From the very extensive factual material available on steric interactions in organic chemistry, the present paper reviews the investigations that provide information about the "size" ("spatial requirement") of the substituents. After an introductory section on the results obtained with the aid of spectroscopic data and by analysis of chemical reactions, attention is turned to conformational processes, including hindered rotation in ethanes, in the biphenyl system, in butadienes, multiply substituted arenes, molecular propellers, and triptycenes. The main emphasis is placed on the explanation of sterically hindered ring inversions in bridged arenes, a class of compounds particularly suitable for the study of steric interactions. The results obtained by variation of the parameters in this system are also discussed with respect to the spatial requirements of organic substituents.
A general protocol is described for structure determinations of organic sorbate-zeolite complexes based on the selective, through-space, distance-dependent transfer of magnetization from protons in selectively deuterated organics to framework silicon nuclei. The method was developed using the known structure of the high-loaded ZSM-5/p-xylene complex containing p-xylene-d(6) or p-xylene-d(4). It was then applied to determine the unknown structure of the low-loaded ZSM-5/p-xylene complex using NMR alone. For the high-loaded complex improved data were obtained below 273 K, where slow motions and exchange processes of the p-xylene are eliminated. The general approach was validated by the exact agreement of the experimental (1)H-(29)Si CPMAS spectra obtained at a specific contact time and the complete 24-line spectra simulated using 1/T(CP) vs M(2) correlations from only the six clearly resolved resonances. For the low-loaded complex the (29)Si resonances were assigned at 267 K, and variable contact time CP experiments were carried out between 243 and 173 K using the same specifically deuterated p-xylenes. All possible locations and orientations of the p-xylene guests were sampled, and those solutions that gave acceptable linear 1/T(CP) vs M(2) correlations were selected. The optimum p-xylene location in this temperature range was determined to be in the channel intersection with the long molecular axis parallel to [0,1,0] (ring center fractional coordinates {-0.009, 0.250, 0.541}) with the ring plane oriented at an angle of 30 +/- 3 degrees about the crystallographic b axis. A subsequent single-crystal X-ray study confirmed this predicted structure.
It is demonstrated on a polycrystalline powder sample of
Cd3(PO4)2, given a known
single-crystal X-ray
structure, that two-dimensional 31P double-quantum
single-quantum MAS correlation experiments allow unique
assignment of multiple 31P resonances to specific P sites
in the structure of such inorganic orthophosphates.
The
eigenvalues of the 31P and 113Cd shielding
tensors of Cd3(PO4)2 are reported,
the determination of which requires
usage of a 2D MAS spinning sideband separation experiment.
SynopsisPrimary and tertiary amine-initiated polymerizations of L-alanine-N-carboxyanhydride (L-Ala-NCA) were conducted a t 20 or 100°C in a variety of solvents. The 75.5-MHz '3C-nmr CP/MAS spectra of the resulting poly(L-alanines) revealed that all samples contain both &-helix and pleated-sheet structures. Depending on the reaction conditions the a-helix content varied between ca. 1 and 99%. Reprecipitation from aprotic nonsolvents does not change the a-helix/@-sheet ratio, indicating that this ratio is thermodynamiccy controlled.Since relatively large amounts of oligopeptides of degree of polymerization (DP) 4-6 can be extracted by means of acetic acid, it is concluded that (a) most poly(L-alanines) possess a bimodal molecular weight distribution, (b) the oligopeptide fraction with 5 11 is responsible for the @-sheet fraction of all samples, and (c) the two-stage crystal growth proposed by Komoto and Kawai is not correct. Solubilizing initiators such as poly(ethy1ene oxide) NH;I prevent the precipitation of oligoalanine and, thus, the formation of a @-sheet structure. 13C-nmr CP/MAS measurements also show that tri-and tetra-L-alanines form insoluble @-sheet structures.
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