Centrifugal distortion in the microwave spectra of cis- and trans-nitrous acids. Determination of the quadratic potential functions and average structures

Abstract

Centrifugal distortion shifts in the microwave spectra of cis and trans nitrous acids and their deuterated species have been analyzed using the first-order treatments of Hill and Edwards, and Watson. The two methods are in close agreement for the ground vibrational state of planar molecules. Centrifugal distortion constants thus derived have been used, together with the infra-red fundamentals and their isotopic shifts, to determine the quadratic potential function for each isomer. Eleven out of a possible sixteen force constants have been determined as follows (in mdyn Å^–1): for cis nitrous acid, ƒ₁₁(OH)= 6.55, ƒ₂₂(NO)= 11.18, ƒ₃₃(N—O)= 2.73, ƒ₄₄(NOH)= 0.88, ƒ₅₅(ONO)= 2.46, ƒ₂₃= 1.80, ƒ₂₄= 0.22, ƒ₃₄= 0.53, ƒ₃₅= 0.35, ƒ₄₅= 0.18 and ƒ₆₆(torsion)= 0.167; and for trans nitrous acid, ƒ₁₁= 7.18, ƒ₂₂= 12.28, ƒ₃₃= 2.22, ƒ₄₄= 0.76, ƒ₅₅= 2.45, ƒ₂₃= 1.67, ƒ₂₄= 0.12, ƒ₃₄= 0.04, ƒ₃₅= 0.41, ƒ₄₅= 0.14 and ƒ₆₆= 0.131.

Using this potential function, average moments of inertia have been evaluated for cis and trans nitrous acid and their isotopic species allowing structures of the average configurations to be determined: for cis nitrous acid, O—H = 0.989, N—O(H)= 1.399, NO = 1.186 Å, ∠ NOH = 103.9 and ∠ ONO = 113.6°, and for trans nitrous acid, O—H = 0.959, N—O(H)= 1.442, NO = 1.169 Å, ∠ NOH = 102.1 and ∠ ONO = 110.6°. The near-zero inertial defects for the average moments of inertia show both isomers to be accurately planar in their average configurations. An equilibrium structure has been estimated for trans nitrous acid on the basis of a model cubic potential function.

Differences in the harmonic force fields of the two isomers substantiate the marked difference in their structural parameters and have been taken as evidence for attraction between hydrogen and cis oxygen. The force fields and structures of the nitrous acids are compared with those of a range of related compounds.