Rotational spectrum of CH3C13CCCH: Determination of the equilibrium structure of methyldiacetylene from microwave spectroscopy and ab initio calculations

The rotational spectrum of CH3C13CCCH, observed in natural abundance, has been recorded and assigned for the first time. The present investigation, carried out in the millimeter- and submillimeter-wave frequency region, allows us to provide accurate ground state rotational and centrifugal distortion constants. The anharmonic force field of methyldiacetylene has been calculated at the MP2 level of theory employing a basis set of triple-zeta quality. Semi-experimental equilibrium structure has then been derived from the experimental ground-state rotational constants available for various isotopologues and the corresponding vibrational corrections calculated from the ab initio force field. This structure has been found in good agreement with pure theoretical best estimates of equilibrium geometry as well as with previous experimental structures, namely r0, rs, and rm.