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 2 ISOMERS OF PROTONATED ETHYL NITRATE

The protonation of C2H5ONO2 has been studied in the gas phase by the j oint application of mass spectrometric and ab initio theoretical metho ds. The MIKE and CAD spectra of(CH2H5ONO2H+ ions from various sources and their reactivity toward selected nucleophiles, investigated by FT- ICR mass spectrometry, point to the existence of two protomers, the C2 H5OHNO2+ ion-dipole complex (1c) and the covalently bound C2H5ONOOH+ s pecies (2c), and to the tendency of the latter to isomerize into 1c in the presence of neutral C2H5ONO2. The BE of NO2+ to C2H5OH, independe ntly measured by the kinetic and the equilibrium methods, amounts to 2 2.2 +/- 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 oxyg en. 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 Del ta G degrees change of ca. -3 kcal mol(-1). The PA of C2H5ONO2 is comp uted to be 177 +/- 2 kcal mol(-1) at 298 K, irrespective of whether pr otonation occurs at the ethereal O or at the NO? group, in excellent a greement with the experimental value. The results are discussed in con nection with the general problem concerning the preferred protonation site and the PA trend along the RONO(2) homologous series. It is shown that entirely different factors control the local PA of the RO and th e NO2 groups.