GAS-PHASE THERMOCHEMICAL PROPERTIES OF THE BICARBONATE AND BISULFITE IONS

A combined experimental and theoretical study of the gas-phase thermochemical properties of bicarbonate ion HOCO2-, bisulfite ion HOSO2-, and their conjugate acids is presented. The formation and qualitative identification of the sulfonate ion HSO3- is also reported. Threshold energies for collision-induced dissociation of OH- ion from HOCO2- and HOSO2- have been determined to be 50.3 +/- 2.5 and 61.3 +/- 2.5 kcal mol-1 respectively, from which the standard heats of formation DELTAH(f,298)-degrees [HOCO2-, g] = -177.8 +/- 2.5 kcal mol-1 and DELTAH(f,298)-degrees [HOSO-] = -165.6 +/- 2.5 kcal mol-1 are derived by means of simple thermochemical cycles. The measured HO- binding energies of CO2 and SO2 are in good agreement with ab initio calculations and with the expected values based on empirical correlations between Bronsted and Lewis basicities of negative ions. Reactions between bicarbonate ion and a series of neutral acids in the flowing afterglow indicate an apparent proton affinity of 356 +/- 2 kcal mol-1. Bicarbonate ion reacts with H2S to produce both HS- and HS- (H2O), and the deuterium-labelled ion, DOCO2-, reacts also by H/D exchange. Bisulfite ion is found to have an apparent proton affinity of 337 +/- 3 kcal mol-1 from its behavior in proton transfer reactions with a series of reference acids. The reaction between HOSO2- and HCl yields Cl-(SO2) as the major ionic product, along with a small amount of Cl-. DOSO2- does not exhibit any H/D exchange with HCl or carboxylic acids. Sulfonate ion can be formed at room temperature in the flowing afterglow either by oxygen atom transfer from NO2 to HSO2-, or by hydride transfer from CH3O- to SO3. High level ab initio calculations predict gas-phase acidities (DELTAH(acid)) for carbonic acid (HO)2CO and sulfurous acid (HO)2SO of 339 and 330 kcal mol-1 respectively, and enthalpy changes for their dehydration of -5 and -2 kcal mol-1 respectively. The origin of the large differences between the calculated and apparent basicities of HOCO2- and HOSO2- is proposed to be due, in part, to a dissociative neutralization mechanism wherein dehydration of the nascent carbonic acid and sulfurous acid molecules accompanies proton transfer.