ACCOUNTING FOR THE TORSIONAL SPLITTING IN THE NU-(5) AND 2-NU-(9) BANDS OF NITRIC-ACID

In an attempt to account for the large-amplitude torsional v(y) mode o f nitric acid, a formalism is set up, based on an internal-axis-method -like approach previously developed for high-barrier tunneling problem s. This formalism is applied to the calculation of the tunneling-rotat ional-vibrational energy of the interacting v(9) = 2 and v(5) = 1 vibr ational states and accounts for the relational dependence of the tunne ling splitting and for the strong Coriolis and Fermi resonances betwee n these two vibrational states. With the help of this formalism, it is shown that the effects of the large-amplitude motion may be very diff erent depending on the way the rotational states are clustered. For la rge K-a values, shifts of about 0.001 cm(-1) are predicted, while spli ttings of roughly the same value occur for large K-c values. This form alism is used to carry out a simultaneous analysis of the microwave da ta relevant to the two vibrational states (Goyette et al., J. Mol. iol . Spectrosc. 167, 365-374. 1994) and of the infrared transitions belon ging to the 2v(9) and v(5) bands (Perrin et al., J. Mol. Spectrosc. 15 7, 112-1211 1993). The frequency of the microwave transitions and the wavenumber of the infrared transitions are satisfactorily reproduced a nd it is possible to point out the effects of the large-amplitude moti on in the latter data set. (C) 1995 Academic Press, Inc.