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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

General pulse sequence elements that achieve sensitivity-enhanced coherence transfer from a heteronucleus to protons of arbitrary multiplicity are introduced. The building blocks are derived from the sensitivity-enhancement scheme introduced by Cavanagh et al. ((1991) J. Magn. Reson., 91, 429-436), which was used in conjunction with gradient coherence selection by Kay et al. ((1992) J. Am. Chem. Soc., 114, 10663-10665), as well as from a multiple-pulse sequence effecting a heteronuclear planar coupling Hamiltonian. The building blocks are incorporated into heteronuclear correlation experiments, in conjunction with coherence selection by the formation of a heteronuclear gradient echo. This allows for efficient water suppression without the need for water presaturation. The methods are demonstrated in HSQC-type experiments on a sample of a decapeptide in H2O. The novel pulse sequence elements can be incorporated into multidimensional experiments.

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Experiments in coherent magnetic resonance, microwave, and optical spectroscopy control quantum-mechanical ensembles by guiding them from initial states toward target states by unitary transformation. Often, the coherences detected as signals are represented by a non-Hermitian operator. Hence, spectroscopic experiments, such as those used in nuclear magnetic resonance, correspond to unitary transformations between operators that in general are not Hermitian. A gradient-based systematic procedure for optimizing these transformations is described that finds the largest projection of a transformed initial operator onto the target operator and, thus, the maximum spectroscopic signal. This method can also be used in applied mathematics and control theory.

A novel and general concept of restricting coherence transfer in nuclear spin systems is described. It opens new possibilities for editing one- and two-dimensional NMR spectra. For example, the widely applied two-dimensional correlation experiment COSY can be modified such as to restrict coherence transfer to take place exclusively between connected transitions in the energy level diagram. Such two-dimensional spectra possess ideal features for assignment of complex scalar coupling networks and for computer assisted analysis. Experimental 1H spectra of a cyclic decapeptide are presented. Other applications of the general filtering concept are briefly discussed.

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