Tackling the stacking disorder of melon—structure elucidation in a semicrystalline material

Abstract

In this work we tackle the stacking disorder of melon, a layered carbon imide amide polymer with the ideal composition (C₆N₇(NH)(NH₂)). Although its existence has been postulated since 1834 the structure of individual melon layers could only recently be solved via electron diffraction and high-resolution ¹⁵N solid-state NMR spectroscopy. With only weak van der Waals interactions between neighboring layers its long range stacking order is poorly defined preventing an efficient use of diffraction techniques. We, therefore, rely on a combination of solid-sate NMR experiments and force field calculations. The key information is obtained based on heteronuclear (¹H–¹³C) and homonuclear (¹H–¹H) second moments M₂ acquired from ¹H–¹³C cross polarization experiments. To allow for an interpretation of the polarization transfer rates the resonances in the ¹³C MAS spectra have to be assigned and the hydrogen atoms have to be located. The assignment was performed using a two-dimensional ¹⁵N–¹³C iDCP experiment. For the determination of the position of the hydrogen atoms NH and HH distances were measured via¹H–¹⁵N Lee–Goldburg CP and ¹H–¹H double-quantum build-up curves, respectively. Furthermore, the homogeneity of the material under examination was investigated exploiting ¹⁵N spin-diffusion. Based on force field methods 256 structure models with varying lateral arrangements between neighboring layers were created. For each model the M₂ were calculated allowing them to be ranked by comparing calculated and measured M₂ as well as via their force field energies. This allows the creation of markedly structured hypersurfaces with two distinctly favored shift vectors for the displacement of neighboring layers.