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 th e MP2 level. The sensitivity of the computed vibrational frequencies t o details of the polarisation functions employed in this basis set has been tested both for diacetylene itself and acetylene. It is found th at computed vibrational frequencies display considerable sensitivity t o the inclusion and detailed nature of polarisation functions and that , as other authors have noted, the inclusion of f functions for such m olecules can be crucial to obtaining acceptable comparison with experi ment. The experimental harmonic force field of diacetylene has also be en 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.