Gas-phase solvation of NO2− and NO3− by ethyl nitrate

The gas-phase clustering reactions NO₂⁻· (C₂H₅ONO₂)_n−1 + C₂H₅ONO₂ = NO₂⁻· (C₂H₅ONO₂)n (n = 1–3) NO₃⁻· (C₂H₅ONO₂)_n−1 + C₂H₅ONO₂ = NO₃⁻· (C₂H₅ONO₂)n (n = 1 and 2) have been studied by means of a high-pressure mass spectrometer at temperatures from 200 to 380 K and at total pressures of 0.5–2 torr. It is demonstrated that under the conditions used here these reactions, with the exception of NO₂⁻ + C₂H₅ONO₂ = NO₂⁻· C₂H₅ONO₂, achieve thermodynamic equilibrium above 1 torr. On the basis of the temperature dependence measurements of the equilibrium constants, thermodynamic data have been determined. The following ΔH_n−1⁰,_n values were obtained: NO₂⁻·(C₂H₅ONO₂)_n; ΔH_0,1⁰ =−20.9 (indirectly obtained), ΔH_1,2⁰ = −8.5 and ΔH_2,3⁰ = −7.3 kcal mol⁻¹; NO₂⁻·(C₂H₅ONO₂)_n; ΔH_0,1⁰ = −17.2 and ΔH_1,2⁰ = −7.2 kcal mol⁻¹.

Semiempirical INDO MO calculations were performed for NO₂⁻· (C₂H₅0NO₂)_n = _1,2 and NO₃⁻·(C₂H₅ONO₂)_n = _1,2 to test the structures and binding energies of these systems. The experimental observations are discussed in terms of the structure stabilities of the solvated ions.