Published in

International Union of Crystallography, IUCrJ, 4(4), p. 466-475, 2017

DOI: 10.1107/s205225251700687x

Links

Tools

Export citation

Search in Google Scholar

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

This paper is made freely available by the publisher.
This paper is made freely available by the publisher.

Full text: Download

Green circle
Preprint: archiving allowed
Green circle
Postprint: archiving allowed
Red circle
Published version: archiving forbidden
Data provided by SHERPA/RoMEO

Abstract

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 (νR≥ 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.