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
✍ Scribed by Volker Jonas; Gernot Frenking
- Publisher
- Elsevier Science
- Year
- 1991
- Tongue
- English
- Weight
- 761 KB
- Volume
- 177
- Category
- Article
- ISSN
- 0009-2614
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✦ Synopsis
Ab initio quantum-mechanical calculations at the MP2/6-31G(2d)//MP2/6-31G(d) level of theory predict that the tram (la) conformation of chlorocarbonyl isocyanate ClC(O)NCO is 0.73 kcal/mol lower in energy than the cis (lb) isomer. This is in excellent agreement with experimental results obtained by gas-phase infrared and electron-diffraction experiments which show that la is 0.6 (3) kcal/mol more stable than lb. The addition of a second set of d-type polarization functions is found to be crucially important for calculating the correct stability order la> lb. 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,Zdithiete (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 tram isomer 2s. .4t MP2/6-3IG(3df), 3 is 5.36 kcal/mol marestable 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.