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Solvent-corrected reduced isotropic spectra of carbonate and bicarbonate in light and heavy water have been measured from 150 to 325 °C at 21 MPa using a confocal Raman microscope and a custom-built titanium flow cell with sapphire windows. The positions of the symmetric vibrational modes of CO32–and HCO3–/DCO3–were compared to density functional theory (DFT) calculations with a polarizable continuum model in light and heavy water. The experimental Raman peak positions shifted linearly toward lower wavenumbers with increasing temperatures. Raman scattering coefficients, measured relative to a perchlorate internal standard, were used to determine equilibrium molalities of the carbonate and bicarbonate species. These yielded quantitative thermodynamic equilibrium quotients for the reaction CO32–+ H2O ⇌ HCO3–+ OH–and its deuterium counterpart. Ionization constants for HCO3–and DCO3–, K2a,H,mand K2a,D,m, calculated in their standard states using the Meissner–Tester activity coefficient model, were combined with critically evaluated literature data to derive expressions for their dependence on temperature and pressure, expressed as solvent molar volume, over the range 25 to 325 °C from psatto 21 MPa. These are the first experimental values to be reported for this reaction in light water above 250 °C and in heavy water above 25 °C. The value of the deuterium isotope effect on the chemical equilibrium constant, ΔpK2a,m= pK2a,D,m– pK2a,H,m, decreased from ΔpK2a,m= 0.67 ± 0.07 at 25 °C to ΔpK2a,m= 0.17 ± 0.13 at 325 °C and psat.

Quantitative 13C nuclear magnetic resonance (NMR) spectroscopy was used to investigate the speciation in (2-methylpiperidine + H2O + CO2) systems at 283.2–313.2 K. The carbamate of 2-methylpiperidine(2-methylpiperidine-N-carboxylate) was shown for the first time to be a stable species in aqueous solutions. The spectroscopic results were used to obtain temperature-dependant formation constants for the carbamate using a simplified model for the activity coefficients from which the standard molar enthalpy of reaction was estimated. The results were incorporated into a self-consistent chemical equilibrium model, which includes vapor–liquid equilibria and all aqueous species, including the formation of carbamate. The predominant conformation of the sterically hindered carbamate, which was determined using two-dimensional exchange spectroscopy NMR, has the methyl group in the axial orientation and is in agreement with the density functional theory quantum chemical calculations.

The molar conductivity (Λ°) of hydrochloric acid, potassium hydroxide, and sodium hydroxide has been measured in both light and heavy waters from 298 to 598 K at p= 20 MPa using a high-precision flow-through alternating current (AC) conductance instrument. The results were used to explore the deuterium isotope effect on ionic transport by proton hopping mechanisms under hydrothermal conditions. Extrapolations of published transport number data to elevated temperature were used to calculate the individual ionic contributions (λ°) for H3O+, D3O+, OH–, and OD–, from which the excess molar conductivities due to proton hopping were calculated. These are the first reported values for the excess conductivities for D3O+and OD–at temperatures above 318 K. The excess conductivities indicate a strong deuterium isotope effect whereby the transport of D3O+by proton hopping is reduced by ∼33% relative to H3O+, and OD–is reduced by over 60% relative to OH–, over the entire temperature range. A well-defined maximum in the excess conductivities of D3O+and H3O+at ∼420 K suggests that the Eigen cation (H2O)4H+and the Zundel transition-state cation (H2O)2H+are destabilized at elevated temperatures as the three-dimensional, tetrahedrally hydrogen-bonded networks in water break down. The less pronounced maximum for OD–and OH–suggested that their Eigen and Zundel anions, (H2O)3OH–and (H2O)OH–, are less destabilized in the two-dimensional networks and chains that dominate the “structure” of liquid water under these conditions.