Abstract
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.
Highlights
Quantum chemical calculations provide a powerful tool to investigate molecular properties and support the interpretation of experimental findings
For 19 F chemical shieldings, for example, which range from about −200 to +500 ppm, Harding et al presented a detailed benchmark study [4]: For the level of electron correlation they find, that compared to CCSD(T), HF leads to 10–20 ppm deviation and MP2 leads to about 5 ppm deviation
We demonstrate how calculations of secondary isotope shifts can be used in order to distinguish different stereoisomers
Summary
Quantum chemical calculations provide a powerful tool to investigate molecular properties and support the interpretation of experimental findings. These shifts occur when a lighter nucleus l different from the resonant one is substituted by a heavier isotope h [6] If this effect is calculated using quantum chemical methods, it has to be noted that the electronic Schrödinger equation in the framework of the Born–Oppenheimer approximation is independent of the nuclear masses. While calculations of secondary isotope shifts are far from routine applications of quantum chemical methods, previous work has shown that for small molecular systems these calculations can yield quantitative results, provided the correct treatment of vibrational effects on nuclear shieldings is used [3]. For which quantum chemical calculations can resolve ambiguities in the literature, we would like to revisit in the second part of this work one of the first studies in which various effects on secondary isotope shifts on NMR chemical shieldings were systematically investigated. Mixtures of exo-2-fluoronorbornanes are obtained with deuterium nuclei bound to the 3- and 7-carbon atom
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