Origine et structure d'ions C4H9O+ issus de la methylation de l'acetone par l'iodomethane ionise

Abstract Electron impact ionization of a mixture of acetone and iodomethane in a high-pressure ion source induces the protonation of acetone and also the methylation yielding C 4 H 9 O + ( m/z 73) ions. The analysis of the collisional activation spectra of these ions and appropriate deuterium labelled analogues indicates the formation of dimethylmethoxycarbenium ions, a . These ions are also formed by protonation of 2-methoxypropene; however, depending upon the experimental conditions, an isomeric methyl-2-propenyl-oxonium species, b , was also detected. Ion cyclotron resonance experiments show that the C 4 H 9 O + ions are formed by methyl cation transfer from the molecular ions of iodomethane; dimethyliodonium ions (CH 3 ) 2 I + were found to be unreactive. The gas-phase basicity of 2-methoxypropene has also been measured (206.2±0.2 kcal mol −1 ) and indicates preferential protonation of the double bond to give the carbenium ions a . These results are supported by ab initio molecular orbital calculations (3–21 G) which show that ion a is more stable than b by 18 kcal mol −1 . The proposed existence of hydrogen-bonded molecule pairs of the kind propyne-H + −methanol, c , is also supported by calculations.