VIBRONIC SPECTRA AND S(1) EXCITED ELECTRONIC-STATE MOLECULAR-STRUCTURES FOR ACETYL-CHLORIDE AND ACETYL FLUORIDE

Vapor-state absorption spectra have been recorded for acetyl fluoride and acetyl chloride and also for deuterated derivatives with path leng ths up to 40 m. The origins of the S1 <-- S0 transitions have been der ived, together with the torsional-vibration energy levels in the groun d state S0 and excited singlet state S1. Fitting the calculated and ob served rotational contours of the vibronic bands has been used to esti mate the geometrical parameters in the S1 states. The carbonyl groups in the S1 states are nonplanar. The internal-rotation potentials have been determined for acetyl fluoride and acetyl chloride in the S1 and S0 states. The relative intensities of the torsional transitions in th ose states indicate that the minima in the potential energy are apprec iably displaced along the torsional coordinate in the S0 and S1 states .