Effects of oxygen and water on the resonance electron capture reactions of low electron affinity compounds

The reactions of the molecular anions of several low electron affinity (EA) compounds, including anthracene, quinazoline, benzophenone, quinoxaline, and azulene, with oxygen and water have been studied by pulsed high pressure mass spectrometry (PHPMS). It is shown that in the simultaneous presence of oxygen and water, these molecular anions, M-, are rapidly destroyed and the ion, O-2(-)(H2O), is rapidly formed. It is shown that the high rate with which this transition occurs can not be explained by the simplest model envisioned that is based on well-known ion molecule reactions. These results can be explained, however, by inclusion into the model of a previously uncharacterized reaction between the molecular ion-oxygen complex, MO2-, and water. The results reported here explain why the molecular anions of compounds that have lower EA's than that of azulene are not readily observed in electron capture ion sources of 1 atm buffer gas pressure. In addition, it is shown that the reactions characterized here lead to a state of chemical equilibrium between the M- and O-2(-)(H2O) ions within the PHPMS ion source from which the EA values of the low-EA compounds can be determined. By this method the electron affinities of anthracene, quinazoline, benzophenone, and quinoxaline are found to be 0.54, 0.56, 0.61, and 0.68 eV, respectively.