The rotational spectra of the Ï 8Â =Â Ï 9Â =Â 1 and Ï 6Â =Â Ï 7Â =Â 1 interacting vibrational states of nitric acid (HNO3)

The analysis of the rotational spectrum of HNO3 has been extended to include the υ8 = υ9 = 1 state at 1205.7 cm−1 and the υ6 = υ7 = 1 state at 1223.4 cm−1. Based on 78-519 GHz data, the assignments in the 8191 vibrational state have been significantly expanded from the previously reported microwave measurements [T.M. Goyette, F.C. De Lucia, J. Mol. Spectrosc. 139 (1990) 241-243]. A new microwave analysis is also reported for the 6171 vibrational state. A simultaneous analysis takes into account the localized ΔKa = ±2 Fermi resonances between the vibrational states, describes the torsional splitting of 3.3 and 1.4 MHz for the 8191 and 6171 states respectively, and fits to experimental accuracy over 1500 rotational transition frequencies that extend up to J = 59. Infrared energy levels [A. Perrin, J.-M. Flaud, F. Keller, A. Goldman, R. D. Blatherwick, F. J. Murcray, C. P. Rinsland, J. Mol. Spectrosc. 194 (1999) 113-123] were also included in the analysis and fit to experimental accuracy. Measurement of strongly perturbed transitions in each vibrational state provide a determination of the band origin difference of 17.733184(17) cm−1. The rotational constants agree well with those predicted by vibrational-rotational constants of the fundamental modes. Furthermore, the analysis will provide a very accurate simulation of the infrared spectrum of HNO3 in the 8.3 μm region.