Ground and Excited-State Acetic Acid Catalyzed Double Proton Transfer in 2-Aminopyridine

Abstract

Theoretical calculations on the ground and excited state double proton transfer in the 2-aminopyridine (2AP)/acetic acid dual hydrogen-bonded system have been performed. Comparisons have been made between thermodynamic parameters deduced from the theoretical approach and those extracted by absorption and fluorescence titration studies. Incorporating the electron correlation, only one transition geometry was resolved in the ground state. The barrier for the 2(1H)-pyridinimine/acetic acid → 2AP/acetic acid ground-state reverse proton transfer was estimated to be as small as 1.60 and 0.40 kcal/mol at MP2/6-31G(d‘,p‘) and B3LYP/6-31+G(d‘,p‘) levels, respectively. The first excited singlet state of the 2AP/acetic acid system possesses a ππ* configuration, in which two transition-state geometries were resolved for the 2AP/acetic acid → 2(1H)-pyridinimine/acetic acid double proton transfer at the CIS level. The barriers were estimated to be 9.48 and 8.67 kcal/mol (relative to the reactant) using the CIS/6-31+G(d‘,p‘) method, whereas two barriers merge to a single, wide barrier upon inclusion of the zero-point energy. In both ground and excited states, the sequence of the asynchronous double proton transfer correlates with the hydrogen-bonding strength. The results provide a theoretical basis for picosecond dynamics of the 2AP/acetic acetic system recently reported by Ishikawa et al. (J. Phys. Chem. A 2002, 106, 2305). Similarities and differences between the theoretical approaches and the experimental results were discussed.