Electronic structure of pi systems: XX. Electronic structure and keto-enoltautomerism of dihydrofuro[2,3-h]coumarin-9-ones by photoelectron spectroscopy

Photoelectron spectroscopy and semiempirical quantum-chemical calculations were used to study the electronic structure and keto-enol tautomerism of 4- methyldihydrofuro[2,3-h]coumarin-9-one and its 8-substituted derivatives. A nalysis of the electronic structure of 8,8-dibromo-4-methyldihydrofuro[2,3- h]coumarin-9-one (fixed keto form) and 9-acetoxy-4-methylangelicin (fixed e nol form) allowed to reveal ionization ranges for the corresponding tautome ric forms. All dihydrofuro[2,3-h]coumarin-9-ones studied were found to exis t in the gas phase exclusively in the keto form.