Stability and structure of H3CO+ formed from COH++H2 at low temperature

Abstract

The gas phase equilibrium (1) HCO++H2 = H3CO+ was studied in the temperature range −100 to −165 °C with a pulsed electron beam high pressure ion source mass spectrometer. Van’t Hoff plots of K1 led to ΔH °1 = −3.9 kcal/mole and ΔS °1 = −20.5 e.u. (stand. state 1 atm). This ΔH1 and standard thermochemical data lead to ΔHf(H3CO+) = 196.8 kcal/mole. Thus the H3CO+ formed by (1) is not protonated formaldehyde H2COH+ whose ΔHf = 169.8 kcal/mole. The reactivity of H3CO+ observed in ion molecule reactions and the theoretically predicted charge distribution in HCO+ suggest that the structure of H3CO+ is an HCO+ in which the partially positive hydrogen forms a weak three center bond with an H2. This bond is similar to, but weaker than the bond occurring between H3+ and H2 in the H5+ molecule.