Theoretical studies on structure, energetic and intramolecular proton transfer of alkannin

The structure and energetic of four tautomers (‘normal’ tautomer NT, ‘double’ tautomer DT, M1 and M2) and four rotamers (NT-R1, NT-R2, NT-R3 and DT-R1) of alkannin had been studied theoretically at the B3LYP/6-31+G(d, p) level. In the ground-state, the calculated results predicted that NT is dominant, and DT is less stable with 1.31 kcal/mol than NT. However, NT and M2 are two most stable tautomers in the excited-state, and NT is more stable with 0.59 kcal/mol than M2. Furthermore, ground- and excited-state intramolecular proton transfer occurring in the four tautomers (the processes NT ↔ M1, NT ↔ M2, DT ↔ M1 and DT ↔ M2) was investigated by DFT.