Rotational barrier in 3-butynal and 3-butynoyl fluoride and chloride based on ab initio calculations

The structure and conformational stability of 3-butynal and 3-butynoyl fluoride and chloride were investigated using ab initio calculations. The 3-21G (or 3-21G* for the chloride) and 6-31G* (or 6-311G* for the aldehyde) basis sets at both RHF and MP2 levels were employed. For 3-butynal, the trans conformer (acetylene group trans with respect to the carbonyl group) was predicted to be slightly more stable than the cis conformer. For 3-butynoyl halides, the cis conformer was predicted to be the thermodynamically preferred conformation by all basis sets with the gauche conformation being the high-energy form. Full optimization was performed at the transition states and the barrier to internal rotation was calculated for the three molecules. The inclusion of electron correlation in the calculations was shown to be important in the case of the aldehyde only.