Density functional theory and RRKM calculations of the gas-phase unimolecular rearrangements of methylfuran and pyran ions before fragmentations.

The potential energy profiles for the mutual conversion of the isomeric molecular ions [C5H6O]+* of 2-methylfuran, 3-methylfuran and 4H-pyran and the fragmentations that lead to [C(5)H(5)O](+) ions were obtained from calculations at the B3LYP/6-311G++(3df,3pd)//B3LYP/6-31G(d,p) level of theory. The various competing unimolecular processes were characterized by their RRKM microcanonical rate coefficients, k(E), using the sets of reactant and transition state frequencies and the kinetic barriers obtained from the density functional method. In either a high- or a low-energy regime, the pyrylium ion [C5H5O]+ is generated directly from the 4H-pyran molecular ion by a simple cleavage. In contrast, in the metastable kinetic window, the molecular ions of methylfurans irreversibly isomerize to a mixture of interconverting structures before dissociation, which includes the 2H- and 3H-pyran ions. The hydrogen atoms attached to saturated carbons of the pyran rings are very stabilizing at the position 2, but they are very labile at position 3 and can be shifted to adjacent positions. Once 4H-pyran ion has been formed, the C-H bond cleavage begins before any hydrogen shift occurs. According to our calculation, there would not be complete H scrambling preceding the dissociation of the molecular ions [C5H6O]+*. On the other hand, as the internal energy of the 2-methylfuran molecular ion increases, H* loss can become more important. These results agree with the available experimental data.