Computational Investigations of Position-Specific Vapor Pressure Isotope Effects in Ethanol—Toward More Powerful Isotope Models for Food Forensics

Abstract

With the advent of new experimental techniques, measurements of individual, per-position, vapor pressure isotope effects (VPIEs) became possible. Frequently, they are in opposite directions (larger and smaller than unity), leading to the cancellation when only bulk values are determined. This progress has not been yet paralleled by the theoretical description of phase change processes that would allow for computational prediction of the values of these isotope effects. Herein, we present the first computational protocol that allowed us to predict carbon VPIEs for ethanol—the molecule of great importance in authentication protocols that rely on the precise information about position-specific isotopic composition. Only the model comprising explicit treatment of the surrounding first-shell molecules provided good agreement with the measured values of isotope effects. Additionally, we find that the internal vibrations of molecules of the model to predict isotope effects work better than the entire set of normal modes of the system.

Isotopic Consequences of Host–Guest Interactions; Noncovalent Chlorine Isotope Effects

“…Therefore, the reported experimental structural results provided us with a unique opportunity to evaluate the quality of different theoretical models used in quantum-chemical calculations of isotope effects associated with changes in weak interactions. We have resorted to the theory level, which proved successful in our recent studies on carbon vapor pressure isotope effects of ethanol, 42 carbon isotope effects on adsorption on graphene, 43 and isotope effects of oxygen and sulfur in phosphates. 44 This level has also been used recently for studies of over 12 000 chemical reactions 45 and thus provides an excellent reference level for future studies.…”

Vapour Pressure Isotope Effect on Evaporation from Pure Organic Phases - a PIMD Approach

“…Although SCC-DFTB offers a promising alternative to any force field method its performance with respect to prediction of isotope effects of very small magnitude like VPIEs is questionable and requires improvement. Very recent study from our laboratory also on VPIEs from ethanol but under distillation conditions clearly indicates necessity of using quantum method along with proper solvation model providing an adequate first solvation shell description (using of at least cluster model of a size of several ethanol molecules) 82 .…”

Intramolecular non-covalent isotope effects at natural abundance associated with the migration of paracetamol in solid matrices during liquid chromatography

“…Within this approach we have used 14 water molecules comprising the first solvation shell of paracetamol (see Figure 1) as a model of well-defined, strong hydrogen bonding network with availability of both proton donor and proton acceptor sites. Calculations were carried out at the DFT level of theory, using ωB97X-D functional [43] expressed in the def2-TZVP basis set [44] as implemented in the Gaussian16 program [45], which proved successful in our recent studies on vapor pressure isotope effects of ethanol [46]. We have carried out calculations for structures of paracetamol in the gas phase, continuum solvent models of diethyl ether and acetone, and surrounded by 14 explicitly treated water molecules.…”