Molecular structure, spectral constants, and fermi resonances in chlorine nitrate

Chlorine nitrate has two low-lying vibrational modes that lead to a series of Fermi resonances in the 99 77 family of levels that include the 9 , 7 and 9 , 79 dyads and the 9 , 97 , 7 and 9 , 97 , 97 triads. These states, along with the ground and 9 vibrational states, have been previously analyzed with millimeter and submillimeter wave spectroscopy and provide a substantial body of data for the investigation of these resonances and their impact on calculated spectroscopic constants and structural parameters. Due to fitting indeterminacies, these previous analyses did not include the main Fermi resonance interaction term. Consequently, the fitted rotational constants are linear combinations of the unmixed rotational constants of the basis vibrational states. In this paper, we have calculated the contributions of the Fermi resonances to the observed rotational constants in a model that determines the vibrational–rotational constants, the Fermi term and the mixing between interacting vibrational states, the cubic potential constant ðf997Þ that connects interacting levels through a Fermi resonance, and the inertial defects. These results agree with predictions from ab initio and harmonic force field calculations and provide further experimental information for the determination of the fundamental molecular properties of chlorine nitrate. q 2003 Elsevier B.V. All rights reserved.