Microwave spectrum of DNO₃, and average structures of nitric and nitrous acids

Abstract

The rotational spectrum of DNO₃ in the ground vibrational state has been investigated in the region 16–40 GHz and analysed to give rotational constants and centrifugal distortion constants. Microwave data for eight isotopomers of HNO₃ have been used to derive the zero-point average structure after the necessary vibrational corrections: r(OH) = 0.941(3) Å, r(NOH) =1.410(2) Å, r(NO)syn = 1.213(2) Å, r(NO)_anti = 1.198(2) Å, ∠HON =102.6(3)°, ∠ONO_syn = 115.7(2)°, ∠ONO_anti = 114.1(2)°. The tilt of the NO₂ group away from the H atom and the difference in the syn and anti NO bond lengths is clearly established in the average structure.

For comparison, zero-point average structures for the two conformers of nitrous acid have been calculated from microwave data for five isotopomers of trans-HONO, and three isotopomers of cis-HONO. For trans-HONO: r(OH) = 0.947(3) Å, r(NOH) = 1.441(2) Å, r(NO) = 1.173(2) Å, ∠HON = 102.1(3)°, ∠ONO = 110.5(2)°. For cis-HONO: r(OH) = 0.975(3) Å, r(NOH) = 1.397(6) Å, r(NO) = 1.190(5) Å, ∠HON = 104.4(4)°, ∠ONO = 113.5(3)°.

Inertial defect calculations demonstrate all three molecules to be strictly planar in the ground vibrational state. In each case for the parent and deuterated species, the inertial defect for the average configuration has been shown to be zero, after the appropriate electronic correction.