The Rotational Spectra of Propyne in the Ground, V(10) = 1, V(10) = 2, and V(9) = 1 Vibrational States

The problem of a vibrating-rotating polyatomic molecule is treated, with emphasis given to the case of molecules with C(,3V) sym- metry. It is shown that several of the gross features of the rotational spectra of polyatomic molecules in excited vibrational states can be predicted by group theoretical considerations. Expressions for the rotational transition frequencies of molecules of C(,3V) symmetry in the ground vibrational state, singly excited degenerate vibrational states, and doubly excited degenerate vibrational states are given. The deri- vation of these expressions by fourth order perturbation theory as given by Amat, Nielsen, and Tarrago is discussed. The ground and V(,10) = 1 rotational spectra of propyne have been investigated in the 17 to 70 GHz, and 17 to 53 GHz regions, respec- tively, and compared with predictions based on higher frequency measurements. The V(,9) = 1 and V(,10) = 2 rotational spectra of propyne have been investigated and assigned for the first time. A perturbation of the V(,9) = 1 rotational spectra for K = -L has been discovered and discussed. Methods of assignment for excited vibrational state spectra are discussed in detail. The methods of assignment given here allow the analysis of spectra where only a relatively small number of experi- mental points are available. A discrepancy has been found in the interpretation of recently reported infrared results on the rotational spectra of propyne in the V(,10) = 1, 2, 3, and 4 vibrational states. The constants of anharmoni- city which describe the change of the rotational constant, B, with the level of excitation of the V(,10) vibrational mode are rederived from microwave data. It is shown that microwave measurements increase the accuracy of one of these constants by over an order of magnitude.