13C Chemical Shift Assignment of Solid 2-Picolinic Acid by DFT/Crystallography Integrated Approach

The tautomerism of organic molecules is widely observed in solution. While for solid organic chemicals, molecules often exist in the most stable tautomeric form. 2-Picolinic acid (PCA) is a very rare case which contains both the neutral molecules and zwitterions in the same crystal structure. Chemical shift assignment for PCA by experimental approach, e.g., 2D NMR methods, is extremely time consuming because the 1H spin-lattice relaxation time (T1) is too long. Density functional theory (DFT) calculation, especially using a periodic model, is an alternative protocol to fix this issue. However, the original crystal structure of PCA cannot be submitted directly for the calculation task due to the proton positional disorder. In this contribution, a virtual structure was constructed via crystallography approach. Theoretical 13C chemical shifts were obtained basing on this virtual model, which are consistent with the experimental values. Also, both neutral PCA and zwitterion demonstrated their featured chemical shifts, such information can be utilized to analyze the molecular states of PCA in its crystalline complexes.