14 N– 1 H Heteronuclear Multiple-Quantum Correlation Magic-Angle Spinning NMR Spectroscopy of Organic Solids
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 (20...
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Published in | Zeitschrift für physikalische Chemie (Neue Folge) Vol. 226; no. 11-12; pp. 1187 - 1204 |
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Main Authors | , , , |
Format | Journal Article |
Language | English |
Published |
01.12.2012
|
Online Access | Get full text |
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Summary: | 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. |
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ISSN: | 0942-9352 2196-7156 |
DOI: | 10.1524/zpch.2012.0308 |