The ν1 and ν2 + ν4 bands in the infrared spectrum of HN3

Abstract

The ν 1 fundamental (3339 cm -1) and the ν 2 + ν 4 combination band (3251 cm -1) in the infrared spectrum of NH 3 have been analyzed from high-resolution Fourier transform spectra. Extensive perturbations in ν 2 + ν 4 have been assigned to interactions with ν 2 + ν 5 + ν 6 (A″) on the basis of detailed intensity considerations: it is found that the matrix element between the two vibrational states is roughly proportional to K, which indicates an a-axis Coriolis mechanism. Although the ν 2 + ν 5 + ν 6 level has A″ symmetry it gives rise to an apparently normal A0type band (except for the absence of K = 0), where even the K = 1 asymmetry splitting is reversed so that it is in the sense expected for an A′ level. The reason for this reversal is shown to be the unusually strong Coriolis interaction between ν 5 and ν 6, caused by the large A-rotational constant: the central member of a Coriolis triad such as [2ν 5(A′), ν 5 + ν 6(A″), and 2ν 6(A′)] becomes so distorted that its rotational structure resembles that of an A′ level. Data from the B-type component of ν 1 have been used to derive a highly accurate set of effective ground state constants for K ≤ 7; these however contain various high-order centrifugal distortion terms because they do not explicitly consider the centrifugal distortion resonance that affects the very high K″ levels. © 1988.