The Heterolytic Dissociation of Neutral and Protonated Nitrous Acid

Structures, energies, and vibrational properties of HONO and H ₂ ONO ⁺ and of their dissociation products NO ⁺ , HO ^- , and water were studied with the DFT methods B3LYP and MPW1PW91 and with the configurational methods MP2, QCISD, QCISD(T), CCSD, and CCSD(T) in conjunction with the basis sets 6-31G**, 6-31++G**, 6-311G**, 6-311++G**, 6-311++G(2d,p), and 6-311++G(2df,2p). The multilevel methods G1, G2, G2(MP2), G3, and CBS-Q also were employed. The experimental trans-preference energy Δ E ₀ ^pref = 1.555 ± 0.167 kJ/mol of HONO is reproduced well at all levels with good basis sets. On the basis of the CCSD(T)/6-311++G(2df,2p) energies and the thermochemical data, our best computed estimate is Δ E ₀ ^pref = 1.83 kJ/mol. The experimental heterolytic bond dissociation energy of Δ E ₀ = 917.80 kJ/mol for HONO and of Δ H ₂₉₈ = 77.3 kJ/mol for H ₂ ONO ⁺ are more difficult to reproduce. At the CCSD(T)/6-311++G-(2df,2p) level, we found Δ E ₀ = 925.5 kJ/mol and Δ H ₂₉₈ = 81.6 kJ/mol. The MP2, QCISD, and CCSD methods yield reasonable results when used with larger basis sets. The G2 method as well as the G2-mimics G2(MP2) and G3 also yield reasonable results. The density functional methods performed significantly worse than the MP2(full), QCISD, and CCSD methods. The structures of E - and Z -HONO, of the electrostatic complex H ₂ ONO ⁺ , and of the dissociation products also are discussed. Effects of aqueous solvation were examined with the IPCM method at DFT, MP2, and QCISD levels with the 6-311++G(2df,2p) basis set. Only the proton-catalyzed heterolysis plays a role at ambient temperatures, and our results suggests different mechanisms for NO ⁺ formation in gas-phase and in solution.