Solvent effect on Hammett sigma constants

Abstract

Ispitan doprinos specifičnih interakcija DMSO-a prilikom računa Hammettovih sigma konstanti reakcije disocijacije para-supstituiranih benzojevih kiselina. Korištene su homodezmotske reakcije kako bi se izbjegao značajno zahtjevniji račun solvatacije protona u DMSO-u. Račun geometrija, slobodne energije solvatacije i termičkih korekcija obavljen je na PBE0/def2-TZVPD/SMD razini teonije dok su finalne energije računate na B2PLYPD3/ def2QZVPPD razini. Solvatacija je opisana kontinuumom te klaster-kontinuum metodom s do 3 molekule vode, odnosno jednom molekulom DMSO-a u klasteru. Valjanost metode potvrđena je usporedbom izračunatih pKa vrijednosti u vodi s literaturnima. Metoda se pokazala uspješnom u računu pKa vrijednosti u vodi uz upotrebu 2 ili 3 molekule vode u klasteru. Račun kontinuum solvatacijom u DMSO-u pokazao je izvrsno slaganje s literaturnim podacima. Klasteri s molekulom DMSO-a dali su lošije rezultate od samo kontinuum solvatacije uzrok čega bi trebalo istražiti.The disociacion of para-benzoic acids in water and DMSO was investigated to estimate the contribution of specific solvent interactions in the calculation of Hammett σ constants. Homodesmic reactions were used to achieve the maximal cancelation of error caused by deficiencies at this level of theory. Calculation of geometries, standard Gibbs solvation energies and thermal corections was done at the PBE0/def2-TZVPD/SMD level, while single point energies were computed at the B2PLYP-D3/def2-QZVPPD level of theory. Solvation was modeled by continuum and cluster-continuum methods with up to three discrete molecules of water or one molecule of DMSO pressent in the clusters. Validation of the method was done by pKa calculation in water which was in good agreement with values taken from literature for clusters containing two or three molecules of water. Calculations in DMSO using continuum solvation showed agreement with literature as good as the best water clusters. DMSO clusters predicted worse results which merits further investigation