Ground and excited state intramolecular proton transfer in salicylic acid:an ab initio electronic structure investigation

Energetics of the ground and excited state intramolecular proton transfer i n salicylic acid have been studied by ab initio molecular orbital calculati ons using the 6-31G** basis set at the restricted Hartree-Fock (RHF) and co nfiguration interaction-single excitation (CIS) levels and also using the s emiempirical method AM1 at the RHF level as well as with single and pair do ubles excitation configuration interaction spanning eight frontier orbitals (PECI = 8). The ab initio potential energy profile for intramolecular prot on transfer in the ground state reveals a single minimum corresponding to t he primary form, in the first excited singlet state, however, there are two minima corresponding to the primary and tautomeric forms, separated by a b arrier of similar to 6 kcal/mol, thus accounting for dual emission in salic ylic acid. Electron density changes with electronic excitation and tautomer ism indicate no zwitterion formation. Changes in spectral characteristics w ith change in pH, due to protonation and deprotonation of salicylic acid, a re also accounted for, qualitatively. Although the AM1 calculations suggest a substantial barrier for proton transfer in the ground as well as the fir st excited state of SA, it predicts the transition wavelength in near quant itative accord with the experimental results for salicylic acid and its pro tonated and deprotonated forms.