Symmetry breaking and the molecular structure of NO3+

The equilibrium structure and vibrational frequencies of the nitrate cation , NO3+, have been investigated with an extensive set of ab initio calculati ons. Two stationary points were identified on the NO3+ potential energy sur face: a symmetric D-3h structure and a C-2 upsilon ring structure similar t o that found for the isoelectronic CO3 molecule. Geometry optimizations exe cuted at the CCSD(T)/aug-cc-pVTZ level of theory yielded the following data . NO3+(D-3h): E-rel = 2130 cm(-1), r(e)= 1.238 Angstrom. NO3+(C-2 upsilon): E-rel = 0 cm(-1), r(1) = 1.131 Angstrom, r(2) = r(3) = 1.309 Angstrom, the ta = 142.3 degrees. Calculations performed at the B3LYP, QCISD, CCSD, and C CSD(T) levels of theory all predict the C-2 upsilon, structure to be lower in energy than the D3h structure, Relative energy calculations performed wi th the Gaussian and complete basis set model chemistry algorithms also pred ict the C-2 upsilon structure to be the most stable NO3+ conformation. Thes e results are supported by vibrational frequency calculations which suggest that the D-3h structure may correspond to a second-order saddle point rath er than a true minimum on the NO3+ potential energy hypersurface. The symme try breaking observed in the present NO3+ calculations is similar to that o bserved in ab initio studies of the NO3 equilibrium structure and is used t o examine symmetry breaking across the nitrate series NO3-, NO3, NO3+.