GASEOUS BORATE AND POLYBORATE ANIONS

Borate and polyborate anions have been studied in the gas phase by an approach involving, instead of vaporization of preformed ions, gas-phase synthesis of increasingly complex polyanions by stepwise addition of H3BO3, followed by H2O loss, to H2BO3-, obtained in turn from gaseous H3BO3 via dissociative electron attachment or by suitable ion-molecule reactions. Chemical ionization and ion-cyclotron resonance techniques demonstrate the formation, in addition to BO2- and H2BO3-, of polyanions belonging to three different classes, (Hn-1BnO2n)-, (Hn+1BnO2n+1)-, and (Hn+3BnO2n+2)-, containing up to 11 B atoms. The results provide the first evidence for the existence as discrete entities in the gas phase of free polyanions, e.g., the triborate ion H4B3O7- and the pentaborate ion H4B5O10-, whose salts exist in solid borates and in solution. Furthermore, previously unknown anions have been obtained, including gaseous H2BO3-, H3B2O5-, H5B4O9-, etc. The character of the H3BO3 acidity changes in passing from solution to the gas phase. In solution H3BO3 behaves exclusively as a Lewis acid, giving B(OH)4- upon ionization, whereas in the gas phase it behaves instead as a Bronsted acid, as shown by the formation of its conjugate base H2BO3- and by its gas-phase Bronsted acidity, 354 +/- 4 kcal mol-1, typical of a relatively weak gaseous acid whose strength is comparable to that of HClO.