Abstract
We propose a novel quantum chemical method, called the harmonic solvation model (HSM), for calculating thermochemical parameters in the condensed phase, particularly in the liquid phase. The HSM represents translational and rotational motions of a solute as vibrations interacting with a cavity wall of solvent molecules. As examples, the HSM and the ideal-gas model (IGM) were used for the standard formation reaction of liquid water, combustion reactions of liquid formic acid, methanol, and ethanol, vapor-liquid equilibration of water and ethanol, and dissolution of gaseous CO2 in water. The numerical results confirmed the reliability and applicability of the HSM. In particular, the temperature dependence of the Gibbs energy of liquid molecules was accurately reproduced by the HSM; for example, the boiling point of water was reasonably determined using the HSM, whereas the conventional IGM treatment failed to obtain a crossing of the two Gibbs energy curves for gaseous and liquid water.
Highlights
Quantum chemistry, which attempts to solve the electronic Schrödinger equation accurately and efficiently, has been well developed over the 90 years since its inception
Modern quantum chemical calculations can lead to the systematic achievement of high accuracy in evaluating a molecule’s electronic energy Eelec, and molecular properties such as geometric parameters and vibrational frequencies
We have described a novel quantum chemical method, called the harmonic solvation model (HSM), for calculating thermochemical parameters
Summary
Quantum chemistry, which attempts to solve the electronic Schrödinger equation accurately and efficiently, has been well developed over the 90 years since its inception. Modern quantum chemical calculations can lead to the systematic achievement of high accuracy in evaluating a molecule’s electronic energy Eelec, and molecular properties such as geometric parameters and vibrational frequencies. The contribution of solvent effects obtained using the SCRF is normally added to the electronic energy using standard quantum chemical programs. Other contributions to the enthalpy and entropy are evaluated using the formalism based on the IGM. We need an alternative, because simple applications of the IGM are expected to lead to unphysical behaviors in the liquid phase. We propose a simple but novel technique, called the harmonic solvation model (HSM), for evaluating the enthalpy and entropy components of molecular motion in the liquid phase.
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