Department of Biology, Chemistry and Pharmacy, Institute of Chemistry and Biochemistry, Freie Universität Berlin, Fabeckstr. 36a, D-14195 Berlin, Germany.
J Phys Chem A. 2012 Jun 28;116(25):6885-93. doi: 10.1021/jp302569g. Epub 2012 Jun 15.
Gas-phase energies of 36 tautomer/isomer pairs of 18 six-membered N-heterocyclic compounds were computed quantum chemically. Among the considered B3LYP, BH&HLYP, BH&HLYP(G), and PW6B95 DFT functionals, the latter two provide accurate tautomer/isomer pair energies with root-mean-square deviations (rmsd) relative to experiments of 0.2 and 0.3 kcal/mol, respectively. Since only few (namely five) experimental data are available, 15 tautomer/isomer pair energies were computed with the very precise QCISD(T)(quadruple-ζ) method serving as reference. Relative to this reference the PW6B95 DFT functional is slightly superior to the BH&HLYP(G) functional, yielding an rmsd of 0.7 and 0.8 kcal/mol, respectively. In contrast to BH&HLYP(G), the PW6B95 DFT functional yields also accurate tautomer/isomer pair energies if zwitterionic structures are involved. The tautomer/isomer pair states possess different amounts of aromaticity. This is characterized by nucleus-independent chemical shift (NICS) values. The tautomer/isomer pair reference energies, from which the energies computed with PW6B95 are subtracted, correlate linearly with the corresponding differences in the NICS values. This correlation is used to construct a correction term for the pair energies computed with PW6B95, yielding tautomer/isomer pair energies with rmsd of 0.3 kcal/mol with respect to the more CPU time demanding QCISD(T)(quadruple-ζ) method.
气相中 36 个六元氮杂环化合物的互变异构/异构体对的能量进行了量子化学计算。在所考虑的 B3LYP、BH&HLYP、BH&HLYP(G)和 PW6B95DFT 函数中，后两个函数提供了准确的互变异构/异构体对能量，其相对实验的均方根偏差（rmsd）分别为 0.2 和 0.3kcal/mol。由于仅有少数（即 5 个）实验数据可用，因此使用非常精确的 QCISD(T)（四重-ζ）方法计算了 15 个互变异构/异构体对的能量作为参考。相对于这个参考，PW6B95DFT 函数略优于 BH&HLYP(G)函数，分别产生 0.7 和 0.8kcal/mol 的 rmsd。与 BH&HLYP(G)不同的是，如果涉及两性离子结构，PW6B95DFT 函数也能产生准确的互变异构/异构体对能量。互变异构/异构体对态具有不同程度的芳香性。这由核独立化学位移（NICS）值来表征。互变异构/异构体对参考能量减去用 PW6B95 计算的能量，与相应的 NICS 值差异呈线性相关。此相关性用于构建 PW6B95 计算的对能量的校正项，从而产生与更耗 CPU 时间的 QCISD(T)(四重-ζ)方法相比 rmsd 为 0.3kcal/mol 的互变异构/异构体对能量。
