Natural abundance 2H NMR spectra of molecules oriented in liquid crystals

Multinuclear Residual Quadrupolar Couplings for Structure and Assignment

Most stable isotopes have a nuclear spin >1/2, but the quadrupole interaction poses challenge on their detection by nuclear magnetic resonance (NMR). On the other hand, the quadrupole interaction is a rich source of structural information that may be exploited for solution NMR in the form of residual quadrupolar couplings (RQCs) of weakly oriented samples. While 2H RQCs are now well established for structure verification and enantiomeric discrimination of organic molecules, we will in this article highlight some recent work on RQCs of other nuclei (especially 7Li and 11B).

Direct estimation of an element of order matrix from 1H NMR spectra of strongly dipolar coupled spins

NMR spectra of molecules oriented in thermotropic liquid crystalline media provide information on the molecular structure and order. The spins are generally strongly dipolar coupled and the spectral analyses require the tedious and time consuming numerical iterative calculations. The present study demonstrates the application of multiple quantum spin state selective detection of single quantum transitions for mimicking the homonuclear decoupling and the direct estimation of an element of ordering matrix. This information is utilized to estimate the nearly accurate starting dipolar couplings for iterative calculations. The studies on the spectra of strongly dipolar coupled five and six interacting spin systems are reported.

Spin State Selective Detection of Single Quantum Transitions Using Multiple Quantum Coherence: Simplifying the Analyses of Complex NMR Spectra

NMR spectra of molecules oriented in liquid-crystalline matrix provide information on the structure and orientation of the molecules. Thermotropic liquid crystals used as an orienting media result in the spectra of spins that are generally strongly coupled. The number of allowed transitions increases rapidly with the increase in the number of interacting spins. Furthermore, the number of single quantum transitions required for analysis is highly redundant. In the present study, we have demonstrated that it is possible to separate the subspectra of a homonuclear dipolar coupled spin system on the basis of the spin states of the coupled heteronuclei by multiple quantum (MQ)-single quantum (SQ) correlation experiments. This significantly reduces the number of redundant transitions, thereby simplifying the analysis of the complex spectrum. The methodology has been demonstrated on the doubly 13C labeled acetonitrile aligned in the liquid-crystal matrix and has been applied to analyze the complex spectrum of an oriented six spin system.

2H Natural-Abundance MAS NMR Spectroscopy: An Alternative Approach to Obtain1H Chemical Shifts in Solids

(2)H NMR was examined as an approach to determine (1)H chemical shifts in solids. For high-resolution observation, the line width due to (2)H quadrupole interaction and chemical-shift anisotropy was removed by magic-angle spinning and that due to (1)H-(2)H dipolar interactions by (1)H decoupling. Further, we showed that the sensitivity can be enhanced by applying (1)H to (2)H cross polarization and by adding spinning-sideband spectra. These make it possible to obtain (2)H natural-abundance MAS spectra revealing highly resolved (2)H signals. The second-order quadrupole effects of (2)H are also examined.

Indirect NMR detection in solids with multiple cross-polarization periods

The use of indirect detection for signal enhancement in solids is much less common than in liquids, but has attracted renewed interest recently. In this work we describe an indirect detection scheme that offers a large signal enhancement for rare spins in solids. The method uses multiple periods of cross polarization, each followed by an evolution period. The latter is increased stepwise in a pseudo 2D experiment, in which the signal of the rare spin is detected as modulation of the abundant spin. As an illustration of this method, the natural abundance deuterium NMR spectrum of a static powder sample of 1,2,4,5-tetramethylbenzene is presented.