How to Compute Isomerization Energies of Organic Molecules with Quantum Chemical Methods

Theoretische Organische Chemie, Organisch-Chemisches Institut der Universität Münster, Corrensstrasse 40, D-48149 Münster, Germany.
The Journal of Organic Chemistry (Impact Factor: 4.72). 03/2007; 72(6):2118-26. DOI: 10.1021/jo062446p
Source: PubMed

ABSTRACT The reaction energies for 34 typical organic isomerizations including oxygen and nitrogen heteroatoms are investigated with modern quantum chemical methods that have the perspective of also being applicable to large systems. The experimental reaction enthalpies are corrected for vibrational and thermal effects, and the thus derived "experimental" reaction energies are compared to corresponding theoretical data. A series of standard AO basis sets in combination with second-order perturbation theory (MP2, SCS-MP2), conventional density functionals (e.g., PBE, TPSS, B3-LYP, MPW1K, BMK), and new perturbative functionals (B2-PLYP, mPW2-PLYP) are tested. In three cases, obvious errors of the experimental values could be detected, and accurate coupled-cluster [CCSD(T)] reference values have been used instead. It is found that only triple-zeta quality AO basis sets provide results close enough to the basis set limit and that sets like the popular 6-31G(d) should be avoided in accurate work. Augmentation of small basis sets with diffuse functions has a notable effect in B3-LYP calculations that is attributed to intramolecular basis set superposition error and covers basic deficiencies of the functional. The new methods based on perturbation theory (SCS-MP2, X2-PLYP) are found to be clearly superior to many other approaches; that is, they provide mean absolute deviations of less than 1.2 kcal mol-1 and only a few (<10%) outliers. The best performance in the group of conventional functionals is found for the highly parametrized BMK hybrid meta-GGA. Contrary to accepted opinion, hybrid density functionals offer no real advantage over simple GGAs. For reasonably large AO basis sets, results of poor quality are obtained with the popular B3-LYP functional that cannot be recommended for thermochemical applications in organic chemistry. The results of this study are complementary to often used benchmarks based on atomization energies and should guide chemists in their search for accurate and efficient computational thermochemistry methods.

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    • "The relative free energy of this TS represents the proton migration activation energy, and was found to have an energy of +41.6 kJ/mol relative to 2-aminoethanolate (confirmed by intrinsic reaction coordinate (IRC) calculations). DFT occasionally performs poorly for barrier heights [53], so coupled cluster calculations with perturbative triples corrections and an augmented correlation-consistent triple-ζ basis set [CCSD(T)/aug-cc-pVTZ//M06-2X/6-311++G(d,p)] were used to determine the single point energies for the three participating structures, i.e., the two minima and the connecting TS. The relative free energies at the two levels of theory are presented in Table 2 for comparison. "
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