The Third and Fourth Excited Torsional States of Acetic Acid

We present the first study of the millimeter and submillimeter wave spectra of the third and fourth excited torsional states of acetic acid (CH3COOH). New measurements have been carried out in the 149-183 GHz and 255-420 GHz ranges using the millimeter wave spectrometer in the Institute of Radio Astronomy of NASU. The new data were combined with previously published measurements and fitted using the rho-axis-method torsion-rotation Hamiltonian and RAM36 code. The current fit uses 109 parameters to give an overall weighted root-mean-square deviation of 2.1 for a dataset consisting of 11463, 9298, 7725, 2663, and 1243 transitions belonging, respectively, to the ground, first, second, third, and fourth excited torsional states of CH3COOH. The new dataset also contains numerous intertorsional transitions (2407) that are observed due to intensity borrowing via avoided crossing interactions that provide direct measure of torsional energy spacings associated with torsional potential barrier. The search for the optimal set of parameters is in progress.