Exploring the salt–cocrystal continuum with solid-state NMR using natural-abundance samples: implications for crystal engineering

There has been significant recent interest in differentiating multicomponent solid forms, such as salts and cocrystals, and, where appropriate, in determining the position of the proton in theX—H...A—YX⁻...H—A⁺—Ycontinuum in these systems, owing to the direct relationship of this property to the clinical, regulatory and legal requirements for an active pharmaceutical ingredient (API). In the present study, solid forms of simple cocrystals/salts were investigated by high-field (700 MHz) solid-state NMR (ssNMR) using samples with naturally abundant¹⁵N nuclei. Four model compounds in a series of prototypical salt/cocrystal/continuum systems exhibiting {PyN...H—O—}/{PyN⁺—H...O⁻} hydrogen bonds (Py is pyridine) were selected and prepared. The crystal structures were determined at both low and room temperature using X-ray diffraction. The H-atom positions were determined by measuring the¹⁵N—¹H distances through¹⁵N-¹H dipolar interactions using two-dimensional inversely proton-detected cross polarization with variable contact-time (invCP-VC)¹H→¹⁵N→¹H experiments at ultrafast (ν_R≥ 60–70 kHz) magic angle spinning (MAS) frequency. It is observed that this method is sensitive enough to determine the proton position even in a continuum where an ambiguity of terminology for the solid form often arises. This work, while carried out on simple systems, has implications in the pharmaceutical industry where the salt/cocrystal/continuum condition of APIs is considered seriously.