Microwave spectrum, structure, dipole moment and quadrupole coupling constants of cis and trans nitrous acids

Abstract

Microwave spectra for eight isotopic species of cis and trans nitrous acids are reported. The planarity of both isomers has been accurately established on the basis of the inertial defects. Accurate structures, electric dipole moments and quadrupole coupling constants at nitrogen have been determined for both isomers. The structure of cis nitrous acid is found to be: O—H = 0.982, N—O(H)= 1.392, N—O = 1.185 Å, ∠ NOH = 104.0 and ∠ ONO = 113.6°, and for trans nitrous acid: O—H = 0.958, N—O(H)= 1.432, N—O = 1.170 Å, ∠ NOH = 102.1 and ∠ ONO = 110.7°. Stark effect measurements yield the following values for the components of the dipole moment for cis nitrous acid: µ_a= 0.306, µ_b= 1.389 and the total dipole moment µ= 1.423±0.005 D. Previous investigations of the Stark effect in the trans isomer have been refined to give µ_a= 1.378, µ_b= 1.242 and µ= 1.855±0.016 D. Orientation of the dipole moment has been established for both isomers from isotopic measurements, despite a moderately large vibrational change in µ_a for trans DNO₂. Quadrupole coupling constants in the principal inertial axis system have been determined for cis nitrous acid to be: χ_aa= 2.05, χ_bb=–5.83 and χ_cc= 3.78 MHz, and for trans nitrous acid: χ_aa= 1.73, χ_bb=–5.28 and χ_cc= 3.55 MHz.

The structural parameters are markedly different in the two isomers and have been discussed, together with the torsional barrier and relative stability of the isomers, in terms of a cis interaction. The structures and charge properties of the nitrous acids are compared with a range of related compounds.