Theoretical Atomic Charges vs Experimental Observables: Assessment of Predictions by Density Functionals

Atomic partial charges have been described as <i>proxy variables, vague concept or noumenon</i> (in contrast to <i>phenomenon</i>). Still, these quantities find wide application in chemistry, physics, and biology research. In this study, we assess the physical relevance of the partial charges concept by examining the correspondence between theoretically derived charges and experimentally measured vibrational frequency shifts of the methanol OH stretching mode Δν(OH)_exp induced by hydrogen bonding with variously substituted nitriles. The Δν(OH)_exp shifts in 4-fluorophenol upon hydrogen bonding with mono- and disubstituted pyridines have been shown to correlate nearly perfectly with an alternative atom-centered theoretical parameter─the electrostatic potential at the basic nitrogen in the respective monomers. We explore the correspondence between the Δν(OH)_exp shifts for methanol complexes with nitriles and the partial charges at the CN nitrogen derived from different models. The theoretical charges are evaluated using several frequently used density functionals─B3LYP, ωB97X-D, PBE, PBE0, M06, and M06-2X combined with the aug-cc-pVTZ basis set. The effects of the basis set choice were also considered. Applications of the different density functionals revealed remarkable consistencies for both the magnitude of estimated charges and the strength of correlations with the Δν(OH)_exp shifts. Altogether, the results obtained provide clear support for the physical significance of the partial charges concept.