High-Speed Magic-Angle Spinning13C MAS NMR Spectra of Adamantane: Self-Decoupling of the Heteronuclear Scalar Interaction and Proton Spin Diffusion
✍ Scribed by Matthias Ernst; Aswin Verhoeven; Beat H. Meier
- Publisher
- Elsevier Science
- Year
- 1998
- Tongue
- English
- Weight
- 262 KB
- Volume
- 130
- Category
- Article
- ISSN
- 1090-7807
No coin nor oath required. For personal study only.
✦ Synopsis
We have investigated the carbon line shape of solid adamantane 30 kHz, the fastest rate available to us, does still not lead under high-speed magic-angle sample spinning (MAS) acquired to a ''liquid-like'' spectrum with a 1:2:1 J-splitted triplet without proton decoupling. The CH-group shows a spinningfor the CH 2 group and a 1:1 doublet for the CH group (Fig.
speed-dependent line broadening while the CH 2 -group consists of 1). The scalar heteronuclear couplings, measured in the liqa spinning-speed-independent sharp component and a spinninguid state, are J CH Å 131 Hz and J CH 2 Å 126 Hz (4).
speed-dependent broader part. These phenomena can be explained
Figure 1 shows the experimental 13 C spectra of solid adaby self-decoupling of the J-interaction due to proton spin diffusion.
mantane as a function of the spinning speed ranging from Such a self-decoupling process can be described by a magnetization £ r Å v r /(2p) Å 10 to 30 kHz. The CH-group shows at all exchange process between the multiplet lines. Changing the spinspeeds a single line but the line becomes significantly diffusion rate constant by off-resonance irradiation of the protons broader at higher speeds with some indications of a multiplet allows us to observe the full range from slow exchange to coalescence to fast exchange of the carbon spectra. One of the multiplet splitting at the highest MAS frequency. The CH 2 -group apcomponents in the CH 2 -group corresponds to a group spin of the pears to be a single line at lower spinning speeds (£ r Å 10 protons of zero and therefore does not couple to the other protons. and 15 kHz) but at higher speeds (£ r ú 15 kHz) the line This gives rise to the sharp central line. The magnetization exshape appears to be a superposition of a sharp line in the change rate constant between the different multiplet lines can be center of the peak and a quite broad line. A closer investigadetermined from the spectra and is a measure for the spinningtion reveals the presence of a sharp component for all five speed-dependent proton spin-diffusion rate constant. Even at an spinning speeds. The line width of the sharp component is MAS speed of 30 kHz, proton spin diffusion is still observable largely independent of the spinning speed while the broad despite the relatively weak intermolecular proton dipolar-coupling component broadens significantly with higher spinning network in adamantane which results in a static proton line width speed.
of only 14 kHz (full width at half height).