IS THE PROTON AFFINITY OF NITRIC-ACID LARGER THAN THE PROTON AFFINITY OF METHYL NITRATE - A DIRECT EXPERIMENTAL ANSWER

A direct experimental approach based on the evaluation of the ligand exchange equilibrium H2O-NO2+ + CH3OH reversible arrow CH3(H)O-NO2+ + H2O has been exploited for the determination of Delta PA = PA(HNO3) - PA(CH3ONO2) at 298 K. The result, Delta PA = 4.0 +/- 1.2 kcal mol(-1), is sufficiently accurate to provide firm experimental support to the counterintuitive prediction, based on high-level ab initio calculations, that the PA of HNO3 exceeds that of CH3ONO2 by 5.6 +/- 5 kcal mol(-1). Combination of the experimental Delta PA with the known PA of CH(3)ONO2 gives PA(HNO3) = 182.0 +/- 2.3 kcal mol(-1), in excellent agreement with the theoretically computed value, 182.5 +/- 3 kcal mol(-1). An explanation of the considerably lower PA(HNO3) value derived from earlier ICR bracketing experiments is offered, based on the R(H)O-NO2+ vs RONO(2)H(+) isomerism, whose role in determining the observed PA trend along the RONO(2) series (R = H, CH3, C2H5) is discussed.