NH3 trapped in nitrogen and rare gas matrices. II. High-resolution spectroscopy of ν2 and calculation of the inversion-translation doubling and proper rotation quadrupling

Abstract

Listen

Translate article

Abstract The high-resolution IR spectrum of the ν 2 absorption band of NH 3 embedded in solid N 2 at 5.5 K exhibits a quadruplet structure. This structure includes the previously mentioned inversion doublet while the additional splitting shows striking nuclear spin species conversion over a very large timescale. The inversion doubling, ⋍ 1.65 cm −1 , is considerably smaller than in rare gas matrices (⋍ 24 cm −1 ) and in the gas phase (37 cm −1 ). The temperature dependence of the quadruplet frequencies shows in N 2 a larger blue shift than is usually expected and a typical motional narrowing for the doublet structure in the range 8–17 K. The a priori determination of the motions of NH 3 around the equilibrium configurations of the potential surface described in the first paper of this series, shows that the strong coupling between intrinsic inversion and translational space inversion is responsible for the doubling decrease. Such a feature is due to the large equilibrium eccentricity in a N 2 matrix. As this eccentricity is much smaller in rare gas matrices, the coupling is much weaker and the spacing closer to the gas-phase value. The quadruplet structure is due to the vibrational dependence of the hindered proper rotational (spinning) motion in the three-fold wells, characteristically coupled to the nuclear spin species. All numerical predictions are in agreement with experimental measurement.