5:15 PM - 6:45 PM
[U11-P04] Calculation of equilibrium isotope fractionation ratio considering pressure/temperature-dependent water properties
Keywords:Equilibrium Isotope Fractionation, Solvation, Quantum Chemistry, Nitrogen Cycle
The difference in isotopic ratio between molecules is a fundamental information to deduce molecular processes and ancient environments. Equilibrium isotope fractionation describes temperature-dependent separation of isotopes between two molecules A and B. Bigeleisen and Mayer proposed reduced partition function ratio, β, that describes isotope fractionation between to isotopologues A and A’. βis calculated using vibrational frequency of the two isotopologues, which is obtained by quantum chemical calculation in a solvent (water). The standard method to include solvation effect is called “water droplet” model, in which electrostatic interaction and inter-molecular interactions such as van der Waals force and hydrogen bonding are implemented by polarized continuum model (PCM) and around 30 explicit surrounding water molecules, respectively. The coordinates of the molecule and the surrounding water molecules are optimized, and vibrational analysis is followed to obtain vibrational frequencies under ambient temperature and pressure. The T-dependence is calculated from the vibrational frequencies of ambient conditions. The water droplet model is considered accurate though it is time-consuming, difficult to converge structure even for small molecules, and require several calculations to normalize local water configurations. However, we doubt the accuracy because the calculated β smoothly depends on temperature even though water boils at high temperature, and there is no way to include pressure (p) effect, which determines the water phase.
We investigated the calculation methods of p/T-dependent β considering p/T dependence of water properties and minimum explicit hydrogen bonding to minimize calculation cost. Calculation was performed on Gaussian 16 using density functional theory (DFT), where B3LYP density functional and 6-31+G(d,p) basis sets were applied. PCM uses dielectric constant and optical dielectric constant, and these parameters were calculated based on equations of the International Association for the Properties of Water and Steam (IAPWS). We optimized the molecules at each p/T, and calculated vibrational frequencies. Water, nitrate anion, and nitrite anion were investigated for 15N and 18O substitution.
We found jump in β around boiling point of water, and the bond length and angle of coordinating water was observed at higher temperature. This is in accordance with the experimentally observed decrease in the average number of coordinating water. The number of coordinating water affected less for β15N than β18O, and the calculated values got close to saturation at one or two explicit water. The reduction of the number of explicit surrounding water contributes to shorten the calculation time to ca. 1/500 since the calculation time is proportional to the number of basis powered by ca. 3.3. Our proposed methodology require calculation at multiple points, but the calculation time is still small, and it is easier to converge atomic coordinates for smaller systems than the water droplet model.
Thiis study was supported by JSPS KAKENHI 23K13211, and calculations were performed on Earth Simulator.
We investigated the calculation methods of p/T-dependent β considering p/T dependence of water properties and minimum explicit hydrogen bonding to minimize calculation cost. Calculation was performed on Gaussian 16 using density functional theory (DFT), where B3LYP density functional and 6-31+G(d,p) basis sets were applied. PCM uses dielectric constant and optical dielectric constant, and these parameters were calculated based on equations of the International Association for the Properties of Water and Steam (IAPWS). We optimized the molecules at each p/T, and calculated vibrational frequencies. Water, nitrate anion, and nitrite anion were investigated for 15N and 18O substitution.
We found jump in β around boiling point of water, and the bond length and angle of coordinating water was observed at higher temperature. This is in accordance with the experimentally observed decrease in the average number of coordinating water. The number of coordinating water affected less for β15N than β18O, and the calculated values got close to saturation at one or two explicit water. The reduction of the number of explicit surrounding water contributes to shorten the calculation time to ca. 1/500 since the calculation time is proportional to the number of basis powered by ca. 3.3. Our proposed methodology require calculation at multiple points, but the calculation time is still small, and it is easier to converge atomic coordinates for smaller systems than the water droplet model.
Thiis study was supported by JSPS KAKENHI 23K13211, and calculations were performed on Earth Simulator.