Hydrogen and Carbon Vapour Pressure Isotope Effects in Liquid Fluoroform Studied by Density Functional Theory

Abstract

H/D and 12C/13C vapour pressure isotope effects (VPIEs) in liquid fluoroform (CHF3) were studied at the MPW1PW91/6-31 ++ G(d) level of theory. The CHF3 monomer and CHF3 molecules surrounded by other CHF3 molecules in every direction in CHF3 clusters were used as model molecules of vapour and liquid CHF3. Although experimental results in which the vapour pressure of liquid 12CHF3 is higher than that of liquid 12CDF3 and the vapour pressure of liquid 13CHF3 is higher than that of liquid 12CHF3 between 125 and 212 K were qualitatively reproduced, the present calculations overestimated the H/D VPIE and underestimated the 12C/13C VPIE. Temperature-dependent intermolecular interactions between hydrogen and fluorine atoms of neighbouring molecules were required to explain the temperature dependences of both H/D and 12C/13C VPIEs.

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

“…It is vital to have in mind that isotope effects are solely an aftereffect of molecule quantum mechanical fluctuations, thence, the theoretical study of such, must be performed by the means of proper computational quantum chemistry tools. For example, Yanase and Oi performed simulations using electronic structure calculations of molecular orbitals 34,35 , where vapour pressure isotope effects (VPIE) of some halogenated systems 20,27 were predicted successfully using a modified version of Bigeleisen and Mayer's approximation 34,35 . Their findings highlighted that Bigeleisen and Mayer's approximation provides unreliable predictions of deuterium isotopic substitution 3,34 .…”

“…Hence it is obvious that proper and/or ultimate ways for theoretical prediction of isotope effect, surrogate to Bigeleisen and Mayer's approach are impelling needed, especially when isotope effects modelling can provide information regarding structure, intermolecular interactions 34,35 and forces dominating the process.…”

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

“…Another exceptional functional is MPW1PW91, [16][17][18] which has permitted the accurate predictions of hydrogen and carbon VPIEs for fluoroform. 19,20 Important, and a point often overlooked, is the quality of stationary point atomic positions. It must be remembered that the Hessian is the second derivative of the molecular energy with respect to nuclear coordinates (Cartesian coordinates), therefore undergoing an accurate reproduction of them intentionally serves as a pivotal step in order to calculate reliable isotope effects for the precise reason.…”