Ion-polar-molecule reactions in the methanol-acetaldehyde system



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Effect of the permanent dipole moment on the mechanism of ion-molecule reactions occurring in the methanol-acetaldehyde system have been evaluated. The essential elements of this study encompassed the following: firsta the determination of the important reactions in the chosen system; second, the determination of the effect of ion energy on the reaction cross sections; and third, the correlation of these results with predictions obtained from computer calculations of the dynamics of the methanol self-reactions. A quantitative method of evaluating relative reaction cross sections was used to evaluate the important processes in the methanol-acetaldehyde system. The method, which utilizes high pressure mass spectroscopy, can only be used for mixtures where the ionization potentials of the parent ions differ appreciably. Proton and hydrogen atom transfer were investigated using combinations of deuterium-labeled reactents. Variations of ionization voltage near the onset of methanol ion formation were used to distinguish between proton and hydrogen atom transfer. Proton transfer appears to be determined chiefly by the energetics of the competitive processes, while hydrogen atom transfer is favored from the aldehyde and hydroxyl groups by a factor of three. A tandem mass spectrometer has been employed to further investigate the transfer processes in collisions involving methanol, acetaldehyde, and their molecular ions at ion energies between 1 and 5 eV (laboratory system). At the lowest incident ion energies hydrogen atom transfer is preferred from the electronegative end of the molecule, while randomness is approached at higher energies. Ion transfer is the most important process at all energies, and transfer of the hydrogen ion from the electronegative group is always favored. The non-randomization of equivalent atoms in a possible complex suggests that the ion retains its identity in the reaction. In an effort to understand more completely the effect of the orientation of the two reactants in an ion-molecule reaction, the dynamics of motion of the methanol self-reactions were investigated in detail. These calculations took into account the long-range forces in the system and the three degrees of rotational freedom of the rigid molecule. By assuming that reaction occurred between the ion and the portion of the molecule nearest the ion at the time of closest approach, relative reaction cross sections could be obtained. Results from these calculations and the tandem mass spectrometer experiments are compared.