Ligand substitution: An assessment of the reliability of
ab initio calculations
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Frankcombe, KE and Cavell, KJ and Yates, BF and Knott, RB, Ligand substitution: An assessment of the reliability of
ab initio calculations, The Journal of Physical Chemistry, 100, (47) pp. 18363-18370. ISSN 0022-3654 (1996) [Refereed Article]
Density functional methods and conventional ab initio molecular orbital methods have been compared in an investigation of the model ligand substitution reaction Pd(N-O)(CH3)(PH3) + CO → Pd(N-O)(CH3)(CO) + PH3 (N-O = NHCHCOO-). Full geometry optimizations were performed for each stationary point using a range of different basis sets, relativistic effective core potentials (RECPs), and levels of theory in order to ascertain a reliable and computationally inexpensive method for probing such reaction mechanisms. For geometries that compare well to experiment, electron correlation and double-ζ valence basis sets with d-type polarization functions on the main group elements are essential. Geometries determined using small-core RECPs and nonlocal density functional theory compared favorably with those produced at the second-order Møller-Plesset (MP2) level, and good agreement with available experimental data was observed. The density functional reaction energetics, however, were poor, apparently due to an unrealistically high Pd-CO binding energy relative to the Pd-PH3 binding energy. This error was reduced by the use of the Becke three-term hybrid exchange functional. The reaction barrier and reaction energy were predicted to be +15.0 kJ/mol and +5.1 kJ/mol, respectively, at the MP2 level with a medium basis set. Approximate large basis set CCSD(T) energetics were determined using an additivity scheme. This method predicted a stabilization of the 5-coordinate transition structure with respect to the 4-coordinate species, reducing the reaction barrier to +4.9 kJ/mol relative to the separated reactants. The associated reaction energy was +8.9 kJ/mol. © 1996 American Chemical Society.
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