On the crucial importance of polarization functions for the calculation of molecules with third-row elements: the conformations of chlorocarbonyl isocyanate CIC(O)NCO and the equilibrium of 1,2-dithioglyoxal with its cyclic isomer 1,2-dithiete

Ab initio quantum-mechanical calculations at the MP2/6-31G(2d)//MP2/6-31G(d) level of theory predict that the trans (1a) conformation of chlorocarbonyl isocyanate CIC(O)NCO is 0.73 kcal/mol lower in energy than the cis (1b) isomer. This is in excellent agreement with experimental results obtained by gas-phase infrared and electron-diffraction experiments which show that 1a is 0.6(3) kcal/mol more stable than 1b. The addition of a second set of d-type polarization functions is found to be crucially important for calculating the correct stability order 1a1b. An even more dramatic effect of the addition of more polarization functions on the computed energy difference is found for 1,2-dithioglyoxal (2) and its cyclic valence isomer 1,2-dithiete (3). With standard values of the exponents, 3 is predicted at MP2/6-31G(3d) to be 3.40 kcal/mol less stable than the trans isomer 2a. At MP2/6-31G(3df), 3 is 5.36 kcal/mol more stable than 2a. A second set of f-functions leads to a predicted higher stability of 3 by 8.48 kcal/mol (MP2/6-31G(2d2f)). Our best estimate, which includes zero-point corrections, is that 3 is 6.3 kcal/mol more stable than 2a. The calculated higher thermodynamical stability of 3 is in agreement with experimental results. The importance of adding a sufficient number of polarization functions is emphasized.