eckart tunneling correction (CVT/Eckart) and fitted to the Arrhenius expression. The reaction mechanism and kinetics revealed at the microscopic level provide efficient and clean production of DCD with certain theoretical guidance. [ABSTRACT FROM AUTHOR]

Copper (Cu) is an important transition metal, and its isotopes have important applications in geology, environmental science, soil science, and other fields. Cu isotope fractionation can occur in many natural processes. However, the mechanism of Cu isotope fractionation in solution/hydrothermal solution systems is not very clear. In this study, the fractionation effects of complexes of Cu(I) and Cu(II) in solution/hydrothermal solution systems were systematically studied by means of an ab initio method based on first principles. In the simulation of an aqueous solution system, the theoretical treatment method used is the "water-droplet" method. The results show that the heavy Cu isotope (65Cu) enrichment capacity of the Cu-bearing complex solutions is greatly affected by the ligand types both for Cu(I) and Cu(II). For Cu(I) complex solutions, the heavy Cu isotope enrichment sequence is [Cu(HS)2]−·(H2O)42 > [Cu(HS)(H2O)]·(H2O)42 ≈ [Cu(HS)(H2S)]·(H2O)42 > [CuCl]·(H2O)42 > [CuCl2]−·(H2O)42 > [CuCl3]2−·(H2O)42. For the aqueous solutions of Cu(II) with an inorganic ligand (such as H2O, OH−, NO3−, SO42− and CN−), the order of heavy Cu isotope enrichment is as follows: [Cu(H2O)6]2+·(H2O)42 > [Cu(NO3)2]·(H2O)42 > [Cu(OH)2]·(H2O)42 > [CuSO4(H2O)3]·(H2O)42 > [CuNO3(H2O)4]+·(H2O)42 > [CuCN]+·(H2O)42. For the Cu(II) complex solutions with a halogen as ligands, the change order of 1000lnβ is [CuCl]+·(H2O)42 > [CuCl2]·(H2O)42 > [CuBr2]·(H2O)42 > [CuCl3]−·(H2O)42. The sequence of 1000lnβ for Cu(II) organic complex aqueous solutions is [Cu(HOC6H4COO)]+·(H2O)42 > [Cu(CH3CH2COO)]+·(H2O)42 > [Cu(COOHCOO)]+·(H2O)42. The calculation also found that for Cu(I) complex aqueous solutions, the difference in Cu isotope fractionation parameters (1000lnβ) between [CuCl2]−·(H2O)42 and [Cu(HS)2]−·(H2O)42 is relatively large. At 100 °C, the 1000lnβ of the two species are 1.14 and 1.55 (‰), respectively. The difference between the two could be reached up to 0.41 (‰). The Cu isotope fractionation parameter obtained with the "water droplet" method is also very different from the results of previous studies, which indicate that the Cu isotope fractionation behavior of the two is similar. At the same time, the exciting discovery is that the enrichment capacity of heavy Cu isotopes is significantly different between Cu(I) complex aqueous solutions and Cu(II) complex aqueous solutions. At 100 °C, the 1000lnβ of 6 Cu(I) complex aqueous solutions and 13 Cu(II) complex aqueous solutions ranged from 0.90 to 1.55 and 2.24 to 3.25(‰), respectively. It also shows that the REDOX reaction has a significant effect on the Cu isotope fractionation, especially in ore-forming fluids. Therefore, the ligand type is a factor that cannot be ignored when considering the mechanism of Cu isotope fractionation in solution/hydrothermal solution systems. Whether the solvation effect of an aqueous solution is considered or not has a great influence on the numerical values of the final Cu isotope fractionation factors. Hence, the solvation effect of an aqueous solution is an essential determinant in the theoretical calculation of the Cu isotope fractionation factors for Cu-bearing complex solutions. [ABSTRACT FROM AUTHOR]

Protein unfolding is a ubiquitous process responsible for the loss of protein functionality (denaturation), which, in turn, can be accompanied by the death of cells and organisms. The nature of enthalpy–entropy compensation (EEC) in the kinetics of protein unfolding is a subject of debate. In order to investigate the nature of EEC, the "completely loose" transition state (TS) model has been applied to calculate the Arrhenius parameters for the unfolding of polyglycine dimers as a model process. The calculated Arrhenius parameters increase with increasing dimer length and demonstrate enthalpy–entropy compensation. It is shown that EEC results from the linear correlations of enthalpy and entropy of activation with dimer length, which are derived directly from the properties of the transition state. It is shown that EEC in solvated (hydrated, etc.) proteins is a direct consequence of EEC in proteins themselves. The suggested model allows us also to reproduce and explain "exotic" very high values of the pre-exponential factor measured for the proteins unfolding, which are drastically higher than those known for unimolecular reactions of organic molecules. A similar approach can be applied to analyzing the nature of EEC phenomena observed in other areas of chemistry. [ABSTRACT FROM AUTHOR]