Analysis of a Solid-State Conformational Rearrangement Using ¹⁵N NMR and X-ray Crystallography

Abstract

A solid azobenzene dyestuff has been studied by a combination of single-crystal X-ray diffraction and 15N CP/MAS NMR. The X-ray work establishes that the crystal structure of one polymorph is disordered. The 15N NMR proves that this is a temporal, rather than a spatial, effect. Three 15N-enriched isotopomers (together with a second polymorph) were examined by NMR, and assignments established by rotational resonance. The shielding tensor components were determined, and the angles of change between their orientations were established by a two-dimensional exchange experiment, the theory of which is discussed. Exchange rates were measured both by selective polarization inversion and by band shape analysis. Thermodynamic parameters for the barrier to exchange are presented. It is demonstrated, by the combination of X-ray and NMR work, that the process is essentially a crankshaft motion of the azo linkage.

Effects of polymorphic differences for sulfanilamide, as seen through 13C and 15N solid‐state NMR, together with shielding calculations

“…We also measured the high-resolution 15 N CP/MAS spectra of 15 N-enriched samples for all the three solid forms for the first time. The value of 15 N spectra as an aid to the understanding of polymorphic behaviour has been shown previously in respect of dyestuffs 28,29 and pharmaceuticals. 30 In the present work, the separate enrichment of sulfanilamide at the N(1) and the N(2) positions allowed an insight into molecular mobility.…”

The Accuracy of Distance Measurements in Solid-State NMR

“…Diffraction-based techniques are unable, however, to distinguish between spatial disorder and temporal disorder caused by molecular motion. For such systems, NMR studies of single crystals, combined with results from diffraction experiments, are a potentially rich source of information about dynamics in the solid phase (23).…”

Conformational flexibility and hydrogen‐bonding patterns of the neotame molecule in its various solid forms

“…[3][4][5] In another example, 2(1H)-quinoxalinone O-(2 0 -quinoxalinyl)oxime crystallizes in two easily distinguishable polymorphs with different colors, due to differences in conformation of the molecules in these two crystal forms. 6 Approaches available to study the conformational flexibility of a molecule include computational chemistry, [7][8][9] X-ray crystallography, 8,10 and solid-state nuclear magnetic resonance (SSNMR) spectroscopy. 10,11 Monte Carlo simulations have been used to determine the stable conformations of a series of N-7-oxoacyl-L-alanyld-isoglutamines with 0-6 methylene groups between the terminal methyl and the 7-oxo groups.…”

“…6 Approaches available to study the conformational flexibility of a molecule include computational chemistry, [7][8][9] X-ray crystallography, 8,10 and solid-state nuclear magnetic resonance (SSNMR) spectroscopy. 10,11 Monte Carlo simulations have been used to determine the stable conformations of a series of N-7-oxoacyl-L-alanyld-isoglutamines with 0-6 methylene groups between the terminal methyl and the 7-oxo groups. 9 Carbon-13 solid-state nuclear magnetic resonance ( 13 C SSNMR) spectroscopy has been used to study the conformations of cimetidine molecules in its polymorphs.…”