Modeling molecular boiling points using computed interaction energies.

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Molecular Interactions

Background:

• Traditional understanding attributes liquid intermolecular forces to interactions between total molecular dipoles (permanent, induced, or transient).
• This model is commonly presented in chemistry textbooks for describing van der Waals forces in liquids.

Purpose of the Study:

• To test the prevailing textbook notion of molecular dipole interactions governing van der Waals forces in liquids.
• To identify the primary contributors to cohesive forces in halogenated hydrocarbon liquids and their impact on boiling points.

Main Methods:

• Examined boiling points of 67 halogenated hydrocarbon liquids.
• Utilized quantum chemical calculations (AM1, PM3, Hartree-Fock, DFT/B3LYP) to determine molecular dipole moments, ionization potentials, and polarizabilities.
• Employed calculated interaction energies and an empirical hydrogen bonding measure in regression analyses to model boiling points.

Main Results:

• Regression analyses indicated that only London dispersion energies and hydrogen bonding significantly correlated with boiling points.
• Model performance improved with higher levels of quantum chemical computation.
• Optimal models for predicting boiling points utilized empirical polarizability or B3LYP/6-311G(d,p) calculated polarizabilities, alongside a hydrogen-bonding parameter.

Conclusions:

• Cohesive forces in halogenated hydrocarbons are better described by highly localized interactions, not global molecular dipoles.
• London dispersion forces and hydrogen bonding are the dominant factors influencing the boiling points of these compounds.
• Advanced quantum chemical methods enhance the accuracy of models predicting liquid properties.