Heteronuclear multiple bond correlation chemical shifts

In order to assign the chemical shifts of the carbon atoms of the conjugated diene system of each CLA isomer, it was necessary to conduct INADEQUATE, HMBC (heteronuclear multiple bond correlation) and two-dimensional 1H-13C correlation spectroscopy (COSY) techniques on the carbon signals of the diene system of the ,Z-isomers. The results of these experiments for the CLA isomers are summarized in Table 13. 

Complete N NMR chemical shifts together with H- N coupling constants (referenced externally to NH4C1 in DMSO-1 6 DMSO - dimethyl sulfoxide) have been determined for toxoflavin 7 and fervenulin 8 in CDCI3 using pulsed field gradient heteronuclear multiple bond correlation (HMBC) techniques <2000H(52)811>, and are presented in Table 1. 

Notably, two isomeric products can be generated. The usual infrared (IR) and mass spectra as well as H and 13C NMR chemical shifts could not define which isomer was formed. The authors used different NMR techniques, such as 2-D heteronuclear multiple bond correlation (HMBC) experiments and phase-sensitive nuclear overhauser enhancement spectroscopy (NOESY) measurements to elucidate the product s structure. 

The fatty acyl substituents were mainly of three types saturated straight-chain C,6-C,9 acids C21-C25 mycosanoic acids and C24-C28 mycolipanolic acids. Analysis of one of the major 2,3-di-O-acyltrehaloses by two-dimensional H-chemical-shift-correlated and H-detected heteronuclear multiple-bond correlation spectroscopy established that the C18 saturated straight-chain acyl group was located at the 0-2 position and that the C24 mycosanoyl substituent was at the 0-3 position of the same nonglycosylated terminus (structure 8). At least six molecular... 

Fourier transform or transformation heteronuclear chemical-shift correlation heteronuclear multiple bond correlation heteronuclear multiple quantum correlation heteronuclear single quantum correlation... 

The two-dimensional, heteronuclear multiple quantum correlation (HMQC) and heteronuclear multiple bond correlation (HMBC) spectra were taken with standard Bruker pulse programs. The HMQC and HMBC spectra are given in Figures 8 and 9 (9). The chemical shifts and spectral assignments are provided in Table 2 (9,10). The effect of Al3+ on the carbon spectrum of lomefloxacin is shown in Figure 10 (9). 

NMR is the tool most widely used to identify the structure of triterpenes. Different one-dimension and two-dimension techniques are usually used to study the structures of new compounds. Correlation via H-H coupling with square symmetry ( H- H COSY), homonuclear Hartmann-Hahn spectroscopy (HOHAHA), heteronuclear multiple quantum coherence (HMQC), heteronuclear multiple bond correlation (HMBC), distortionless enhancement by polarisation transfer (DEPT), incredible natural abundance double quantum transfer experiment (INADEQUATE) and nuclear Overhauser effect spectroscopy (NOESY) allow us to examine the proton and carbon chemical shift, carbon types, coupling constants, carbon-carbon and proton-carbon connectivities, and establish the relative stereochemistry of the chiral centres. 

Bax A, Davis DG (1985) MLEV-17 Based two-dimensional homonuclear magnetization transfer spectroscopy. J Magn Reson 65 355-360 Bax A, Drobny G (1985) Optimization of two-dimensional homonuclear relayed coherence transfer NMR spectroscopy. J Magn Reson 61 306-320 Bax A, Marion D (1988) Improved resolution and sensitivity in H-detected heteronuclear multiple-bond correlation spectroscopy. J Magn Reson 78 186-191 Bax A, Subramanian S (1986) Sensitivity-enhanced two-dimensional heteronuclear chemical shift correlation NMR spectroscopy. J Magn Reson 67 565-569 Bax A, Summers MF (1986) and Assignments from sensitivity-enhanced detection of heteronuclear multiple bond connectivity by 2D multiple-quantum NMR. J Am Chem Soc 108 2093-2094... 

To date, the only inverse-detected heteronuclear chemical shift correlation experiment available for multiple-bond correlation is the HMBC experiment of Bax and Summers (1986). The pulse sequence is shown in Fig. 5 and... 

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