ETHER OXIDES - A NEW CLASS OF STABLE YLIDES - A THEORETICAL-STUDY OF METHANOL OXIDE AND DIMETHYL ETHER OXIDE

The potential energy surfaces of neutral methanol oxide and dimethyl e ther oxide and their anion and cation radicals have been calculated at the BECKE3LYP/6-311++G(d,p) level of theory. Both neutral singlet met hanol and dimethyl ether oxides are predicted to correspond to local m inima on their potential energy surfaces. Natural bonding orbital (NBO ) population analysis reveals a distinct ylidic character for these sp ecies. Upon increasing methyl substitution, the R2O+-O- ylide structur e is stabilized energetically because of the better charge distributio n of the formally positive central oxygen atom; thus, the energy diffe rences relative to the R2O + O-3 exit channel decrease significantly. The barrier for the 1,2-hydrogen migration in methanol oxide to yield methyl hydroperoxide amounts to only 5 kcal/mol, whereas the methyl sh ift in dimethyl ether oxide to afford dimethyl peroxide demands >40 kc al/mol and can proceed by retention or inversion of the configuration at the migrating carbon. The kinetic stabilization of the latter is in stead determined either by the loss of a methyl radical or by spin cro ssing to the triplet surface followed by O atom loss. For this process , the minimal-energy crossing point of the two neutral surfaces was lo cated. The corresponding cation radicals of methanol and dimethyl ethe r oxide rest in rather deep wells, and their geometries are not too di fferent from those of the neutrals, Therefore, neutralization-reioniza tion mass spectrometry may allow generation and identification of the neutral species, provided that the cation-radical precursors can be ma de. Furthermore, the kinetic stability of neutral dimethyl ether oxide may be sufficient for its detection in matrix isolation experiments.