Gaseous protonated acids fragment in the first drift region of a double focussing mass spectrometer to yield the corresponding acylium ion and water. The metastable peaks for this fragmentation reaction have been recorded for the protonated acids from acetic to valeric and the kinetic energy release distributions evaluated from the metastable peak shapes. The protonated acids were produced by dissociative ionization of the ethyl, propyl, and butyl esters. The results provide evidence for two structures for gaseous protonated acids. Fragmentation of the hydroxyl protonated structure, a minor contributor to the metastable peak intensity, shows a low kinetic energy release (T(most probable) = 0.02 eV) as would be expected for a simple bond fission reaction. Fragmentation of the carbonyl protonated acid, which represents the major part of the metastable peak, is accompanied by a much larger kinetic energy release (T(most probable) = 0.30 to 0.43 eV). This result is interpreted in terms of an activation barrier for fragmentation of the carbonyl protonated acid which is considerably greater than the reaction endothermicity, with the excess energy being partitioned between internal energy and kinetic energy of the fragments. The results indicate that the addition of the acylium ion to water in the gas phase to produce the carbonyl protonated acid has an activation energy barrier.NORA E. MIDDLEMJSS et ALEX G. HARRISON. Can. J. Chem. 57.2827 ( 1979 Les acides protones I'ttat gazeux se fragmentent dans la premiere region de derive d'un spectrometre de masse a double champ en donnant les ions acylium correspondants et de l'eau. On a enregistre les pics metastables provenant de la fragmentation d'une serie d'acides protones allant de l'acide acetique a l'acide valerique et on a evalue, a partir de la forme de ces pics, les distributions de I'energie cinetique liberee. Les acides protones proviennent d'une dissociation ionique des esters ethylique, propylique et butyrique. Ces resultats prouvent que les acides protones existent sous deux formes structurales. La fragmentation de la forme protonee au niveau du groupe hydroxyle, forme qui contribue faiblcment a 1'intensitC du pic metastable, libere une faible tnergie cinitique (T = 0.02 eV probablement) ce qui correspond a I'energie liberee lors d'une simple rupture de liaison. La fragmentation de I'acide protone au niveau du groupe carbonyle, forme qui contribue principalement a l'intensite du pic metastable, est accompagnee d'une plus grande libtration d'energie cinetique ( T = 0.30 a 0.43 eV probablement). On interprete ces risultats en fonction d'une barriere d'energie d'activation, de la reaction des acides protones au n i~e a u du carbonyle considerablen~ent plus ClevCe que l'endothermicite de la reaction. L'exces d'energie ktant reparti entre I'energie interne et l'energie cinetique des fragments. Ces resultats revelent que la formation de I'acide protone au niveau du groupe carbonyle, par suite de l'addition de l'ion acylium l'eau dans la phase gazeuse, a une barriere d...

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