¹⁴N–¹H Heteronuclear Multiple-Quantum Correlation Magic-Angle Spinning NMR Spectroscopy of Organic Solids

Abstract

Abstract 14 N– 1 H heteronuclear multiple-quantum correlation (HMQC) solid-state magic-angle spinning (MAS) NMR spectra recorded at a 1 H Larmor frequency of 850 MHz are presented for the dipeptide β -AspAla. A modified version of the pulse sequence presented by Gan et al. (Chem. Phys. Lett. 435 (2007) 163) that utilises rotary resonance recoupling (R 3 ) at the n = 2 condition ( ν 1 = 2 ν R ) is employed. Spectra recorded with a short recoupling period (under 200 μs) show two correlation peaks corresponding to the NH and NH 3 moieties in the dipeptide. The quadrupolar product, P Q = C Q √ [1 + ( η Q 2 /3)] , is determined experimentally as 3.1 MHz (NH) and 1.0 MHz (NH 3 ) by a comparison of the 14 N and 15 N isotropic chemical shifts which differ due to the isotropic second-order quadrupolar shift for the spin I = 1 14 N nucleus. It is shown that the peak sensitivities increase markedly upon increasing the MAS frequency from 30 to 45 to 60 kHz due to a combination of the reduced residual dipolar broadening of the 1 H resonances and a lengthening of the coherence lifetimes under R 3 recoupling. Increasing the recoupling period leads to the observation of additional peaks corresponding to longer range intra- and intermolecular NH proximities. Reasonable agreement is evident upon comparing the experimental build-up of correlation peak intensity to that observed for eight-spin density-matrix simulations.