Barrier to internal rotation and structural stability of cyanoacetaldehyde and cyanoacetyl halides

The structure and conformational stability of cyanoacetaldehyde, cyanoacetyl fluoride and chloride, were investigated using ab initio calculations with the 3-21G, 3-21G^* and 6-31G^* basis sets. For cyanoacetaldehyde the cis form (cyano group is cis with respect to the aldehydic hydrogen) is predicted by the 6-31G^* basis sets to be slightly more stable than the trans conformer. For cyanoacetyl halides, the trans is predicted to be thermodynamically the preferred conformation by all basis sets. Only for cyanoacetyl chloride was the gauche conformation predicted to be the high energy form. Full optimization was performed at the transition states and the barriers to internal rotation were calculated. For the aldehyde, the cis to trans barrier was calculated to be 2679 cal mol^-1 at the 6-31G^* basis set. For the fluoride the trans to cis barrier was calculated to be 4345 cal mol^-1 and 2353 cal mol^-1, while for the chloride, the trans to gauche barrier was calculated to be 3788 cal mol^-1 and 2576 cal mol^-1 at the 3-21G (or 3-21G^* for the chloride) and 6-31G^* basis sets respectively.