Microwave Spectrum and Structure of Nitric Acid

Abstract

The microwave spectra of five further isotopic species of nitric acid, H¹⁸ONO₂, cis‐HON¹⁸OO, trans‐HONO¹⁸O, cis‐HO¹⁵N¹⁸OO, and D¹⁵NO₃, have been investigated. The data have been used to refine the previous structural determination of Millen and Morton. In the calculation of structure, particular consideration has been given to the use of ground‐state moments of inertia in Kraitchman's equations for near‐oblate planar molecules. The structure was calculated using several different methods, including the double‐substitution method due to Pierce, in order to substantiate the following structural parameters: O–H=0.96₄, (H)O–N=1.40₆, cis–N–O=1.21₁, trans‐N–O=1.19₉ Å, ∠HON=102°9′, ∠cis‐ONO=115°53′, ∠trans‐ONO=113°51′, and O···O (NO₂ group)=2.18₄ Å. This determination clearly demonstrates a 2° tilt of the NO₂ group away from the hydrogen atom. The NO₂ parameters are close to those found in a number of related molecules, but there is some evidence that the cis‐NO bond in HNO₃ is slightly longer than the trans‐NO bond. The NOH configuration is very similar to that found in formaldoxime. Further evidence for the exact planarity of nitric acid is given. Stark‐effect measurements limit the out‐of‐plane component of the dipole moment μ_C to a value less than 1.1×10⁻³ D. The other components have been redetermined to have the values μ_A=1.99 D and μ_B=0.88 D, giving a total dipole moment μ=2.17±0.02 D.