The structure of 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene-silver(I) chloride, 1, has been determined to be a monomer with weak head-to-tail H...Cl interactions in the solid state. A multinuclear NMR study using a (13)C(carbene) labeled derivative, 1((13)C), exhibits (13)C-(107,109)Ag coupling in solution. Further, the solid state CP/MAS NMR parameters, including the principal components of the chemical shift tensors for both the (13)C and (109)Ag centers, have been determined. With the aid of DFT calculations, the orientation of the chemical shift tensors have been assigned.

less mobile on the electrode surface and perhaps cannot orient for most effective electron and energy transfer between the species.The rates of the reactions leading to R*, eq 1-6, may also differ in the monolayer. For example, production of the critical intermediate, C02", may be more effective in the monolayer, if the direct oxidation at the electrode surface of it and C2041 2 3" are effectively blocked by the monolayer. In this case any C02" at the monolayer will produce either R* directly via eq 6, or indirectly via (4) and ( 5); other processes leading to loss of this very reducing species, eg., reduction of protons or dimerization, must also be occurring in both monolayer and solution reactions.While a fuller elucidation of the processes described above requires further studies, the availability of an excitable monolayer assembly on conductive substrates should allow a number of novel experiments to be carried out. For example, the study on the effect of electrode material and potential on the quenching of R* following photoexcitation should be possible. ECL has been used as a highly sensitive analytical method;4 the monolayer approach may provide for even higher sensitivity, with a lesser amount of material concentrated on an electrode surface. It may also be useful in the fabrication of a fixed probe for ECL. When combined with the techniques of STM or scanning electrochemical microscopy,13 with video detection and imaging,14 it should be possible to detect optically the distribution of small numbers of molecules or the arrival of suitable reactant molecules at the monolayer/solution interface. Experiments of this type are under way in our laboratory.Acknowledgment. The support of this research by the Robert A. Welch Foundation (F-079) and the Army Research Office (DAAG29-85-K-0104) is gratefully acknowledged.(13) Bard, A.

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We have examined the acidic properties of a SiO 2 -on-Al 2 O 3 monolayer catalyst by 31 P NMR of adsorbed tricyclohexylphosphine and triphenylphosphine. These large probe molecules permit the assessment of steric effects, by comparison with trimethylphosphine, and reveal differences in acid strengths of surface sites. We find that there are significant constraints to accessing Lewis sites, but not Brønsted sites. The concentration of strong Brønsted sites is slightly less than half of the total concentration of Brønsted sites.