*3.4. X-ray Powder Diffraction (XRPD) Analysis*

The formation of cocrystals could be verified by XRPD. In Figure 11, each XRPD pattern of mandelic acid cocrystals is different from either that of **D**, **L-H2ma** or the corresponding coformer. All of the peaks displayed in the measured XRPD patterns of the **D**, **L-H2ma** cocrystal bulk powder are in close accordance with those in the simulated patterns acquired from single-crystal diffraction data, which confirm the formation of high-purity phases. Similarly, in both cocrystals, the XRPD patterns simulated from the single-crystal structures matches well with the XRPD patterns of powder obtained (Figure S12).

**Figure 11.** XRPD patterns of mandelic acid (red), pyridinecarboxamides (green) and cocrystal (blue) in (**a**) **1** and (**b**) **3**.
