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diamide's behavior on the basis of spectroscopic data should be more straightforward than for 1. Previously reported variabletemperature 'H NMR measurements (A5(NH)/Ar) indicated that the intramolecularly hydrogen-bonded and non-hydrogen-bonded6 states of 2 are of very similar enthalpy in CD2Cl2.2a In contrast, when an attempt was made to account for solvation by including three CH2C12 molecules in a "supermolecule" calculation, AMI predicted the minimum energy intramolecularly hydrogen-bonded conformation of 2 to be 1.9 kcal/mol more enthalpically favorable than the minimum energy non-hydrogen-bonded conformation.3In order to provide a more quantitative comparison with the calculations, we have now carried out an IR-based van't Hoff analysis of the intramolecular hydrogen bonding equilibrium occurring in a 1 mM CH2C12 solution of 2 over the temperature range -69 to 23 °C. Figure 1 shows the N-H stretch region of the IR spectra obtained at high and low temperatures. Both hydrogen-bonded (3340-50 cm"1) and non hydrogen-bonded6 (3443-8 cm"1) bands are observed at each temperature. No hydrogen-bonded N-H stretch band can be detected at any temperature for a 1 mM sample of iV-methylcyclohexylacetamide (3) in CH2C12; therefore, we used this compound to estimate the extinction coefficient of the non-hydrogen-bonded N-H stretch band of 2 as a function of temperature, van't Hoff analysis (intramolecularly hydrogen-bonded vs non-hydrogen-bonded states; each "state" comprises a set of conformations) indicated that the internally hydrogen-bonded state of 2 is 0.25 ± 0.06 kcal/mol less enthalpically favorable and 0.67 ± 0.48 eu more entropically favorable than the non-hydrogen-bonded state.7 (CHE-9157510), the Eastman Kodak Company, and the Upjohn Company for support. The FT-IR spectrometer was purchased with funds provided by the Office of Naval Research (N00014-90-1902).Supplementary Material Available: Representative van't Hoff plot for intramolecular hydrogen bonding of 2 in CH2C12 (1 page). Ordering information is given on any current masthead page.(10) (a) Baker, E.

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NMR spectroscopy and computer modeling were used to characterize tiopronin monolayer-protected gold clusters (MPCs). These MPCs contain gold cores with a distribution of radii ranging from 0.4 to 2.6 nm. NOESY and HMQC spectra yielded assignments for all NMR sensitive nuclei in the tiopronin ligands. DOSY and T 2 experiments provided information about the particle size distribution as a function of proton frequency shift. Further information was obtained from hole-burning and amide-exchange experiments. The spectroscopic data reveal two classes of ligands, a network of hydrogen bonds, and considerable inhomogeneous and homogeneous line broadening. The methyl and methine protons clearly exhibit two components with separations that decrease strongly with the number of bonds separating the proton from the gold core. Spin−echo experiments clearly show that a range of T 2 values is associated with each resonance frequency in both the upfield and downfield components for each type of proton but that the most probable value is larger for the upfield component. Various models that may be consistent with the NMR data and the properties of reported crystal structures were considered. It is suggested that bimodal frequency distributions result from chemical shifts that are associated with a mixture of primarily two gold cluster structure types that differ in the mode of core packing. It is suggested that the Knight shift contributes to the large downfield shift observed for the methine protons in the larger particles.

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