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This paper reports the principal values of the¹³C chemical shift tensors for five nitrogen‐dense compounds (i.e., cytosine, uracil, imidazole, guanidine hydrochloride, and aminoguanidine hydrochloride). Although these are all fundamentally important compounds, the majority do not have¹³C chemical shift tensors reported in the literature. The chemical shift tensors are obtained from¹H→¹³C cross‐polarization magic‐angle spinning (CP/MAS) experiments that were conducted at a high field of 18.8 T to suppress the effects of¹⁴N‐¹³C residual dipolar coupling. Quantum chemical calculations using density functional theory are used to obtain the¹³C magnetic shielding tensors for these compounds. The best agreement with experiment arises from calculations using the hybrid functional PBE0 or the double‐hybrid functional PBE0‐DH, along with the triple‐zeta basis sets TZ2P or pc‐3, respectively, and intermolecular effects modeled using large clusters of molecules with electrostatic embedding through the COSMO approach. These measurements are part of an ongoing effort to expand the catalog of accurate¹³C chemical shift tensor measurements, with the aim of creating a database that may be useful for benchmarking the accuracy of quantum chemical calculations, developing nuclear magnetic resonance (NMR) crystallography protocols, or aiding in applications involving machine learning or data mining. This work was conducted at the National High Magnetic Field Laboratory as part of a 2‐week school for introducing undergraduate students to practical laboratory experience that will prepare them for scientific careers or postgraduate studies.

 

Competitive milling (CM) and stability milling (SM) mechanochemical reactions are used to comprehensively assess the relative thermodynamic stabilities and cocrystallization affinities of three pharmaceutical cocrystals (PCCs) of fluoxetine HCl ( X ) with three different pharmaceutically acceptable coformers (PACs, i.e. , benzoic acid ( B ), fumaric acid ( F ), and succinic acid ( S )). CM reactions, which involve milling X in the presence of two or more different PACs, were used to determine cocrystallization affinities, whereas SM reactions, which involve milling a PCC of X with a different coformer, were used to determine relative thermodynamic stabilities. In certain cases, SM reactions exhibited a remarkable solid-state exchange of coformers, yielding new cocrystalline forms. 35 Cl (spin I = 3/2) SSNMR is used as the primary probe of the products of CM and SM reactions, providing a reliable means of identifying and quantifying chloride ions in unique hydrogen bonding environments in each reaction mixture ( 13 C SSNMR spectra and pXRD patterns are used in support of these data). On the basis of these reactions and data, the PAC cocrystallization affinities with X are B > F ≈ S (most to least preferred), and the PCC stabilities are XB > X 2 F ≈ X 2 S (most to least preferred), corresponding to enthalpies of cocrystallization ranked as Δ H CCXB < ≈ . PAC affinities and PCC stabilities were found to be the same for products of analogous slow evaporation experiments and mechanochemical reactions with extended milling times ( i.e. , 90 minutes). Preliminary plane-wave DFT-D2* calculations are supportive of cocrystal formation; however, challenges remain for the quantification of relative enthalpies of cocrystallization. This work demonstrates the great potential of CM and SM reactions for providing pathways to the rational design, discovery, and manufacture of new cocrystalline forms of APIs.