Methods are described for determining the mutual orientation of molecular interaction tensors based on twodimensional NMR powder spectra. Necessary conditions are derived under which this determination can be made, procedures for the computation and analysis of 2D powder spectra are indicated, and a technique for the construction of ridge plots is worked out. Chemical shift resolved dipolar powder spectra provide an easily interpreted representation of the mutual orientation of chemical shielding and dipolar coupling tensors. The various features and practical applications of such spectra are demonstrated with 2D powder spectra of benzene, lead formate. and methyl formate.It is sufficient to consider a system of two nuclear species, I and S, including Zeeman, dipolar, and possibly quadrupolar interactions. The general Hamiltonian of the system is 1C =JC OI + JC ZI +·JC II + JC IS + JC ss +·JC zs +JCos with the two quadrupolar interaction terms the Zeeman terms and the three dipolar interaction terms

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It is shown that the introduction of well-selected dipolar interactions into carbon-13 spin systems of liquid crystals can lead to well-resolved spectra with a rich content of structural information. Two possibilities are investigated: (a) selective isotope substitution, in particular by deuterium and nitrogen-15, and simultaneous proton decoupling; (b) two-dimensional separated local field carbon-13 spectroscopy. The methods are demonstrated by investigations of benzylidene anilines in their nematic phases. I. INTRODUCTIONIt has been recognized since the early days of nuclear magnetic resonance that the dipolar interactions in oriented media, such as solids and liquid crystals, can provide a wealth of information on molecular structure and on molecular dynamics. Unfortunately, this information is mostly inaccessible by conventional means because of the multitude of interactions leading to broad lines without any noticeable structure. A successful analysis is possible only after drastic simplification of the network of interactions.Numerous techniques have been proposed to simplify N.M.R. spectra of oriented samples. Perhaps the most successful of the presently-known methods is high resolution natural abundance carbon-13 resonance with complete heteronuclear decoupling [1,2]. The remaining anisotropic carbon-13 chemical shifts provide information on molecular structure [3]. However, a quantitative interpretation is often impossible because the calculation of chemical shielding tensors requires complete knowledge of the molecular state functions and this can be an exceedingly demanding task, particularly for complicated molecules or solid systems [4]. The interpretation of chemical shielding tensors is therefore based mostly on empirical rules, similar to those used for the isotropic shifts in solution, rather than on a theoretical analysis. The same situation exists for homonuclear high resolution studies utilizing multipulse schemes [2,5] to suppress the homonuclear dipolar interaction. Again, chemical shielding tensors remain as the only informative quantities.The selective reintroduction of dipolar interactions or the selective removal of dipolar interactions proved to be a very fruitful idea. A sufficiently transparent network of interactions is established leading to well-resolved spectral features and permitting complete analysis and interpretation. In favourable systems, a natural simplicity already exists due to a finite number of localized interactions

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