Homonuclear NOE

In the case of proton-proton interactions, both nuclei S and /will have the same gyromagnetic ratios, and an implication of the Equation (4) then is that there is an upper limit of 50% on the nOe obtainable, whatever the distance between nuclei S and I. This means that the observation of an nOe between two nuclei does not necessarily mean they are spatially close to one another, and nOe results must therefore be interpreted with caution. Similarly, as will be seen later, the absence of nOe between two nuclei does not necessarily mean they are far apart. In the case of heteronuclear nOe, since the gyromagnetic ratio of proton (yv) is four times the gyromagnetic ratio of carbon y,), js/ jt can be four times greater than that obtainable in homonuclear nOe. 

So far we have been concerned with homonuclear nOe effects. nOe between nuclei of different elements can also be a useful tool for structural investigations. Such heteronuclear nOe effects—for instance, between protons and carbons—can be used with advantage to locate quaternary carbon atoms. Normally, beteronuclear nOe effects are dominated by interactions between protons and directly bonded carbon atoms, and they can be recorded as either ID or 2D nOe spectra. 

In a heteronuclear nOe experiment, the first step may be identical to the homonuclear nOe experiment (i.e., involving an incoherent transfer of coherence from H to H/), while the second step could involve a coherence transfer from H to C nucleus by an INEPT sequence. These methods suffer from poor sensitivity and have therefore not been used extensively. 

We have concentrated on the proton-proton, homonuclear NOE experiments in this section but the potential use of analogous heteronuclear experiments should not be overlooked, if you have the appropriate hardware available to you. The 19F- 1H NOE experiment, for example, can be very useful in certain situations as demonstrated in the following example. You have one of two possible positional isomers (Structure 8.6). 

NOESY Homonuclear NOE (nuclear Overhauser) spectroscopy To elucidate structure of organic molecules To determine the spatial proximity of nuclei... 

The relative values of /, J, y/, yj are important for determining the size and the sign of heteronuclear NOE obviously, in homonuclear NOE they cancel each other and Eq. (7.7) takes the simple form... 

In Appendix VII, where the steady state NOE has been derived, the equation for the NOE as a function of the irradiation time is also derived. In the case of homonuclear NOE, it is... 

The homonuclear NOE, almost always between two protons, is used to measure distances and determine stereochemical relationships. The NOE intensity (the percent increase or decrease of z magnetization observed at a nearby proton) is proportional to the inverse sixth power of distance between the two protons (1/r6), and is generally too weak to be observed for distances over 5 A. 

NOES Y and ROESY (Fig. 10.13) correlate protons with other protons via their homonuclear NOE interactions. A NOESY spectrum looks very much like a COSY, except that the crosspeaks correspond to pairs of protons that are close in space (<5 A) and not necessarily close in the bonding network. The intensities of crosspeaks are roughly proportional to 1/r6, where r is the direct through-space distance between the two protons correlated by the crosspeak. 

In the reaction of 3-nitro-177-1,2,4-triazole with benzyl chloride it was possible to obtain l-benzyl-3-nitro-177-l,2,4-triazole (60%), l-benzyl-5-nitro-17/-l,2,4-triazole (10%), and 4-benzyl-3-nitro-177-l,2,4-triazole (4%), whose structures were determined by H and 13C NMR with homonuclear NOE difference spectroscopy (Table 3.21) [559], The tautomerism of C-nitro-substituted 1,2,4-triazoles was studied by NMR and IR spectroscopy [582], 31P NMR spectroscopy was employed for the identification of nitrated triazolyl-1 phosphates [583, 584],... 

We will discuss homonuclear NOE in Section 12.7, but before we leave the topic of heteronuclear NOE, try the example below. 

EXAMPLE 12.7 What is the maximum NOE signal enhancement expected in a homonuclear NOE experiment ... 

The nuclear Overhauser effect (NOE), caused by dipole-dipole crossrelaxation, has great potential in the elucidation of the molecular structure and conformation (Noggle and Shirmer, 1971 Hall and Sanders, 1980). A homonuclear NOE can theoretically be as large as 50%, but is usually much smaller, and depends on the inverse sixth power of the distance between the nuclei, so that the relative magnitudes of enhancements reflect the spatial relationships of the atoms involved. 

Having taken the trouble to see how the relaxation rates in a two-spin system depend on molecular motion, we are now in a position to predict the behaviour of the NOE itself as a function of this motion and of intemuclear separation. Taking the rale constant Eq. (8.4) and substituting these into that for the NOE Eq. ((8.2)) produces the curve presented in Fig. 8.8 for the theoretical variation of the homonuclear NOE as a function of molecular tumbling rates as defined by (where u>o is the spectrometer observation frequency, approximately equal to uii and ujs). Note this is for a two-spin system, which relaxes solely by the dipole-dipole mechanism and as such represents the theoretically maximum possible NOE. The curve has three distinct regions in it, which we shall loosely refer to as the fast, intermediate and slow motion regimes. For those molecules that tumble rapidly in solution (short those in the extreme narrowing limit), the NOE has a... 

Seebach D, Ko SY, Kessler H, Kock M, Reggelin M, Schmieder P, Walkinshaw MD, Bolsterli JJ, Bevec D (1991) 185. Thiocyclosporins preparation, solution and crystal structure, and immunosuppressive activity. Helv Chim Acta 74 1953-1990 Sklenar V, Bax A (1987) Two-dimensional heteronuclear chemical-shift correlation in proteins at natural abundance and levels. J Magn Reson 71 379-383 Sohn K, Opella SJ (1989) Determination of homonuclear nOe between amide sites in protins with heteronuclear correlation spectroscopy. J Magn Reson 82 ... 

The dipole-dipole cross-relaxation mechanisms that give rise to homonuclear NOEs can just as easily produce heteronuclear NOEs. The heteronuclear NOE is often used to increase the intensity of the heteronuclear signal. For example, saturation of protons during acquisition of spectra gives rise to an NOE on carbon of just under 2 (eqn [6]). Thus, the net intensity of carbon signals in small molecules is up to three times greater than it would be without the NOE. 

The same homonuclear NOE methods have been used to investigate the interactions between different chains. These typically become obvious in solution at weight concentrations in excess of 40%. Polystyrene (PS) and poly(vinylmethyl ether) have been shown to blend on the molecular scale in concentrated solution in toluene, but not in chloroform [40, 41]. The ether methoxyl protons closely approach all the aromatic protons. With a similar experiment where PS is replaced by polycaprolactone [40], all the latter s chain methylene protons sense the proximity of the ether methoxyl. 

This chapter describes the application of the complete relaxation matrix method to homonuclear NOE data of the SRP 28mer and the determination of a preliminary high fresolution structure of the latter. 

NOE is widely used in three-dimensional structure determination of molecules, especially for proteins in solution. NOE-based NMR experiments provide an effective means to investigate nanostructural organizations in ionic Hquids. Homonuclear NOE experiments (NOESY) and heter-onuclear NOE experiments (HOESY) have been extensively appfred to probe intra- and intermolecular interactions within ILs and interaction of ILs with solvents and inorganic salts. Due to the abihty to probe dipolar coupled through space interactions, NOE is a perfect choice to probe interionic interactions to understand ion-pair dynamics. Since quantum chemical calculations are a convenient method to study ion-pair interactions and spatial information about them [76-78], combinations of this with NOE experiments would be a powerful approach to understand site-specific ion-pair interactions. 

Heteronuclear Chemical Shift Correlation (HETCOR, HM()C).. 539 Homonuclear NOE Correlation.. . 540 NMR Spectral Collections. 

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