How reliable are ab initio calculations?: The structure and conformation of chlorocarbonyl isocyanate (CIC(O)NCO) revisited

Abstract

The theoretical analysis of the conformations ofchlorocarbonyl isocyanate was reinvestigated. Our results show that ab initio molecular orbital (MO) calculations are reliable in determining the cis-trans energy difference CL C(O)NCO (the trans conformer being the more stable), in agreement with experiment. We thus answer positively the recently posed question, “How reliable are ab initio calculations?” (H.-G. Mack, H. Oberhammer and C.O. Della Vedova, J. Mol. Struct., (Theochem), 200 (1989) 277). The failure of the calculations by Mack et al. to reproduce the cis-trans separation gap might be due to the geometrical parameters employed in their work. (In the present work, we employed the MP2/6-31G( d,p) optimized geometries.) We extended the scope of the problem by carrying out similar calculations for HC(O)NCO and FC(O)NCO. The calculated geometries were improved by using a correction procedure in order to predict the corresponding r g structures. In the two latter species, the cis conformers are predicted to have slightly higher stabilities than the trans conformers.