D NMR

Fig. 5. A 2-D nmr experiment of 2-methyl-5-bromopentane [626-88-0] where and correspond to evolution and acquisition time, respectively.

Online detection using 4H nuclear magnetic resonance (NMR) is a detection mode that has become increasingly practical. In a recent application, cell culture supernatant was monitored on-line with 1-dimensional NMR for trehalose, P-D-pyranose, P-D-furanose, succinate, acetate and uridine.33 In stopped-flow mode, column fractions can also be analyzed by 2-D NMR. Reaction products of the preparation of the neuromuscular blocking compound atracurium besylate were separated on chiral HPLC and detected by 4H NMR.34 Ten isomeric peaks were separated on a cellulose-based phase and identified by online NMR in stopped-flow mode. 

Tm data 2-D NMR CD fluorescence propose rigid oxidant in highly-ordered environment... 

If 2-D NMR techniques are really useful then 3-D ones must be even more so... shouldn t they A number of 3-D experiments have been devised which are in fact, produced by merging two, 2-D experiments together. The results could never be plotted in true 3-D format since etching them into an A3-sized block of glass would not be practical and viewing them as some sort of holographic projection, would probably not be feasible In essence, 3-D spectra have to be viewed as slices through the block which effectively yield a series of 2-D experiments. It is possible to combine techniques to yield experiments such as the HMQC-COSY and the HSQC-TOCSY. 

The relative stereochemistry of hyperaspine 93 was determined by 2-D NMR spectroscopic and mass spectrometry (MS) methods. It has a m-fused bicyclic conformation 93a <2001TL4621>. The trans-fused one is disfavored by an axial pentyl group at C-8 and by a destabilizing dipole-dipole interaction between the N- and O-atoms, which does not exist in the alternative //.(-conformation. The geminal coupling constant of C( 1 )H2 in 93 (11.0 Hz), and that of its 6-hydroxy derivative (11.2 Hz), indicates that they exist preferentially in / //-conformations, whereas their 6-epimers adopt trans-conformations (9.3 and 8.4 Hz, respectively) <2005EJ01378>. Nuclear Overhauser enhancement spectroscopy (NOESY) studies also confirmed the stereochemistry of 93 by the marked nuclear Overhauser effect (NOE) correlation between H-3 and H-4a <20030L5063>. 

The disposition of the bridgehead proton with respect to the protons in position-3 and -6 was studied by ROESY 2-D NMR in four sets, 288-291 of 16 compounds bearing different substituents. The compounds were obtained via solid-phase syntheses by tandem cyclization <2004TL6333>. 

Although many pulse sequences have been investigated for 2-D NMR, only a very limited number are of practical interest. One deterrent to their more widespread use is the time required and hence the cost of collecting the data. The time factor arises because the pulse sequences have to be repeated hundreds of times whilst varying the time interval during the evolution period, t13 in order to provide sufficient data for the final 2-D contour plot. Three of the more valuable 2-D techniques are described below. 

Methods for measuring Ti and T2 are discussed in Chapter 5 of reference 21. Suffice it to say here that understanding the method for measuring T2 (the Carr-Purcell pulse sequence or spin-echo method) becomes important for discussing two-dimensional NMR spectra. When spin-spin coupling is present, a modulation of spin echoes is produced, and it is this fact that is important in 2-D NMR spectroscopy. Nuclear relaxation rates and mechanisms become important when discussing the effect of paramagnetic metal centers on NMR spectroscopy. 

Our results confirmed the feasibility of this strategy (Zeng et al. 2002). Peptide segments were attached to an unsymmetrical, 4-H bonded heteroduplex template with the complementary sequences of ADAA/DADD, leading to four hybrid strands 13a, 13b, 14a, and 14b. Combination of strands 13a and 13b with strands 14a and 14b resulted in four different pairs. The formation of well-defined /3-sheets by different combinations of paired peptide sequences was confirmed by 1-D and 2-D NMR, ITC, and VPO smdies. For example, 2-D NMR (NOESY) showed interstrand NOEs corresponding to protons of duplex templates and the... 

The presence of intramolecular H bonds in the cross-linked products was probed by carrying out 2-D NMR (NOESY) experiments on cross-linked 26-27, which contains two interstrand intramolecular H bonds. It was found that the same cross-strand NOEs were detected in CDCI3 (Fig. 9.18a) and in H20/THF-r(8 (80/20, v/v Fig. 9.18b). Furthermore, diluting 26-27 from 9 to 0.2 mM in the same aqueous solvent led to no shift in the two aninline NH signals, which remained at 9.75 ppm. These results suggest the persistence of an intramolecular H bonded conformation for 26-27 in aqueous media, which is consistent with the thermodynamic mechanism shown in Figure 9.17. 

The stereochemistry of a large number of 2- and 5-substituted-l,3-dioxane derivatives has been studied by the full arsenal of 2-D NMR spectroscopy and accompanied by X-ray crystallography, ortho- and ra-Substituted benzenes 74 and 75 adequately show anancomeric structures with the aryl substituent preferring an equatorial position, the 5-position substituents Me and Ph pteferring equatorial orientations, while the ester substituents favor axial dispositions <1997LA2371, 1998HC053, 1998ACS366>. 

The condensation of furo[3,2- ]pyrrole-type aldehydes 8g and 265-267 with hippuric acid was carried out in dry acetic anhydride catalyzed by potassium acetate as is shown in Scheme 26. The product methyl and ethyl 2-[( )-(5-oxo-2-phenyl-l,3-oxazol-5(4//)-ylidene)methyl]furo[3,2- ]pyrrol-5-carboxylates 268a-d were obtained. The course of the reaction was compared with the reaction of 5-arylated furan-2-carbaldehydes with hippuric acid. It was found that the carbonyl group attached at G-2 of the fused system 8 is less reactive than the carbonyl group in 5-arylated furan-2-carbaldehydes in this reaction <2004MOL11>. The configuration of the carbon-carbon double bond was determined using two-dimensional (2-D) NMR spectroscopic measurements and confirmed the (E) configuration of the products. 

Aryl derivatives of thieno[3,2-f]pyridines, 36 and 37, have been the subject of two-dimensional (2-D) NMR studies. Phase-sensitive nuclear Overhauser enhancement spectroscopy (NOESY) and correlation spectroscopy (COSY) experiments confirm the nonplanar conformation of the two aromatic ring systems <1999SAA1035>. 

The identity of a glucosylated derivative of L-biopterin from Synechococcus sp. PGG 7942 has been confirmed as the 2 -0-a-glucosyl derivative by 2-D NMR spectroscopy <2001PTR121>. Tetrahydro-D-monapterin 223 has been identified as the native pteridine in Tetrahymena pyriformis <2001HGA918> the configuration and stmcture were confirmed by comparison with synthetic samples of known configuration. 

The structural assignment of both 29 and 30 was accomplished through extensive two-dimensional (2-D) NMR heteronuclear multiple quantum correlation (HMQC) and heteronuclear multiple bond correlation (HMBC) spectroscopic studies <2004T8189>. In the HMBC spectrum of 29, the proton at 8.64p.p.m. shows a strong correlation Jq-h with the carbonyl carbon (C-10) at 180.9 ppm and the proton at 8.82p.p.m. with the carbonyl carbon (C-5) at 181.7 ppm. The HMBC spectrum of 30 shows a significant strong correlation Vq h of the C-5 carbonyl carbon with the H-6 proton at 8.52 ppm and the H-4 proton at 8.52p.p.m. 

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