Dawson Daniel M, Smellie Iain A, Ashbrook Sharon E
School of Chemistry, EaStCHEM and Centre of Magnetic Resonance, University of St Andrews, St Andrews, KY16 9ST, UK.
Phys Chem Chem Phys. 2024 May 15;26(19):14256-14264. doi: 10.1039/d4cp01127a.
We use a combination of one- and two-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy and density functional theory (DFT) calculations to obtain a full assignment of the H and C signals for solid (+)-usnic acid, which contains two molecules in the asymmetric unit. By combining through-space H-H correlation data with computation it is possible to assign signals not just to the same molecules (relative assignment) but to assign the signals to specific crystallographic molecules (absolute assignment). Variable-temperature measurements reveal that there is some variation in many of the C chemical shifts with temperature, likely arising from varying populations of different tautomeric forms of the molecule. The NMR spectrum of crystalline (+)-usnic acid is then compared with that of ground lichen (the source material of the usnic acid). The abundance of usnic acid is so great in the lichen that this natural product can be observed directly in the NMR spectrum without further purification. This natural sample of usnic acid appears to have the same crystalline form as that in the pure commercial sample.
我们结合一维和二维固态核磁共振（NMR）光谱以及密度泛函理论（DFT）计算，对固态（+）-松萝酸的氢和碳信号进行了完全归属，该化合物的不对称单元中含有两个分子。通过将空间H-H相关数据与计算相结合，不仅可以将信号归属到同一分子（相对归属），还能将信号归属到特定的晶体学分子（绝对归属）。变温测量表明，许多碳化学位移随温度存在一定变化，这可能是由于分子不同互变异构体的相对丰度变化所致。然后将结晶态（+）-松萝酸的NMR谱与地衣粉（松萝酸的原料）的NMR谱进行比较。松萝酸在地衣中的含量非常高，以至于无需进一步纯化就能直接在地衣的NMR谱中观察到这种天然产物。这种松萝酸的天然样品似乎与纯商业样品具有相同的晶体形式。
