The band spectrum of propynal at 4140 Å: Calculation of the structure from the Franck-Condon principle

The in-plane and out-of-plane vibrational modes in the ³A″ and ¹A′ electronic states of propynal were subjected to a normal coordinate treatment in the approximation of the Urey-Bradley force field. From the relative oscillator strengths of the transitions connecting the vibrationless ¹A′ state and the vibronic levels of the ³A″ state, the differences in equilibrium configuration were evaluated by an approximate Franck-Condon analysis based on the ground state normal coordinates. As this treatment gave 512 possible geometrical structures for the upper state, it was necessary to resort to a comparison of the observed and calculated moments of inertia along with chemical intuition to isolate the structure. A test of the correctness of the calculated structural change and the vibrational assignment was made by evaluating the intensities of the in-plane and out-of-plane fundamental, sequence, and cross sequence transitions by the exact Franck-Condon method.