HETCOR coupling-based experiments

Rocha et al. have reviewed recent advances in NMR spectroscopy of quadrupolar nuclei with half-integer spins. In particular, B, O, Na, A1, Ga, Nb MQ MAS applications have been considered. A range of related techniques, such as satellite transition (ST) MAS, inverse-STMAS NMR, fast amplitude modulation, and techniques based on the dipolar interactions between quadrupolar and spin-1/2 nuclei (CP MQ MAS and MQ HETCOR) and the recently introduced /-coupling based experiments (/-HMQC), have also been reviewed. 

As in the case of homonuclear correlations the heteronuclear Li,X TOCSY experiment, pulse sequence (viii), forms an alternative to the HETCOR and HMQC experiments introduced above. It has been tested for H, Li as well as H, Li shift correlations [147]. The MLEV16 decoupling sequence [148] was used for magnetization transfer, and Li as well as H detection was successful. An advantage of these experiments compared with correlation experiments based on the sequences (iii), (iv) and (v) must be seen in the fact that pure absorption spectra are produced. Signal elimination as a consequence of small coupling constants, as observed for antiphase crosspeaks, is thus prevented. On the other hand, adjusting the two decoupler fields to match the Hartmann-Hahn condition Y/Bi(7) = y,Bi(S) is not trivial and needs considerable experimental experience. 

A series of mesostructured aluminophosphate (AlPO)-based lamellar materials have been characterised by several solid-state NMR techniques. The average number of A1 atoms and identify the nature of other ancillary groups in the second coordination sphere for each P site was estimated. A combination of several dipolar coupling-based P/ Al double-resonance techniques e.g. TRAPDOR, REDOR, and HETCOR as well as H P CP can provide more detailed structural information regarding the local environments of P and A1 atoms in AlPO-based mesostructured materials, which is not readily available from straightforward P and Al MAS experiments. ... 

Recently, the combination of /-based correlation methods (INEPT) with MQMAS (MQ-/-HETCOR experiment) was accomplished (Figs. 26b and 29) [105]. Unlike MQ-HETCOR, which is based on the spatial proximity of nuclei probed by the dipolar interaction, MQ-/-HETCOR maps the chemical bonding between the quadrupolar nuclei and the coupled spin under high-resolution conditions. This technique does not require the spin-locking of quadrupolar nuclei as is the case with CP-based HETCOR experiments. The short T2 in solids may prevent the observation of relatively small /-couplings. 

Fig. 14. Contour plot of the 125.8 MHz 2D 13C/1H HETCOR experiment for Pb(OC-Me)4 in C6D6 (polarization transfer was based on 2 7(I3C, H) = 7 Hz which gives approximately the same signal-to-noise ratio for both alkynyl l3C nuclei).131 The tilt of the pairs of cross peaks for the 207Pb satellites compares the coupling signs of J(207Pb,l3C=) (>0) with 4./(207Pb, H) (<0), and of 2y(207Pb,= 3C) (>0) with VpTbJH) (<0).

The COSY, TOeSY, HETCOR. and HMQC ex periments are great aids in interpreting proton and C spectra. However, the INADEQUATE experiment is perhaps the definitive 2D NMR technique. This experiment is based on spin-spin coupling between directly bonded pairs of nuclei. It can trace out the carbon backbone of an organic compound one carbon at a time. The low sensitivity of INADEQUATE has, however, limited its practical utility. 

---