Crystallographic inequivalence in 13C NMR

Hiyam:
Solid State Nuclear Magnetic Resonance
Volume 29, Issue 4 , June 2006, Pages 283-293

Solid-state NMR and 2,3-dicyano-5,7-dimethyl-6H-1,4-diazepine

Jeffrey M. Njus

Center for Advanced Materials, Department of Chemistry, University of Massachusetts Lowell, Lowell, MA 01854-5046, USA

Abstract

Crystalline 2,3-dicyano-5,7-dimethyl-6H-1,4-diazepine (A) was investigated by solid-state NMR spectroscopy, X-ray diffraction, and spectral simulations. The solid-state 13C NMR spectra of A display peculiar splittings for the methyl and cyano resonances. The crystal structure of A indicates that the methyl doublet is a consequence of two crystallographically inequivalent environments. The methyl motions associated with each site was examined via spin-lattice relaxation time (T1) measurements, and the carbon relaxation times (T1C) were used to calculate energy barriers to methyl rotation. The energy barriers to rotation were then used to correlate each methyl 13C shift with a particular crystallographic environment. The complex cyano splittings, however, are a result of both crystallographic inequivalence and residual 13C–14N dipolar coupling. The multiplet patterns of the isotropic shifts (centerbands) are dependent upon the magic-angle spinning (MAS) rate. Spectral simulations, using the perturbation method, of the centerbands and first-order sidebands were used to demonstrate, and elucidate, the observed MAS rate-dependent multiplet patterns of the cyano signals.

Keywords: Crystallographic inequivalence; Asymmetric unit cell; Spin-lattice relaxation; NOE; Spectral simulations