REACTIVITY OF THE RADICAL-ANION OCC-

The characteristic reactivity of the radical anion OCC- has been investigated in the ps phase at 298 K through determination of rate coefficients, products, and branching fractions for each of 29 ion-molecule reactions. A wide variety of reactions is observed including abstraction of H, H+, and H-2+, nucleophilic displacement, charge transfer, and reactions involving electron detachment. Many of the reactions involve cleavage of the C--CO bond, consistent with the relatively small C--CO bond energy and the proposed1 electronic structure of the ground state anion in which both radical and charge are centered on the terminal carbon. Similarities are noted between the chemistry of OCC- and its neutral analogue OCC and between the chemistry of OCC- and the radical anions O- and o-C6H4-. Most reaction products observed are consistent with reaction mechanisms involving initial attack of the terminal carbon in OCC- on the neutral reaction partner. The gas-phase acidity of HCCO is bracketed between those of CH3NO2 and CH3CHO, yielding 1502 +/- 8 > DELTAG-degrees(acid)(HCCO) greater-than-or-equal-to 1463 +/- 8 kJ mol-1 and 1531 +/- 12 > DELTAH-degrees(acid)(HCCO) greater-than-or-equal-to 1491 +/-12 kJ mol-1. Observation of H atom transfer from CH2Cl2 to OCC- indicates that DELTAH-degrees(f)(OCC-) greater-than-or-equal-to 148 +/- 12 kJ mol-1 and gives a larger lower limit of DELTAH-degrees(acid) greater-than-or-equal-to 1507 +/- 15 kJ mol-1. These and related thermochemical values, including the hydrogen bond dissociation energy in HCCO, are compared with literature values.