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

Article abstract of DOI:10.1021/ja00093a048

A direct experimental approach based on the evaluation of the ligand exchange equilibrium H₂O-NO₂⁺ + CH₃OH ?CH₃(H)O-NO₂⁺+H₂O has been exploited for the determination of ΔPA = PA(HNO₃)-PA(CH₃ONO₂) at 298 K. The result, ΔPA = 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 CH₃-ONO₂ by 5.6 ± 5 kcal mol^-1. Combination of the experimental ΔPA with the known PA of CH₃ONO₂ 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 RONO₂H⁺ isomerism, whose role in determining the observed PA trend along the RONO₂ series (R = H, CH₃, C₂H₅) is discussed.