Institute for Technical Chemistry and Macromolecular Chemistry, RWTH Aachen University, Worringerweg 2, D-52074, Aachen, Germany.
Analyst. 2017 May 2;142(9):1459-1470. doi: 10.1039/c7an00020k.
Elucidating the structure of complex molecules is difficult at low magnetic fields due to the overlap of different peak multiplets and second-order coupling effects. This is even more challenging for rigid molecules with small chemical shift differences and with prochiral centers. Since low-field NMR spectroscopy is sometimes presumed as restricted to the analysis of only small and simple molecules, this paper aims at countering this misconception: it demonstrates the use of low-field NMR spectroscopy in chemical forensics for identifying strychnine and its counterions by exploring the chemical shift as a signature in different 1D H and C experiments. Hereby the applied methodologies combine various 1D and 2D experiments such as 1D H, C, DEPT, and 2D COSY, HETCOR, HSQC, HMBC and J-resolved spectroscopy to elucidate the molecular structure and skeleton of strychnine at 1 Tesla. Strychnine is exemplified here, because it is a basic precursor in the chemistry of natural products and is employed as a chemical weapon and as a doping agent in sports including the Olympics. In our study, the molecular structure of the compound could be identified either with a 1D experiment at high magnetic field or with HMBC and HSQC experiments at 1 T. In conclusion, low-field NMR spectroscopy enables the chemical elucidation of the strychnine structure through a simple click with a computer mouse. In situations where a high-field NMR spectrometer is unavailable, compact NMR spectrometers can nevertheless generate knowledge of the structure, important for identifying the different chemical reaction mechanisms associated with the molecule. Desktop NMR is a cost-effective viable option in chemical forensics. It can prove adulteration and identify the origin of different strychnine salts, in particular, the strychnine free base, strychnine hemisulphate and strychnine hydrochloride. The chemical shift signatures report the chemical structure of the molecules due to the impact of the counterions on the chemical shift of the protons adjacent to the heteroatoms. This can serve as a methodology for the structure elucidation of complex molecules at lower-magnetic fields.
在低磁场下，由于不同峰多重峰的重叠和二阶耦合效应，解析复杂分子的结构很困难。对于具有小化学位移差异和前手性中心的刚性分子，这就更加具有挑战性。由于低场 NMR 光谱学有时被认为仅限于仅分析小而简单的分子，因此本文旨在反驳这种误解：它通过探索化学位移作为不同一维 H 和 C 实验中的特征，展示了低场 NMR 光谱学在化学取证中用于识别士的宁及其反离子的用途。为此，应用的方法学结合了各种一维和二维实验，例如一维 H、C、DEPT 和二维 COSY、HETCOR、HSQC、HMBC 和 J 分辨光谱学，以在 1 特斯拉下阐明士的宁的分子结构和骨架。士的宁在这里被举例说明，因为它是天然产物化学中的基本前体，并且被用作化学武器和体育中的兴奋剂，包括奥运会。在我们的研究中，化合物的分子结构可以通过在高磁场下进行一维实验或在 1 T 下进行 HMBC 和 HSQC 实验来识别。总之，低场 NMR 光谱学通过简单的点击计算机鼠标即可实现士的宁结构的化学阐明。在无法获得高场 NMR 光谱仪的情况下，紧凑型 NMR 光谱仪仍然可以生成结构知识，这对于识别与分子相关的不同化学反应机制非常重要。台式 NMR 是一种具有成本效益的可行选择，可用于化学取证。它可以证明掺假并识别不同士的宁盐的来源，特别是士的宁游离碱、士的宁半硫酸盐和士的宁盐酸盐。化学位移特征报告了分子的化学结构，因为反离子会影响邻近杂原子的质子的化学位移。这可以作为在较低磁场下解析复杂分子结构的方法。
