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 <-> M1and DT <-> M2) was investigated by DFT. (c) 2007 Elsevier B.V. All rights reserved.