Partial orientation, structure determination

A zwitterionic (see Zwitterion) carbene-stannylene adduct has been prepared via the cleavage of a dibenzotetraazafulvalene by a stannylene. Thus, reaction of the tetraamine l,2-C6H4-[NHCH2CH2NMe2]2 with bis(bis(trimethylsilyl)amido)tin(II) yielded the N-heterocyclic stannylene (85). Treatment of stannylene (85) with A.MM A -tetramethyl dibenzotetraazafulvalene results in C=C bond cleavage in the dibenzotetraazafulvalene to give the carbene-stannylene adduct (86). An X-ray structure determination showed that compound (86) is zwitterionic with a partially cationic carbene subunit and a partially anionic stannylene unit. The structure of (86) is similar to that of the carbene-silylene (80) such that the carbene plane and the stannylene plane are orientated in an almost perpendicular manner. 

Structural parameters and interatomic distances derived from electron diffraction <77JST(42)121> and x-ray diffraction studies <76AX(B)3178> were given in CHEC-I. The molecular structure of pyrazine has been determined by combined analyses of data obtained by gas-phase electron diffraction (ED) and liquid-crystal NMR (LCNMR) <88JA2758>. The NMR spectrum gives structural information because the solute is partially oriented in the liquid-crystal solvent. The structural parameters determined from the ED, LCNMR data and in a joint analysis of both are listed in Table 2. There the C—C bonded distance is fixed since LCNMR data give no information on the absolute size of the molecule. Since pyrazine itself has no dipole moment, it should not show a microwave (pure rotation) spectrum. 

Structure elucidation requires still the very time consuming analysis of distance information from NOESY spectra. Therefore, it would be highly desirable to obtain information from different sources. This is possible by the measurement of dipolar couplings in partially oriented samples. The use of orientational restraints can enhance the speed of stmcture determination quite dramatically, hi Fig. 54, the amount of time for the different steps in structure elucidation are summarized. 

The measurement and interpretation of RDCs between spatially proximate spin-pairs in biopolymers such as proteins, nucleic acids, and oligosaccharides in partially oriented environments has become a popular method for structure determination." All observed RDCs, D s contain an unknown contribution from the anisotropic part of coupling tensor, J in addition to the direct dipolar contribution, Bryce and Wasylishen" evaluated the influence of J on RDCs... 

Oyler and Tycko have demonstrated that absolute, molecular-level structural information can be obtained from solid state NMR measurements on partially oriented amyloid fibrils.In particular, it has been shown that the direction of the fibril axis relative to a carbonyl CSA tensor can be determined from MAS sideband patterns in C NMR spectra of fibrils deposited on planar substrates. Deposition of fibrils on a planar substrate creates a highly anisotropic distribution of fibril orientations (hence, CSA tensor orientations) with most fibrils lying in the substrate plane. The anisotropic orientational distribution gives rise to distorted spinning sideband patterns in MAS spectra from which the fibril axis direction can be inferred. 

The structure of 1,1-dichlorocyclopropane has been determined from partially oriented n.m.r. spectroscopic data. While the C-1—C-2 and C-2—C-3 bond lengths were not identical (1.480 and 1.544 A, respectively), the data should not be taken as further evidence for a breakdown in the theoretical model, since the values do not agree with those previously obtained by microwave and electron-diffraction techniques. However, the average value of the C-1—C-2 and C-2—C-3 bond lengths (1.512 A) compares very favourably with the earlier results and, as the authors suggest, a thorough reinvestigation of the structure of this compound seems justified. 

R 239 J. Jokisaari, NMR of Noble Gases Dissolved in Liquid Crystals , p. 109 R 230 C.L. Khetrapal and G.A.N. Gowda, NMR of Partially Ordered Solutes with Emphasis on Structure Determination , p. 137 R 231 C. Fares and J.H. Davis, The Search for High-Resolution NMR Methods for Membrane Peptide Structure , p. 191 R 232 E.E. Burnell and C.A. De Lange, Solutes as Probes of Simplified Models of Orientational Order , p. 221 R 233 D.J. Photinos, Molecular Theory of Orientational Order , p. 259 R 234 G. Celebre and M. Longer , NMR Studies of Solutes in Liquid Crystals Small Flexible Molecules , p. 305 R 235 J.M. Poison, Simulations of Orientational Order of Solutes in Liquid Crystals , p. 325... 

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