Chemical shifts sideband suppression

In MAS, the sample is rotated rapidly by mechanical means around an axis that makes a prescribed angle with the static field. If the sialic spectrum (at rotation speed zero) has contributions from second-order tensor interactions such as the chemical shift anisotropy, then MAS breaks these contributions into a series of sidebands, separated from the average resonance frequency by multiples of the rotation frequency (/). The envelope of these sidebands traces out the original second-order tensor. At high rotation speeds, all sidebands are outside the static spectrum and therefore have zero amplitude so that only the average resonance survives. It is not always technically possible to reach such speeds in such cases the intensity of the sidebands can be diminished by applying certain pulse sequences [total suppression of sidebands (TOSS)] (see section IV.D). 

Following the first procedure, mineral standards were synthesized and characterized by P MAS NMR [31,42]. BD, CP and DD spectra of mineral standards and bone samples were compared, giving particular attention to chemical shifts (Table 2),linewidths and rotational sideband patterns [35]. A typical set of spectra, acquired for mineral standards, is given in Fig. 7. Each BD spectrum comes from all P-sites, while its CP counterpart exposes a fraction of the P-sites capable of obtaining polarization from surrounding protons. The DD experiment is performed with a time interval without proton decoupling inserted just after CP. This is called a dipolar suppression period (DS), because... 

Fig. 2. A 2D SLF pulse sequence (A) with S-spin magnetization evolution (B) subject only to heteronuclear dipolar couplings in the q period and detection of chemical shift spectrum in the 2 period. Various multiple pulse (MP) sequences can be used to suppress dipolar coupling among I spins in the laboratory frame during the h period, which enables line-narrowing in the I-S dipolar coupling dimension (i.e., the a>i frequency dimension of the 2D spectrum). This experiment under MAS can be used for separating I-S dipolar sideband patterns by isotropic chemical shifts the re-pulse and the start of the acquisition need to be synchronized with rotational echoes. Other aspects of this pulse sequence are similar to the SLF sequence in Fig. 1.

The sidebands in C solid-state NMR spectra of 13 polycyclic aromatic hydrocarbon model compounds associated with kerogen structure were suppressed with C CP MAS TOSS NMR. The chemical shift values of these model compounds were obtained and were subsequently used to determine the chemical shifts of aliphatic and aromatic carbons in a series of kerogens via C CP MAS TOSS NMR. Dipolar dephasing (DD) was used to obtain the spectra of nonprotonated carbon present. ... 

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