THE 3RD AND 4TH TORSIONAL STATES OF ACETALDEHYDE

At least-squares fit is presented of 1105 far-infrared and 2860 microw ave transitions in acetaldehyde, which sample rotational levels in all torsional states below the lowest-frequency small-amplitude vibration v(10). Four-fifths of these transitions, involving torsional states b elow the barrier (upsilon(t) less than or equal to 2), were treated in an earlier publication; here the theoretical model is further tested by extending the data set to include the upsilon(t) = 3 and 4 states a bove the barrier. The new data set includes (i) 224 upsilon(t) = 3 <-- 2 far-infrared transitions, (ii) Nizhny Novgorod RAD-2 a-typed and b- type submillimeter transitions in upsilon(t) = 2, which make more prec ise Delta K = +/-1 intervals in this last torsional state below the ba rrier, (iii) RAD-3 a-type submillimeter transitions in upsilon(t) = 3 and 4, and (iv) microwave and submillimeter measurements or remeasurem ents from NIST and Lille of transitions with upsilon(t) less than or e qual to 4. The global fit, which uses an improved version of the compu ter program previously applied to the upsilon(t) less than or equal to 2 data, gave a weighted overall standard deviation of 1.21 with 55 ad justed and 2 fixed parameters. Residuals from the fit were close to ex perimental uncertainties for the infrared wavenumber measurements and for microwave and submillimeter frequency measurements below the barri er, but some residuals reached several MHz or more for frequency measu rements involving levels near upsilon(10). We have attempted to achiev e a qualitative understanding of the numerical results by presenting a general overview of internal rotation energy levels above the barrier and a theoretical discussion of torsion-rotation interactions among t hese levels. (C) 1996 Academic Press, Inc.