The role of ortho, meta, para isomerism in measured solid state and derived sub-cooled liquid vapour pressures of substituted benzoic acids

Knudsen Effusion Mass Spectrometry (KEMS) has been used to measure solid state equilibrium vapour pressures of several multifunctional aromatic compounds; phthalic, isophthalic, terephthalic, para-anisic, ortho-amino benzoic, meta-amino benzoic, para-amino benzoic, vanillic, syringic, 1,2,4-tricarboxylic benzoic, 3,5-dihydroxy-4-methyl benzoic and 4-methyl phthalic acids and nitrocatechol. Sub-cooled liquid vapour pressures were derived using Differential Scanning Calorimetry (DSC) measured thermochemical properties for the compounds measured here, as well as for other substituted benzoic acids using literature values. Unusual trends in the sub-cooled liquid vapour pressure, not represented by currently available vapour pressure estimation methods, are explained using a newly constructed Structure-Activity Relationship (SAR) with a combination of resonance and steric effects. This was then tested against further measurements of ortho-dimethyl amino benzoic and meta-dimethyl amino benzoic acids.