Partial Hessian vibrational analysis: vapour pressure isotope effects and their relation with non-covalent interactions

TL;DR

This paper compares full and partial Hessian vibrational analyses in predicting equilibrium isotope effects in organic solvents, emphasizing the role of non-covalent interactions and solvent effects.

Contribution

It demonstrates the effectiveness of partial Hessian vibrational analysis for estimating isotope effects and explores how non-covalent interactions influence these effects using SAPT and IGM methods.

Findings

PHVA qualitatively predicts EIEs similar to FHVA for weakly interacting systems.

In strongly interacting systems, PHVA estimates slightly smaller EIEs than FHVA.

Intermolecular interactions significantly impact isotope effects, especially in strongly interacting solvents.

Abstract

In this study equilibrium isotope effects (EIEs) on evaporation to several organic solvents (bromobenzene, dibromomethane, ethanol, methanol, and trichloromethane) in the pure phase are estimated employing Kohn-Sham Density functional theory (KS-DFT) along with full Hessian vibrational analysis (FHVA) and partial Hessian vibrational analysis (PHVA). Both FHVA and PHVA qualitatively predict compounds EIEs. Weakly interacting systems (bromobenzene, dibromomethane, and trichloromethane) display identical EIE values for both, FHVA and PHVA. Whereas for strongly interacting systems (ethanol and methanol), PHVA estimates slightly smaller EIE compared to FHVA. By employing the symmetry-adapted perturbation theory (SAPT) along with independent gradient model (IGM), it was possible to establish how minimal changes in compounds interaction energy affected carbon and bromine EIEs estimation in dibromomethane. Moreover, SAPT and IGM revealed how in ethanol, apparently insignificant intermolecular interactions possessed an enormous impact in carbon position specific isotope effects, which highlight the importance of solvent effects for strongly interacting compounds. The presented results have important implications for correctly comprehending and characterising the role of non-covalent interactions in the prediction of isotope effects, as well as providing a robust test to PHVA for the prediction of EIEs.