Very low field 19F MRI of perfluoro-octylbromide: Minimizing chemical shift effects and signal loss due to scalar coupling
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19F images have been obtained from perflurooctylbromide (PFOB) at very low magnetic field (50 mT). The small spectral dispersion (in Hz) means that all fluorine nuclei contribute to the signal without chemical shift artifacts or the need for specialized imaging sequences. Turbo spin echo trains with short interpulse intervals and full 180° refocussing pulses suppress scalar coupling, leading to long apparent T2 values and highly efficient data collection. Overall, the detection efficiency of PFOB is very similar that of water in tissue.Keywords:
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Abstract 13 C and 1 H NMR chemical shifts of 9‐methyltriptycene (2), 9‐ethyltriptycene (3), 9‐isopropyltriptycene (4) and 9‐ tert ‐butyltriptycene (5) were measured. The 13 C chemical shifts of the carbon atoms at each corresponding position in 2‐5 are proportional to the number of β‐methyl carbon atoms attached to the 9‐methyl (C‐α) carbon atom. Relatively small changes in the 13 C chemical shifts of C‐2, C‐3 and C‐4 carbon atoms and nearly constant values for the 1 H chemical shifts of the H‐2, H‐3 and H‐4 protons in 2‐5 suggest that the structures of the benzene rings in the 9‐alkyltriptycenes remain almost unaltered, resulting in linear variations of the 13 C chemical shifts of the 9‐alkyltriptycenes.
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35 N-(Phenyl)-, N-(2/4-chlorophenyl)- and N-(2/4-methylphenyl)-substituted acetamides are prepared, characterised and their NMR spectra studied in solution state. The variation of the chemical shifts of the aromatic protons in these compounds follow more or less the same trend with changes in the side chain. The chemical shifts remain almost the same on introduction of Cl substituent to the benzene ring, while that of methyl group lowers the chemical shifts of the aromatic protons. But only 13 C-1 and 13 C-4 chemical shifts in these compounds are sensitive to variations of the side chain. The incremental shifts in the chemical shifts of the aromatic protons and carbons due to -COCH 3−i X i or NHCOCH 3−i X i groups in all the N-(phenyl)-substituted acetamides, C 6 H 5 NHCOCH 3−i X i (where X = Cl or CH 3 and i = 0, 1, 2 or 3) are calculated. These incremental chemical shifts are used to calculate the chemical shifts of the aromatic protons and carbons in all the N-(2/4-chlorophenyl)- and N-(2/4-methylphenyl)-substituted acetamides, in two ways. In the first way, the chemical shifts of aromatic protons or carbons are computed by adding the incremental shifts due to -COCH 3−i X i groups and the substituents at the 2 nd or 4 th position in the benzene ring to the chemical shifts of the corresponding aromatic protons or carbons of the parent aniline. In the second way, the chemical shifts are calculated by adding the incremental shifts due to -NHCOCH 3−i X i groups and the substituents at the 2 nd or 4 th position in the benzene ring to the chemical shift of a benzene proton or carbon, respectively. Comparison of the two sets of calculated chemical shifts of the aromatic protons or carbons of all the compounds revealed that the two procedures of calculation lead to almost the same values in most cases and agree well with the experimental chemical shifts.
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Abstract Proton and carbon‐13 chemical shifts of para‐substituted stilbenes have been measured. 1 H‐ 1 H, 1 H‐ 13 C COSY spectra were obtained to analyze unambiguously the chemical shifts of protons and carbons. A long range coupling between 2‐H and α‐H was observed in a 1 H‐ 1 H COSY spectrum. The observed chemical shifts have been correlated with Hammett substituent parameters. Among ethenyl protons and carbons, all but the chemical shifts of α‐H show good correlation with both dual substituent parameters and single substituent parameters. In addition to this finding, the excellent linear correlations of C‐l, and 4′‐H of 4‐substituted trans‐stilbenes are also reported. Besides the correlations of chemical shifts with Hammett parameters, a good correlation between the chemical shifts and the calculated charges of position C‐4′ are reported.
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Substituent effects in para-substituted 5-arylmethylenehydantoins have been studied by correlations of 1H and 13C chemical shifts with Hammett constants. In addition to the expected correlations for the 1H chemical shift of the vinyl proton and the 13C chemical shift of the β-carbon C-5, good linear correlations are also obtained for the chemical shifts of the N-1 and N-3 protons which are respectively four and five atoms away from the benzene ring as well as for the α-carbon C-6.
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Abstract 15 N, 17 O and 33 S NMR chemical shifts were determined for some aliphatic and aromatic sulphonamides, sulphinamides, sulphenamides and related sulphones and sulphoxides. The 17 O and 33 S NMR chemical shifts change only slightly for the sulphonyl compounds. In the sulphinyl compounds, on the other hand, the presence of nitrogen causes a noticeable shift to higher frequencies in the 17 O resonance. The differences between the 17 O chemical shifts of sulphinyl and sulphonyl compounds are more noticeable than those between sulphinamides and sulphoxides. The 15 N NMR chemical shifts of sulphon‐, sulphin‐ and sulphen‐amides reflect well the effect of the environments of both nitrogen and the adjacent sulphur atom. The correlations between 15 N, 17 O and 33 S NMR chemical shifts and the structures of sulphur amides and related sulphones and sulphoxides are discussed. The chemical shifts of the 13 C nuclei are also presented.
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Abstract 8‐Hydroxyflavone is not found in nature. While the 13 C chemical shifts of 8‐hydroxyflavone have been reported previously, the observed 13 C chemical shifts were not assigned. A previously reported empirical predictive tool has been applied in reverse in order to deconvolute the 13 C chemical shifts for 8‐hydroxyflavone from each of those of 7,8,4′‐trihydroxyflavone and 7,8‐dihydroxyflavone together with those of 7‐hydroxyflavone, 4′‐hydroxyflavone, and flavone. The two sets of calculated 13 C chemical shifts for 8‐hydroxyflavone are in good agreement with each other in that the average absolute difference is 0.4 ppm. The previously reported but unassigned 13 C chemical shifts for 8‐hydroxyflavone have been assigned by matching them with the averages of the two sets of calculated 13 C chemical shifts for 8‐hydroxyflavone such that the minimum average absolute difference is 0.63 ppm. The assigned 13 C chemical shifts of 8‐hydroxyflavone may be used, along with the 13 C chemical shifts of the remaining monohydroxyflavones, as part of a predictive tool to rapidly assess the 13 C NMR spectra of C8‐hydroxylated flavonoids. Copyright © 2008 John Wiley & Sons, Ltd.
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