Collision-induced dissociation and charge reversal experiments on H 3O − and NH 4−

Abstract

It is shown that the hydride solvated water molecule or H 3O − ion can be generated in a chemical ionization source by electron bombardment of a mixture of ammonia, water, and formaldehyde. Collision-induced dissociation of this ion following its acceleration to 8 keV kinetic energy results in the exclusive formation of OH − by elimination of molecular hydrogen. Large isotope effects have been found for this elimination reaction in the cases of hydride or deuteride solvated HDO molecules, suggesting a linear and more or less symmetrical transition state for proton transfer from the water molecule to the hydride ion to generate OH − and H 2. Upon charge reversal and accompanying fragmentation of the H 3O − ion, much more abundant H 2 +· and H 3 + ions are generated than upon collision-induced dissociation of the H 3O + ion directly generated in the chemical ionization source. This observation is in line with the structure of the H 3O − ion, its most stable form being calculated to correspond to a species in which the hydride ion is bonded to one of the hydrogen atoms of the water molecule rather than a species in which the oxygen atom is symmetrically surrounded by the hydrogen atoms as is the case for H 3O +. Similar observations have been made for the NH 4 − ion, which can be generated in a chemical ionization source by electron bombardment of a mixture of ammonia and formaldehyde, upon charge reversal with respect to directly generated NH 4 +.