1,3-HYDROGEN REARRANGEMENTS OF VIBRATIONALLY ACTIVATED ENOLATE IONS IN THE GAS-PHASE

The unimolecular rearrangement reactions of the isomeric enolate ions of 2-butanone have been investigated using Fourier transform ion cyclo tron resonance mass spectrometry and infrared multiple photon activati on techniques. The individual isomers of 2-butanone enolate ions were generated from the corresponding trimethylsilyl enol ethers and photod issociated independently. Infrared multiple photon activation of the 2 -butanone enolate ions induces a 1,3-hydrogen rearrangement which inte rconverts the individual isomers. Because infrared multiple photon act ivation only involves vibrational excitation, the 1,3-hydrogen rearran gement must be a thermal reaction and occur on the ground electronic s tate potential energy surface. The observation of a 1,3-hydrogen rearr angement is unexpected and appears to violate the Woodward-Hoffmann sy mmetry rules. Orbital correlation diagrams show, however, that thermal suprafacial 1,3-hydrogen rearrangements are allowed for enolic system s. Nevertheless, a more probable reaction mechanism involves the rotat ion of the methylene groups so that the 1,3-hydrogen rearrangement cor responds to a simple proton transfer between two unsaturated carbons.