Nuclear Overhauser effect positive

Hence the population difference between the lower and upper energy states of the two I transitions becomes d + x, as compared to the original difference of d at equilibrium. Thus an intensification of the lines for nucleus 1 will be observed by an amount corresponding to this increased difference x. This is the positive nuclear Overhauser effect that is encountered in small, rapidly tumbling molecules, in which Wj is the predominant relaxation pathway. 

An important first step in interpreting the C-13 spectra is to distinguish a-carbons from 3-carbons, i.e. methine from methylene. Observation of multiplicity when the proton decoupler is off is one way, but this is not always easy if the lines are broadened by chemical shift multiplicity. Measurement of has been used for this purpose since the 3-carbon with two bonded protons relaxes about twice as fast as the a-carbon with only one. A very positive way is by deuterium labelling. In Fig. 3 is shown the main-chain 25 MHz carbon spectrum of two styrene-S02 copolymers containing 58 mol% styrene, or a ratio of styrene to SO2 of 1.38 (7 ). In the bottom one, 3,3-d2-styrene has been used, cind all the 3-carbon resonances are distinguishable from the a-carbon resonances since the presence of deuterium has eliminated their nuclear Overhauser effect because of this eind the deuterium J coupling ( 20 Hz), they are markedly smaller eind broader than the a-carbon resonances. 

In solution, dipole-dipole interactions constitute a relaxation mechanism, and the dipolar relaxation which is the basis for the well-known nuclear Overhauser effect (NOE), mostly used in the homonuclear H, H case. The 2D HOESY method between H and Li has been used to obtain structural information of many organolithium systems in solution and this field was reviewed in 1995. Li is commonly used as the relaxation is dominated by the dipole-dipole mechanism and the relaxation time is relatively long. Knowledge of the proximity of the lithium cation relative to protons in the substrate is used to derive information about the structure and aggregation of organolithium systems in solution. In a few cases quantitative investigations have been made °. An average error of the lithium position of ca 0.2 A was reported. 

Steric interaction between the 1- and 9-positions in dibenzofurans has, however, been observed by NMR techniques. Thus saturation of the signal for the 1-methyl group in the H-NMR spectrum of 1,2,3,4-tetramethyl-dibenzofuran (11) produced a 38% nuclear Overhauser effect at the 9-H. This may be compared qualitatively with the 33% nuclear Overhauser effect produced on the 5-H in the spectrum of 1,2,3,4-tetramethylphenanthrene by... 

The nuclear Overhauser method has been used to demonstrate that in the dibenzofuranol 14 the position ortho to the hydroxy group is vacant. Saturation of the signal due to the benzylic protons of the benzyl group in the spectrum of the benzyl ether 15 gave a 39% nuclear Overhauser effect at the aromatic proton. 

The fragmentation of a range of 1,3-dioxolanes has been examined using positive-ion Cl mass spectrometry [95RCM405] and nuclear Overhauser effects in the NMR spectra of 4-trimethylammoniomethyl-2,2-disubstituted-1,3-dioxolanes have been used to study their conformational properties [95MRC167]. 

Tetracoordinate l,2A4-oxaselenetane 47 is synthesized by a ring-closure reaction of /3-hydroxylalkyl selenide in 54% yield <1997CC1671>. Similarly, oxaselenetanes 48, 35, and 36 are obtained in 44%, 56%, and 65% yields, respectively (Scheme 14) <1997CC1671, 1998PS501>. These compounds were found to be air stable, colorless plates at room temperature, and the relative stereochemistry between the 3- and 4-positions of 35 and 36 has been determined by differential nuclear Overhauser effect (NOE) experiments. 

Irie et al.28 obtained experimental evidence for the most stable conformation of the open form by using H NMR nuclear Overhauser effect (NOE) measurements. Irradiation of the protons in the NCH3moiety (3.79 ppm) produced positive NOEs of 10% at the H7 aromatic proton (7.49 ppm) and 19% at the H a olefinic proton. Irradiation of this a hydrogen produced a 12% enhancement of the NCH 3 proton, as illustrated in Figure 2.8. These observations indicated that the geometrical structure of the colored open form of spirooxazine is the TTC form. 

Meyer et al. recently reported transfer NOE (nuclear overhauser effect) (trNOE) (88), which detects the strong negative trNOE effect of receptor-bound molecules compared to the weak, positive trNOE of unbound compounds without the need of receptor labeling the method was used to identify an E-selectin antagonist from an artificially assembled 10-member library of saccharides (89). 

FruA is stereoselective and stereocenter 2 is created with the SNuclear Overhauser Effects (NOEs) observed, and the coupling constants measured, the configurations found for compound 11 were IS, 2S, 3R, and 6R. AH the substituents are equatorially positioned on the cyclohexane ring. The (2S,3R)-configurations are consistent with the usually observed diastereoselectivity of FruA In a second exploration [20], the racemic aldehyde 10 was condensed with DHAP after ketal hydrolysis of rac-14 under acidic conditions. In this case, two major isomers 12 and 13 were isolated in 35 and 29% yields after flash chromatography purification. 

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