The Infrared and Ultraviolet Spectra of Propynal

The infrared and near ultraviolet vapour spectra of three isotopic forms HCCCHO, DCCCHO and HCCCDO of propynal (proplol-aldehyde) have been examined under medium resolution, and the 3820 A ultraviolet band system of these three molecules has been examined under high resolution. On the basis of the infrared and high-resolution ultraviolet measurements, supplemented by liquid-phase infrared spectra, a complete set of fundamental vibrational frequencies of the ground electronic state is proposed, and the isotope effects are shown to be consistent with the Teller-Redlich product rule, using the rotational constants found from the microwave spectrum. The fundamental frequencies associated with the stretching of the various bonds agree with those observed for related molecules, but the frequencies of deformation of the bond angles present some unusual features. Information has also been obtained on the interaction of the vibrational motions with rotation about the axis of least inertia. Four band systems have been found in the ultraviolet spectrum to long waves of 2000 A, and these are described and their possible assignments to electronic transitions are discussed. A detailed vibrational and partial rotational analysis of the 3820 A band system is presented, leading to a set of fundamental vibrational frequencies of the excited state, which lacks only one fundamental frequency for each isotope, although some of the proposed values are tentative. The direction of the electronic transition moment and the changes in the frequencies of vibration of the formyl group show that this transition is the analogue of the 3530 A transition of formaldehyde. However, it is concluded that nothing has been observed here which is inconsistent with the present excited state being planar at equilibrium, in contradistinction to the non-planarity found for excited formaldehyde; but the evidence here presented is probably insufficient to preclude the presence of a potential-energy barrier of at most 100 cm-1 at the planar configuration. In addition to the changes in the formyl frequencies, drastic changes are found to take place, as a consequence of the electronic excitation, in the frequencies of vibrational modes localised in the ethynyl group; the excitation cannot therefore be regarded as localised within the formyl group. A feature of the 3820 A system is the presence of bands whose vibronic transition moments lie along each of the three principal axes of inertia. The possible meaning of this in terms of vibronic interaction is discussed. Perturbations of various types have been observed. Purely vibrational interactions of Fermi type are common in the 3820 A system; and some regular Coriolis perturbations of the sub-band structure have also been found. Two striking examples of irregular perturbations - one of them perhaps involving predis sociation - in the sub-band structure of bands of HCCCDO are described, and their possible causes are examined