A density functional theory study
Vapor pressure isotope effects have been explored for understanding intermolecular interactions of chemical substances and practical isotope separation. Traditionally, cell models coupled with theory of isotope effects in condensed phases have been used to study vapor pressure isotope effects. Subsequent studies have adopted simplified methods such as molecular orbital calculations for analyzing the equilibrium isotope effects between liquid and vapor phases. It is, however, difficult to model condensed phases for accurate elucidation of isotope salt effects using these methods.
To this note, researchers at Sophia University: Mr. Ryota Mitome, Dr. Satoshi Yanase, Professor Yoshikazu Kikawada, and Professor Takao Oi studied the vapor pressure isotope effects of H/D and 12C/13C in liquid methylene difluoride. This was an extension of their previous work in liquid fluoroform where temperature dependence of both H/D and 12C/13C vapor pressure isotope effects were attributed to the intermolecular interactions between hydrogen and fluorine atoms in the liquid phase. Unlike the previous study, the authors employed in the most recent study molecular orbital calculations on methylene difluoride to characterize the vapor pressure isotope effects of liquid methylene difluoride based on density functional theory. The optimized monomer and methylene difluoride molecules surrounded by other methylene difluoride molecules clusters were respectively utilized as model molecules for vapor and liquid methylene difluoride. The work is currently published in the research journal, Chemical Physics Letters.
Molecular orbital calculations reproduced both normal hydrogen and inverse carbon vapor pressure isotope effects even though the former was overestimated while the latter was underestimated. The reduced partition function ratio based method was noted to be effective for isotope effects analysis. Calculated and experimental values were compared. At a given temperature, most calculated values were observed to be larger than experimental values. This resulted in an overestimation of hydrogen vapor pressure isotope effects. Considering the significant effects of intermolecular interactions on the vapor pressure isotope effects, the number of C-H∙∙∙F interactions in methylene difluoride molecule was explored. The value of reduced partition function ratio increased with an increase in the number of C-H∙∙∙F interactions.
Just like in their previous study, the authors confirmed the temperature-dependence of intermolecular H∙∙∙F interactions in liquid methylene difluoride. This was clarified by the number of interactions between the F-C and C-H bonds of neighboring methylene difluoride in liquid phases and their corresponding strengths. Furthermore, blue-shifts in C-H frequency were observed to take place in methylene difluoride monomer aggregates.
In a nutshell, the study by Sophia University scientists explored vapor pressure isotope effects in methylene difluoride molecules. Based on their results, the intermolecular interaction in the liquid methylene difluoride was realized to depend on temperature. In addition to molecular orbital calculations that effectively reproduced the vapor pressure isotopes effects, the authors also identified the potential use of molecular dynamic simulations in such studies. Altogether, the study insights will enable thorough investigation of the temperature dependence of intermolecular interactions.
Mitome, R., Yanase, S., Kikawada, Y., & Oi, T. (2020). H/D and 12C/13C vapor pressure isotope effects in liquid methylene difluoride studied by density functional theory. Chemical Physics Letters, 739, 137036.