PROTON-TRANSFER IN SOLID 1-PHENYLBUTANE-1,3-DIONE AND RELATED 1,3-DIONES AS STUDIED BY C-13 CPMAS NMR-SPECTROSCOPY AND AM1 CALCULATIONS

The intramolecular proton transfer taking place in 1-phenylbutane-1,3-dione was studied by using C-13 NMR data obtained both in solution and in the solid state. Comparison of CPMAS results with carbon chemical shifts measured in solution reveals that this compound exists in both phases as a rapidly interconverting mixture of both possible enol tautomers, with a slight preference for the methyl keto structure. Semiempirical AM1 calculations support this conclusion, leading to a preferred form having the enolic structure and giving two minima with optimized geometries corresponding to two different enol tautomers having almost equal energy. NMR data are also reported for related 1,3-diaryl-1,3-diketones in both phases, which show them to undergo a similar proton-exchange process.