The Heterolytic Dissociation of Neutral and Protonated Nitrous Acid

Structures, energies, and vibrational properties of HONO and H2ONO+ 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
ΔE0Pref = 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
ΔE0Pref = 1.83 kJ/mol. The experimental heterolytic bond dissociation energy of ΔE0 = 917.80 kJ/mol for
HONO and of ΔH298 = 77.3 kJ/mol for H2ONO+ are more difficult to reproduce. At the CCSD(T)/6-311++G(2df,2p) level, we found ΔE0 = 925.5 kJ/mol and ΔH298 = 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 H2ONO+, 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.