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HMBC Spectra (hmbc + spectrum)
Selected AbstractsSuppressing One-Bond Correlations in HMBC Spectra: Improved Performance for the BIRD,HMBC Pulse SequenceMAGNETIC RESONANCE IN CHEMISTRY, Issue 3 2009Julien Furrer Abstract An improved version of the BIRD,HMBC experiment is proposed. In comparison to the original version, the filtering (suppression of 1JCH signals) is accomplished using a double tuned G-BIRD filter positioned in the middle of the long-range correlations evolution period. Compensation of offset dependence by replacing the rectangular 180° pulses with the broadband inversion pulses (BIPs), with superior inversion performance and improved tolerance to B1 field inhomogeneity, significantly improves the sensitivity of the original BIRD,HMBC experiment. For usual one-bond coupling constants ranges (115,180 Hz), optimal results are easily obtained by adjusting the delays, ,, of the BIRD elements to an average J value. For larger ranges (e.g. 110,260 Hz), the use of a double tuned G-BIRD filter allows excellent suppression degrees for all types of one-bond constants present in a molecule, superior to the original scheme and other purging schemes. These attributes make the improved version of the BIRD,HMBC experiment a valuable and robust tool for rapid spectral analysis and rapid checks of molecular skeletons with a minimum spectrometer time. Copyright © 2009 John Wiley & Sons, Ltd. [source] NMR Characterization of Complex p- Oligophenyl Scaffolds by Means of Aliasing Techniques to Obtain Resolution-Enhanced Two-Dimensional SpectraHELVETICA CHIMICA ACTA, Issue 9 2004Damien Jeannerat The usefulness of computer-assisted aliasing to secure maximal resolution of signal clusters in 1H- and 13C-NMR spectra (which is essential for structure determination by HMBC 2D NMR spectroscopy) in minimal acquisition time is exemplified by the complete characterization of the two complementary p -octiphenyls 1 and 2 with complex substitution patterns. The need for digital resolution near 1,Hz/pt to dissect the extensive signal clusters in the NMR spectra of these refined oligomers excluded structure determination under routine conditions. High resolution was secured by exploiting the low signal density in the 13C dimension of HMBC spectra by using computer-assisted aliasing to maximize signal density. Based on the observed shifts in DEPT and 1H-decoupled 13C-NMR spectra of 1 and 2, computer-assisted aliasing allowed to reduce the number of required time increments by a factor of 20 to 30 compared to full-width spectra with identical resolution. Without signal-to-noise constraints, this computer-assisted aliasing reduced the acquisition time for high-resolution NMR spectra needed for complete characterization of refined oligomers 1 and 2 by the same factor (e.g., from over a day to about an hour). With resolved signal clusters in fully aliased HSQC and HMBC spectra, unproblematic structure determination of 1 and 2 is demonstrated by unambiguous assignment of all C- and H-atoms. These findings demonstrate that computer-assisted aliasing of the underexploited 13C dimension makes extensive molecular complexity accessible by conventional multidimensional heteronuclear NMR experiments without extraordinary efforts. [source] Synthesis and structure of some imines containing furoxan ring derived from isosafroleJOURNAL OF HETEROCYCLIC CHEMISTRY, Issue 6 2004Nguyen Huu Dinh A series of 14 imines containing furoxan and benzene rings has been prepared starting from isosafrole. The structure of reported compounds have been confirmed by elemental analysis, EI MS, UV, IR, and NMR spectroscopy. It is shown that, on treatment with Na2S2O4, the nitro group on the benzene ring was reduced to amino group, but the N,O group of the furoxan ring was not. The 1H- and 13C NMR signals are assigned based on their spin-spin splitting patterns, in some cases, NOESY and HMBC spectra are used. The NOESY spectra indicate that for reported imines, the benzene and the furoxan rings could not be co-planar; the imine group has E -configuration. [source] Multiplicity-edited broadband HMBC NMR spectraMAGNETIC RESONANCE IN CHEMISTRY, Issue 4 2006Nils T. Nyberg Abstract A new, edited HMBC experiment is introduced that leads to two subspectra according to the number of protons attached to 13C nuclei being even or odd, i.e. one subspectrum with C + CH2 and another with CH + CH3. This experiment can be useful for resolving spectral overlap among the typically large number of peaks in HMBC spectra. It is implemented in a broadband version similar to broadband HMBC and demonstrated on prednisolone [(11,)-11,17,21-trihydroxypregna-1,4-diene-3,20-dione]. Copyright © 2006 John Wiley & Sons, Ltd. [source] Complete assignments of 1H and 13C NMR data for ten phenylpiperazine derivativesMAGNETIC RESONANCE IN CHEMISTRY, Issue 10 2005Zhihui Xiao Abstract Ten phenylpiperazine derivatives were designed and synthesized. The first complete assignments of 1H and 13C NMR chemical shifts for these phenylpiperazine derivatives were achieved by means of 1D and 2D NMR techniques, including 1H,1H COSY, HSQC and HMBC spectra. Copyright © 2005 John Wiley & Sons, Ltd. [source] Synthesis and NMR spectral study of some t(3)-aryl- r(2),c(4)-bisethoxycarbonyl- t(5)-hydroxy- c(5)-methylcyclohexanonesMAGNETIC RESONANCE IN CHEMISTRY, Issue 5 2005K. Pandiarajan Abstract Six t(3)-aryl- r(2),c(4)-bisethoxycarbonyl- t(5)-hydroxy- c(5)-methylcyclohexanones (6,11) were synthesized by condensing ArCHO (Ar = Ph, p -O2NC6H4, p -CH3OC6H4, p -ClC6H4, m -O2NC6H4 and m -C6H5O6H4) with ethyl acetoacetate in the presence of methylamine and their 1H and 13C NMR spectra were recorded. 1H,1H COSY and NOESY spectra were recorded for 6 and 7 and also HSQC and HMBC spectra for 6 and 8. Elemental analysis was carried out for all compounds. The mass spectrum was recorded for 8. All analytical data are consistent with the proposed molecular formulae. Analysis of NMR spectral data suggests that these compounds largely adopt chair conformations with the hydroxyl group occupying an axial orientation and all the other substituents occupying equatorial orientations. Long-range coupling (2,3 Hz) between the OH proton and the axial methylene proton at C-6 is observed in 6, 7, 8 and 11. Copyright © 2005 John Wiley & Sons, Ltd. [source] Complete assignments of 1H and 13C NMR data for 10 phenylethanoid glycosidesMAGNETIC RESONANCE IN CHEMISTRY, Issue 7 2004Jun Wu Abstract Ten phenylethanoid glycosides, including two new ones, isolated from the aerial parts of the mangrove plant Acanthus ilicifolius were identified. The first complete assignments of the 1H and 13C NMR chemical shifts for these glycosides were achieved by means of 2D NMR techniques, including 1H,1H COSY, TOCSY, HSQC and HMBC spectra. Copyright © 2004 John Wiley & Sons, Ltd. [source] High resolution in heteronuclear 1H,13C NMR experiments by optimizing spectral aliasing with one-dimensional carbon dataMAGNETIC RESONANCE IN CHEMISTRY, Issue 1 2003Damien Jeannerat Abstract In the chemistry literature it is common to provide NMR data on both proton and carbon spectra based on one-dimensional experiments, but often only proton spectra are assigned. The absence of a complete attribution of the carbons is in good part due to the difficulty in reaching the necessary resolution in the carbon dimension of two-dimensional experiments. It has already been shown that high-resolution heteronuclear spectra can be acquired within nearly the same acquisition time using a violation of the Nyquist condition. For a spectral width reduction by a given factor k, the resolution increases by the same factor as long as it is not limited by relaxation. The price to pay for such an improvement is a k -fold ambiguity in the chemical shift of the signal along the folded or aliased dimension. The computer algorithm presented in this paper takes advantage of the peak list stemming from one-dimensional spectra in order to calculate spectral widths for which the ambiguities in the aliased dimension of heteronuclear experiments are eliminated or at least minimized. The resolution improvement factor is only limited by the natural lineshape and reaches a typical value higher than 100. The program may be set to run automatically on spectrometers equipped with automatic sample changers. Applications to short-range HSQC experiments and long-range HMBC spectra of steroids, carbohydrates, a peptide and a mixture of isomers are shown as examples. Copyright © 2002 John Wiley & Sons, Ltd. [source] | ||||||||||