Experimental NMR measurements

Reactivity and reaction models. In the last decade, one of the main innovations in the use of theoretical calculations concerns the investigation of reaction mechanisms. The cyclization mechanism of 2,3-bis(phenylethynyl)-camphor-sultam derivatives (see Section 4.05.7.3, compound 288) was investigated by PM3 calculations providing support to experimental NMR measurements <1996ZNB1655>. Transition structures for the thermal decomposition of 5-azidoisothiazole 11 to 11a were calculated at the MP2/6-31G level. 

In this section we will demonstrate calculations made from two-dimensional images of the emulsions and how they relate to experimental NMR measurements on short time scales. The new methodology has recently been described in detail by Loren et al. (Loren et al., 2005). 

The J(CIN) coupling constant in the HCh pyridine complex has been studied in Ref. by means of a methodology similar to the one employed for the ClH- -NHs complex °. The conclusions are basically the same as for the CIH- - - NH3 complex, with an additional advantage of being partly supported by the experimental NMR measurements for this complex A summary of the CCSD calculations of the spin-spin coupling constants in acid-base pairs is presented in Ref. 

VO(L ° )(OMe)(MeOH)] (2-MeOH) was studied by and NMR spectroscopies and DFT calculations, and the difference between the NMR chemical shifts of the two diastereomers for 1 or 2 is 30 or 28 ppm, respectively. The chemical speciation of 2-amino-N-hydro-(p-alaninohydroxamic acid, HL) and V(v) in aqueous solution was studied using the results obtained by the calculations of the thermodynamic properties and the NMR chemical shifts of the species formed at equilibrium and the experimental NMR measure-ments. The NMR chemical shifts were calculated by DFT approach accounting for relativistic corrections and solvent effects and all the tautomers of the 1 1 and 1 2 V02 /p-ala complexes with different... 

When file polymerization is earned out in the same experimental conditions but without addition of p-TSA (S11CI2 2H20 = 0.4% mass, 180°C and 20 h), the resulting polymer is amorphous with Mw = 40,000 and a degree of racemization equal to 75% (determined from 13C NMR measurements). 

Fig. 6.8 plots experimental data on nitrogen in the gas phase. Intensive NMR measurements have recently turned nitrogen into the system for... 

This conclusion is supported by the experimental result " given by the pulsed-NMR measurement that the spin-spin relaxation time T2 is considerably shorter for the gel than that for the matrix mbber vulcanizate, which of course, indicates that the modulus is considerably higher for the gel than for the matrix mbber. More quantitatively, Maebayashi et al. measured the acoustic velocity of carbon gel by acoustic analysis and concluded that the compression modulus of the gel is about twice that of matrix mbber. Thus, at present, we can conclude that the SH layer, of course without cross-linking, is about two times harder than matrix cross-linked mbber in the filled system. 

The inherent sensitivity of NMR signals to the fluid-substrate interactions via a large number of mechanisms provides a direct connection between the NMR measurables, the pore structure and the motional characteristics of the imbibed fluid. While the large number of potential NMR variables makes the experimental design and analysis complex, it also provides the potential for a measurement method capable of measuring and spatially resolving the parameters of interest to functionalized ceramics. 

Fig. 1 Solid-state NMR structure analysis relies on the 19F-labelled peptides being uniformly embedded in a macroscopically oriented membrane sample, (a) The angle (0) of the 19F-labelled group (e.g. a CF3-moiety) on the peptide backbone (shown here as a cylinder) relative to the static magnetic field is directly reflected in the NMR parameter measured (e.g. DD, see Fig. 2c). (b) The value of the experimental NMR parameter varies along the peptide sequence with a periodicity that is characteristic for distinct peptide conformations, (c) From such wave plot the alignment of the peptide with respect to the lipid bilayer normal (n) can then be evaluated in terms of its tilt angle (x) and azimuthal rotation (p). Whole-body wobbling can be described by an order parameter, S rtlo. (d) The combined data from several individual 19F-labelled peptide analogues thus yields a 3D structural model of the peptide and how it is oriented in the lipid bilayer...

Experimentally determined pA, values have been compared with Hammett substituent constants (Figure 1) and an excellent correlation was found. This result provided a further support - in accord with some nuclear magnetic resonance (NMR) measurements - for existence of one particular tautomeric form (Figure 1, structure a) among the three possible ones (Figure 1, a, b, c). 

The lines in the spectrum of Fig. 1 are, even under optimum experimental conditions, quite broad. This is due to small unresolved hyperfine interaction with the eighteen equivalent hydrogen atoms. Either the width of the lines, or nmr measurements (La Mar et al., 1973) can reveal the magnitude of this y-hydrogen splitting (ca. 0.15 G—dependent on temperature and solvent). 

The principle source of experimental conformational data in an NMR structure determination is constraints on short interatomic distances between hydrogen atoms obtained from NMR measurements of the nuclear Overhauser effect (NOE). NOEs result from cross-relaxation mediated by the dipole-dipole interaction between spatially proximate nu-... 

Which model provides the best representation for local mobility in a particular group remains unclear, as a detailed picture of protein dynamics is yet to be painted. This information is not directly available from NMR measurements that are necessarily limited by the number of experimentally available parameters. Additional knowledge is required in order to translate these experimental data into a reliable motional picture of a protein. At this stage, molecular dynamic simulations could prove extremely valuable, because they can provide complete characterization of atomic motions for all atoms in a molecule and at all instants of the simulated trajectory. This direction becomes particularly promising with the current progress in computational resources, when the length of a simulated trajectory approaches the NMR-relevant time scales [23, 63, 64]. 

Abstract In this chapter we discuss practical techniques and instrumentation used in experimental measurements of kinetic and equilibrium isotope effects. After describing methods to determine IE s on rate constants, brief treatments of mass spectrometry and isotope ratio mass spectrometry, NMR measurements of isotope effects, the use of radio-isotopes, techniques to determine vapor pressure and other equilibrium IE s, and IE s in small angle neutron scattering are presented. 

In the systems that I have examined, I can satisfy the dynamic requirements with a ten second pulse delay. The longest methyl T] may be 3 seconds. In general, the longer the side chain, the longer will be the methyl Tj. We will hear more about this subject later on. We need not be too concerned about NOE factors because they are usually full under the experimental conditions (T = 120-130°C) used for polymer quantitative measurements. The Tj problem can be handled, even under non-equilibrium conditions, by utilizing resonances from the same types of carbon atoms in a quantitative treatment. Such an approach can sometimes lead to more efficient quantitative NMR measurements. Adequate pulse spaclngs will have to be used whenever one wishes to utilize all of the observed resonances. Quantitative measurements in branched polyethylenes are very desirable because this is one of the best applications of analytical polymer C-13 NMR. 

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