AN AB-INITIO AND EXPERIMENTAL-STUDY OF THE HARMONIC FORCE-FIELD OF DIACETYLENE

An ab initio harmonic force field for diacetylene has been determined using a 6-311G(d,p) basis set with electron correlation included at the MP2 level. The sensitivity of the computed vibrational frequencies to details of the polarisation functions employed in this basis set has been tested both for diacetylene itself and acetylene. It is found that computed vibrational frequencies display considerable sensitivity to the inclusion and detailed nature of polarisation functions and that, as other authors have noted, the inclusion of f functions for such molecules can be crucial to obtaining acceptable comparison with experiment. The experimental harmonic force field of diacetylene has also been re-investigated using published data for the normal and deuterated species yielding a force field which is more consistent with ab initio results than earlier force fields for diacetylene.