Proton NMR Experiments

Prior to the advent of homonuclear 2D-NMR experiments, it was necessary to rigorously interpret a proton NMR experiment and identify all of the homonuclear couplings to assemble the structure. Alternatively, there are multi-dimensional NMR experiments that provide similar information in a more readily interpretable way. 

The continuity of the linewidth over a large temperature range, above room temperature, that has previously been reported (IJ) and is further detailed here does not reveal any change in the vicinity of the a transition region (80-100°C). Major changes are found in broad line proton NMR experiments, which measure linewidths or second moments. ( ) There is, however, no discrepancy between these results since the a transition is a property of the crystalline regions. The proton measurements... 

Suppose that a pulse Fourier transform proton NMR experiment is carried out on a sample containing acetone and ethanol. If the instrument is correctly operated and the Bq field perfectly uniform, then the result will he a spectrum in which each of the lines has a Lorentzian shape, with a width given hy the natural limit 1/(7tT2). Unfortunately such a result is an unattainable ideal the most that any experimenter can hope for is to shim the field sufficiently well that the sample experiences only a narrow distribution of Bq fields. The effect of the Bq inhomogeneity is to superimpose an instrumental lineshape on the natural lineshapes of the different resonances the true spectrum is convoluted by the instrumental lineshape. 

In a proton NMR experiment in which 1,4-pentadiene was added to a solution of HNi[P(OMe)3]4, it was possible to watch the isomerization of 1,4- to 1,3-pentadiene, followed by formation of l,3-dimethyl-7t-allyl complexes (53). The observation of 7t-allyl products in the reaction of the hydride with the conjugated diene, but not in the ff-alkyl intermediates involved in isomerization, illustrates the much greater stability of zr-allyl complexes of nickel compared to tr-alkyls, a feature which is also observed in the hydrocyanation reactions. 

In the following sections we shall first present some aspects of structure and function of hemes and hemoproteins, and then proceed to a presentation of some facts on NMR spectroscopy. Sections IV through VI deal with high resolution proton NMR experiments where information was obtained from direct observation of the hemoprotein resonances. This will be follwed by a brief survey of nuclear relaxation enhancement... 

The low relative sensitivity of C is coupled with a low natural abundance of C of 1.1%, so that the overall effect is to greatly reduce the signal intensity that is obtained in natural abundance C NMR measurements compared with those obtained in proton NMR experiments. 

The first NMR experiments came in 1957 (112, 130) some twelve years after the initial proton nmr experiments. Early progress was hampered by poor signal to noise ratios and the resulting need for using high sample concentrations. The development of wide band proton decoupling in the mid 1960 s led to great advancements, but it was the recent availability of the Fourier transform apparatus that has allowed NMR to become a routine analytical tool. 

Where Pr stands for C3H7. Proton NMR experiments at several temperatures have shown that the relative stability of the dihydride increases at higher temperatures. A thermodynamic analysis of the reaction led to A,7/°(59) = 5.0 2.5kJmor and 7 AA °(59)w 1.5 2.6kJmor. 2 As A,//°(59) = D7/ (W-H2)-fi)//°(H-H)-2 DH°)(W-H), the above value of Z)//°(W-H2) yields (Z)7/ ) (W-H) = 236kJ moP. This tungsten-hydrogen mean bond dissociation enthalpy in the W(ll) seven-coordinate complex is about lOOkJmoP lower than DH°(W-H) in W(Cp)(CO)3H, where the metal is in the same oxidation state ... 

Fig. 5.14 presents the same eomparison for the anionic surfactant, SDS. The experiment values were taken from the literature [54], We observe again a very good agreement between theory and experiment. No adjustable parameter was necessary here since the D° values for monomers and micelles were available from Lindman et al. [55]. These data had been obtained from Fourier Transform Proton NMR experiments ( H FT NMR) and tracer-diffusion. 

The first proton NMR experiments of diffusion in an amorphous hydride were done > on TiCuH where various relaxation times were measured. Fig. 2 compares the temperature behavior of for c-TiCuHQ g and a-TiCuHj 4 that were obtained by BPP analyses of the proton data. These results are also compared with similar parameters for cubic TiHj gQ. Significant differences in hydrogen diffusion behavior are immediately apparent in Fig. 2. A structural restriction accounts for the smaller values and larger activation energies... 

The synthesis and characterization of the samples used in this work were reported elsewhere." Proton NMR experiments were performed on a... 

Proton NMR experiments have received a relatively large amount of attention. The study of hydrogen diffusion in the hydrides is of interest both from the technological point of view and from that of fundamental physics. Systematic studies seem to be desirable in order to establish a relationship between the experimentally observed activation energies and the occupancy of particular interstitial sites. The choice should fall on hydrides for which a firm basis regarding the site occupancies has already been laid by neutron diffraction studies on the corresponding deuterides. 

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