Is the proton affinity of nitric acid larger than proton affinity of methyl nitrate? A direct experimental answer.

A direct experimental approach based on the evaluation of the ligand exchange equilibrium H20-N02+ + CH30H CH3(H)O-N02+ + H20 has been exploited for the determination of APA = PA(HN03) - PA(CH30N02) at 298 K. The result, APA = 4.0 f 1.2 kcal mol-', is sufficiently accurate to provide firm experimental support to the counterintuitive prediction, based on high-level ab initio calculations, that the PA of HN03 exceeds that of CH3- ON02 by 5.6 f 5 kcal mol-'. Combination of the experimental APA with the known PA of CH30N02 gives PA(HNO3) = 182.0 f 2.3 kcal mol-', in excellent agreement with the theoretically computed value, 182.5 f 3 kcal mol-'. An explanation of the considerably lower PA(HN03) value derived from earlier ICR bracketing experiments is offered, based on the R(H)O-N02+ vs RON02H+ isomerism. whose role in determining the observed PA trend along the RON02 series (R = H, CH3, C2H5) is discussed.