Heteronuclear spinning-sideband patterns

A comparatively long N- -H hydrogen bond length in the benzoxazine dimer (see below), measured using an advanced solid state NMR technique (DIP-HSQC),has been reported [52]. This technique employs REDOR-type recoupling under fast MAS to recouple the heteronuclear - N dipolar interaction, such that rotor-encoded spinning sideband patterns are obtained. 

Fig. 8. Calculated heteronuclear DQ MAS NMR spinning sideband patterns for rotating phenylene groups and different flip angles or as indicated (Nrcpl DIS/ coR= 1.67).

Figure 15 presents the pulse sequence for a general separated local field (SLF) experiment.133-136 The basic principle of the SLF technique is that a spinning-sideband pattern, from which the heteronuclear dipolar coupling can be extracted, is obtained in the indirect dimension for each resolved resonance in the direct dimension, i.e., the dipolar interaction is separated from the chemical shift interaction. In the original SLF papers, a homonuclear decoupling method is applied in t, but recently McElheny et al. 

Figure 16. Pulse sequence and coherence transfer pathway diagram for the REPT-HDOR heteronuclear ( H-13C) experiment, which is suitable for recording rotor-encoded spinning-sideband patterns. Dark- and light-shaded rectangular blocks represent rf pulses of flip angle 90° and 180°, respectively. For more details, see ref 125.

Fig. 9.30 H—heteronuclear MQ MAS spinning-sideband patterns, obtained at a Vr = 25 kHz, using the REPT-HMQC experiment. The patterns correspond to the sum projections over the resonance due to the aromatic core CH in the 2D spectra of HBC-C12, and HBC-PhCi2. The spectra for the room temperature (solid) and high temperature LC phases were...

One way of studying molecular motions involves monitoring the reduction of dipole-dipole couplings probed by DQ spinning sidebands. The site selectivity is particularly high for heteronuclear DQ coherences. In Fig. 8, simulated sideband patterns are plotted for the C-H group, which is a sensitive probe of phenylene rotational motions, often met in practice. At low temperatures, one would expect... 

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.

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