Böhm Karl-Heinz, Banert Klaus, Auer Alexander A
Max-Planck-Institute for Chemical Energy Conversion, Stiftstraße 34-36, D-45470 Mülheim an der Ruhr, Germany.
Institut für Chemie, Technische Universität Chemnitz, Straße der Nationen 62, D-09111 Chemnitz, Germany.
Molecules. 2014 Apr 23;19(4):5301-12. doi: 10.3390/molecules19045301.
We present ab-initio calculations of secondary isotope effects on NMR chemical shieldings. The change of the NMR chemical shift of a certain nucleus that is observed if another nucleus is replaced by a different isotope can be calculated by computing vibrational corrections on the NMR parameters using electronic structure methods. We demonstrate that the accuracy of the computational results is sufficient to even distinguish different conformers. For this purpose, benchmark calculations for fluoro(2-2H)ethane in gauche and antiperiplanar conformation are carried out at the HF, MP2 and CCSD(T) level of theory using basis sets ranging from double- to quadruple-zeta quality. The methodology is applied to the secondary isotope shifts for 2-fluoronorbornane in order to resolve an ambiguity in the literature on the assignment of endo- and exo-2-fluoronorbornanes with deuterium substituents in endo-3 and exo-3 positions, also yielding insight into mechanistic details of the corresponding synthesis.
我们展示了对核磁共振化学屏蔽中二级同位素效应的从头算计算。如果一个特定原子核被不同同位素取代时所观察到的该原子核核磁共振化学位移的变化，可以通过使用电子结构方法对核磁共振参数进行振动校正来计算。我们证明计算结果的准确性足以区分不同的构象异构体。为此，在HF、MP2和CCSD(T)理论水平下，使用从双ζ到四ζ质量的基组，对处于gauche和反式平面构象的氟（2 - 2H）乙烷进行了基准计算。该方法被应用于2 - 氟降冰片烷的二级同位素位移，以解决文献中关于内型和外型2 - 氟降冰片烷在endo - 3和exo - 3位置带有氘取代基的归属的模糊性问题，同时也深入了解了相应合成的机理细节。
