6297Ni' dehydrogenates propane and isobutane by an apparently exothermic 1,2 process,25 which occurs with moderate cross section at low energy (Table I). It is not clear why the 1,2 process "turns off" with straight-chain hydrocarbons with more than three carbons. If the initial insertion into a C-H bond is reversible, then the metal ion may sample several insertion sites in a single encounter.26 To explain the present results requires that further reaction of the C-C insertion intermediates occurs with greater facility (higher frequency factors, lower activation energies) than that of the C-H insertion intermediates. As noted in the Ex-(26) Multiple encounter reactions are a well-established feature of ionmolecule reactions owing to the strong ion-neutral attractive interaction. See, for example: Dugan, J. V., Jr.; Rice, J. H.; Magee, J. L. Chem. Phys. Left. 1969,3,323. Sullivan, S. A.: Beauchamp, J. L. J. Am. Chem. SOC. 1977,99, 5017.perimental Section, the 1,2 process is observed with n-butane at relative kinetic energies above -1 eV.
Acknowledgment.Abstract: The 270-and 500-MHz IH NMR spectra of cyclo-(~-Pro,-~-Pro) in various solvents have been analyzed with the aid of one-and two-dimensional (2-D) NMR techniques; 500-MHz 2-D-J-resolved spectra were used to determine starting parameters for chemical shift values and to assign overlapping spin multiplets of the individual nuclei in the 21-spin system. 2-D spin-echo-correlated spectra yield connectivities between coupled spins. In particular, many small long-range couplings have been detected by this technique. Stereochemical assignment of geminal proline protons has been performed by 2-D nuclear Overhauser enhancement (2-D NOE) spectroscopy and by interpretation of aromatic solvent-induced shift (ASIS) effects, coupling constants, and carbonyl anisotropy. A twisted-boat backbone conformation similar to the structure determined by X-ray analysis was derived from the data.
Aus der Aquivalenz der fiinf Aminosauren in cyclo(-Phe,-) und cycle(-Gly5-) kann man auf ein im Sinne der NMR-Zeit-Skala schnelles Konformationsgleichgewicht schlieRen. Die Titelverbindung liegt dagegen iiberwiegend in einer Konformation vor und eignet sich daher fur detaillierte Untersuchungen. Die Zuordnung der NMR-Signale erfolgte durch Synthese und Messung von drei Derivaten, in denen jeweils eine Aminosaure der Sequenz in a-Stellung deuteriert wurde. Aus der Temperaturabhangigkeit der chemischen Verschiebung der NH-Signale bzw. deren Veranderung beim Zusatz von Trifluorethanol oder Chloroform folgt, daO in DMSO die NH-Protonen von Phe2 und Gly5 zwei intramolekulare Wasserstoffbrucken bilden. Das Phe2-NH ist Bestandteil einer y-Schleife, wahrend man aus der geminalen Kopplungskonstanten der a-Protonen von Gly4 schliekn kann, daR das NH-Proton von Gly5 an einer P-Schleife vom Typ I1 beteiligt ist. Die an Cyclopentapeptiden gewonnenen Ergebnisse zeigen iibereinstimmend das Vorliegen wenigstens einer y-Schleife, die offenbar in kleineren Peptiden nicht so ungiinstig ist, wie bisher angenommen wurde.
Conformation of Peptides, 11 ')
'H NMR Investigation of the Conformation of cyelo(-Phe,-Gly,-)The equivalence of the five amino acids of cycle(-Phe5-) and cycle (-Gly,-) indicates a fast equilibration on the NMR time scale. However, cyclo(-Phe,-GIy,-) is conformationally homogeneous and hence was selected for detailed NMR investigations. For signal assignments three derivatives, each of which contains one a-deuterated amino acid in the sequence, were synthesized. The temperature dependence of the chemical shift of the N H signals in DMSO proves intramolecular hydrogen bonds of Phe2-and Gly5-NH. This is supported by the spectral change after addition of trifluoroethanol or chloroform. PheZ-NH is involved in a y-loop, whereas it is concluded from the geminal coupling constants of the Gly4 a-protons that Gly5-NH is part of a p-loop of type 11. The results from conformational investigations of so far known cyclopentapeptides show that in each of them at least one y-loop is formed. Obviously in smaller peptides the y-loop is less unfavourable as usually assumed.
The thermodynamic behavior of planar hexagonal model molecules interacting through a 6- and 7-center Lennard-Jones potential is considered. The influence of molecular size on various thermodynamic properties is elucidated. It is shown that the second virial coefficients obey a microscopic scaling, which causes serious ambiguities for devising effective potential models for real substances. By use of a perturbation theory the vapor–liquid coexistence curves and the critical points are established. Some regularities of distinct properties on the pVT surface are found. The reduced coexisting curves can be used to obtain unambiguous potential parameters for real substances with hexagonal molecular scaffold by using only two experimental points. Thus, all results are rationalized by empirical correlations for easy use. For benzene as an example this procedure results in a model that reproduces vapor pressures and dew densities over two orders of magnitude. Bubble densities are given within a few percent and second virial coefficients are predicted within the experimental scatter throughout the measured temperature range.
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