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Gas-Phase Proton Affinity of Nitric Acid and Its Esters. A Mass Spectrometric and ab Initio Study on the Existence and the Relative Stability of Two Isomers of Protonated Ethyl Nitrate

Massimiliano Aschi, Fulvio Cacace,* Giulia de Petris, and Federico Pepi

Received: May 7, 1996

In Final Form: June 20, 1996

FULL TEXT

Abstract:

The protonation of C₂H₅ONO₂ has been studied in the gas phase by the joint application of mass spectrometric and ab initio theoretical methods. The MIKE and CAD spectra of (C₂H₅ONO₂)H⁺ ions from various sources and their reactivity toward selected nucleophiles, investigated by FT-ICR mass spectrometry, point to the existence of two protomers, the C₂H₅OHNO₂⁺ ion-dipole complex (1c) and the covalently bound C₂H₅ONOOH⁺ species (2c), and to the tendency of the latter to isomerize into 1c in the presence of neutral C₂H₅ONO₂. The BE of NO₂⁺ to C₂H₅OH, independently measured by the kinetic and the equilibrium methods, amounts to 22.2 ± 2 kcal mol^-1 at 298 K, leading to a PA of C₂H₅ONO₂ of 178.4 ± 2.6 kcal mol^-1, referred to the protonation at the ethereal oxygen. The computational results at the G2(MP2) level of theory show that protomers 1c and 2c have the same stability at 298 K and that at the same temperature the 2c 1c isomerization is characterized by a G change of ca. -3 kcal mol^-1. The PA of C₂H₅ONO₂ is computed to be 177 ± 2 kcal mol^-1 at 298 K, irrespective of whether protonation occurs at the ethereal O or at the NO₂ group, in excellent agreement with the experimental value. The results are discussed in connection with the general problem concerning the preferred protonation site and the PA trend along the RONO₂ homologous series. It is shown that entirely different factors control the local PA of the RO and the NO₂ groups.