A new general technique for the investigation of exchange processes in molecular systems is proposed and demonstrated. Applications comprise the study of chemical exchange, of magnetization transfer by inter- and intramolecular relaxation in liquids, and of spin diffusion and cross-relaxation processes in solids.

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The possibilities for the extension of spectroscopy to two dimensions are discussed. Applications to nuclear magnetic resonance are described. The basic theory of two-dimensional spectroscopy is developed. Numerous possible applications are mentioned and some of them treated in detail, including the elucidation of energy level diagrams, the observation of multiple quantum transitions, and the recording of highresolution spectra in inhomogenous magnetic fields. Experimental results are presented for some simple spin systems.

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Filtering according to coupling network connectivity 438 8.3.4. Relayed coherence transfer 440 8.3.5. Coherence transfer by an average Hamiltonian in total correlation spectroscopy 444 8.4. Homonuclear two-dimensional multiple-quantum spectroscopy 8.4.1. Excitation and detection of multiple-quantum coherence 8.4.2. Double-quantum spectra of two-spin systems 451 8.4.3. Multiple-quantum spectra of scalar-coupled networks in isotropic phase 8.4.4. Multiple-quantum spectra of dipole-coupled nuclei in anisotropic phase 463 8.4.5. Double-quantum spectra of quadrupolar nuclei with S = 1 in anisotropic phase 465 8.5. Heteronuclear coherence transfer 8.5.1. Sensitivity considerations 8.5.2. Coherence transfer pathways 8.5.3. Heteronuclear two-dimensional correlation spectroscopy in isotropic phase 471 8.5.3.1. Transfer of in-phase magnetization 8.5.3.2. Broadband decoupling 8.5.3.3. Decoupling by refocusing pulses 475 8.5.3.4. Bilinear rotation decoupling 475 8.5.3.5. Editing of heteronuclear correlation spectra 477 8.5.4. Relayed heteronuclear correlation spectroscopy 479 8.5.5. Heteronuclear correlation experiments involving double transfer 482 8.5.6. Heteronuclear correlation in solids 485 9. DYNAMIC PROCESSES STUDIED BY TWO-DIMENSIONAL EXCHANGE SPECTROSCOPY 9.1. Polarization transfer in one-and two-dimensional methods 9.2. Selection of coherence transfer pathways 10.5. Comparison of sensitivity and performance time of various imaging techniques 10.

The combination of sodium borohydride with cobalt(II),1 nickel(II),1,2 copper(II),3 and rhodium(III)4 halides has been employed to reduce functional groups such as nitriles, amides, and olefins,3'5 which are inert to NaBH4 alone. Despite frequent use,6-15 the nature of the actual reducing species in these complex mixtures remains obscure. For example, the reaction of NaBH4

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