Symmetry Breaking and the Molecular Structure of NO₃⁺

Abstract

The equilibrium structure and vibrational frequencies of the nitrate cation, NO 3 + , have been investigated with an extensive set of ab initio calculations. Two stationary points were identified on the NO 3 + potential energy surface: a symmetric D 3h structure and a C 2V ring structure similar to that found for the isoelectronic CO 3 molecule. Geometry optimizations executed at the CCSD(T)/aug-cc-pVTZ level of theory yielded the following data. NO 3. Calculations performed at the B3LYP, QCISD, CCSD, and CCSD(T) levels of theory all predict the C 2V structure to be lower in energy than the D 3h structure. Relative energy calculations performed with the Gaussian and complete basis set model chemistry algorithms also predict the C 2V structure to be the most stable NO 3 + conformation. These results are supported by vibrational frequency calculations which suggest that the D 3h structure may correspond to a second-order saddle point rather than a true minimum on the NO 3 + potential energy hypersurface. The symmetry breaking observed in the present NO 3 + calculations is similar to that observed in ab initio studies of the NO 3 equilibrium structure and is used to examine symmetry breaking across the nitrate series NO 3 -, NO 3 , NO 3 + . † Part of the special issue "Harold Johnston Festschrift".

Charged and Neutral NO3 Isomers from the Ionization of NOx and O3 Mixtures

“…Consistent with previous results, [6] among the species characterized by three N À O bonds, our calculations identify two trigonal NO 3 ( 1 A 1 ) ions, the one of C 2v symmetry (7) being the most stable isomer, slightly lower in energy than 8, which has D 3h symmetry (Figure 7). The 3 B 2 species 9 lies only 14.5 kcal mol À1 higher in energy than 8 at the CCSD(T) level, consistent with the experimental value (see Table 1).…”

“…[11,12] Moreover, the remarkably low dissociation energy of ion 10, 2.9 kcal mol À1 , must be considered. Consistent with this picture, the NO 3 ion is very weak in the NO/O 3 CI plasma: the minor amount observed being traceable to the alternative reaction (6) with O 2 , a minor process that, at low O 2 pressures, does not significantly compete with the reaction (2) of NO with O 3 .…”

“…Finally, let us examine the NO 3 ions of O 2 -NO connectivity from reaction (6). The NO /O 2 BE is too low to account for the stabilization of ion 10 from ground state NO and O 2 species (Table 1).…”

“…The NO /O 2 BE is too low to account for the stabilization of ion 10 from ground state NO and O 2 species (Table 1). On the other hand, reactions other than (6) are unlikely to occur in the NO/O 2 plasma, as the weak (< 5 %) NO 2 ion present in the source undergoes no reaction with O 2 . As to the 11 and 12 singlets, their BE with respect to the dissociation into NO and the first O 2 singlet state (a 1 D g ) are expected to be close to that of the ground-state adducts, since the energy separation between O 2 (a 1 D g ) and the ground O 2 (X 3 S g À ), 22.6 kcal mol À1 , [15] closely approaches the computed energy difference between 10 and 11.…”

“…Indeed, despite its importance, to the best of our knowledge, no experimental studies of the NO 3 ion have been reported, and the theoretical analysis of its equilibrium structure and of the relative stability of its isomers is seriously affected by symmetry-breaking effects. [6] The same applies to the NO 3 . radical, the major tropospheric night-time oxidant, whose theoretical study is still the focus of active interest.…”