ON THE CRUCIAL IMPORTANCE OF POLARIZATION FUNCTIONS FOR THE CALCULATION OF MOLECULES WITH 3RD-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 1a > 1b. 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 for 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.