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An apparatus suitable to measure 13C relaxation times TI and Ta in liquids at 62 MHz is described. The required field of 58 kG is generated by a superconducting magnet. Results of TX measurements on a variety of liquids, including benzene, derivatives of benzene, saturated rings, CSn and others are reported. They are discussed in terms of dipole-dipole interactions, spin-rotation interactions and anisotropic chemical shifts, the latter of which turn out to play only a minor role. Intramolecular dipole-dipole interactions are found to provide by far the most important spin-lattice relaxation mechanism, whenever protons are bound directly to the carbons under investigation, even when the samples contain dissolved oxygen.

Tancredi, T.; Temussi, P. A.; Trivellone, E.; Bradbury, E. M.; Crane-Robinson, C., J. Chem. Soc., Chem.ABSTRACT: This paper reports on a 'H NMR study of the members 11-X (n = 2-10) and XV ( n = 15) of the series Boc-(L-Phe),-(D-Phe-L-Phe)c,-,,,z-OMe (n = number of residues in the oligopeptide; m = 0 or 1) in chloroform solution a t 25 'C. It is shown that there is a species that is strongly preferred by the oligophenylalanines with seven or more residues and that this species is a dimer with the structure of a right-handedt&@6,6-helix with 2(n -1) interstrand H bonds. In the case of XV this species is virtually the only one occurring. The preference of these D,L-alternating oligophenylalanines for a t J@5.6-helix contrasts with the behavior of the corresponding oligovalines, which, as it has been observed in earlier studies, form preferentially P4.*-helices in chloroform. The role of the nature of the side chains in determining this different behavior is discussed.ABSTRACT: The structure and conformational energies of polysilane, H-(SiH&,-H, and poly(dimethylsilylene), Me-(SiMe&,,-Me, have been investigated by using full relaxation empirical force field (EFF) techniques. Gauche conformational states are calculated to be lowest in energy for both polymers. These results contrast with polyethylene hydrocarbon polymers which typically adopt trans conformations in the ground state. Both polysilane and poly(dimethylsily1ene) are calculated to be conformationally more flexible than polyethylene.

4195The application of eq 15 to the 2Jcc, in P-[Dlglucose 16 and a- [D]Glucose 17 gives results in good conformity with the experimental data if we assume that the OH group at the C2 carbon contributes +3 Hz to the total (vide supra) and that the contributions from eq 15 for each of the three oxygen substituents at C1 and C3 are additive. The total of the four contributions are +2.5 Hz and -0.1 Hz, which are in good conformity with the experimental values of 3.5 Hz and C1.5 Hz, respectively. ConclusionsGeminal 13C-'3C coupling constants exhibit dependencies on substituent orientation, hybridization, and CI-C2-C3 bond angle effects. Some of the trends are qualitatively reproduced by means of the INDO-FPT MO method, but the calculated magnitudes are often 5-6 Hz too negative.By means of a valence-bond bond-order formulation, 2Jccr in a propanic fragment are related to geminal, vicinal, and long range 'H-IH or to geminal and vicinal I3C-'H coupling constants. The long range terms make relatively small contributions, but the small magnitudes of 2Jcc. in saturated hydrocarbons must be due to a cancellation between the geminal and vicinal terms. In freely rotating systems a good linear correlation is observed between the experimental data for zJcc, and 2JcH.Substituent effects at the C2 carbon follow the same trends observed for geminal 'H-'H and I3C-'H coupling constants wherein electronegative substituents lead to more positive values of ZJccJ. By means of VB formulation it is shown that substituents at C1 or C3 should lead to an angular dependence of the vicinal type. A single equation of this type is found to provide a good correlation of the experimental data.

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