Heteronuclear multiple bond correlation spectra

FIGURE 2.2 heteronuclear multiple bond correlation spectrum of 8-methoxykaempferol... 

C-NMR, COSY, HMQC (heteronuclear multiple quantum coherence), and HMBC (heteronuclear multiple bond correlation).48 Furthermore, the structure of trimer was confirmed by X-ray crystallography.48 The incorporation of 13C into the indole 3a position proved valuable in these structural determinations and in documenting the ene-imine intermediate. For example, the presence of a trimer was readily determined from its 13C-NMR spectrum (Fig. 7.7). 

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. 

Record the 2-D H-13C heteronuclear multiple bond correlation (HMBC) spectrum (Braun et al., 1998, pp. 489-492). 

The structural assignment of the trithiepines 44-46 has been performed using H, 13C, heteronuclear multiple bond correlation (HMQC), heteronuclear multiple quantum correlation (HMBC), and variable-temperature NMR spectroscopic data. The 60MHz H NMR spectrum of trithiepine 44 exhibits a broad singlet at 3.05 ppm in CDC13, whereas a narrow ABCD multiplet was observed for all of the protons in a 300 MHz spectrum. The two 13C NMR signals at... 

The Si NMR spectrum of pentaorganosilicate 25 was recorded at —50°C to minimize signal broadening <2004AGE3440>. H NMR, NMR, heteronuclear multiple quantum correlation (HMQC), and heteronuclear multiple bond correlation (HMBC) spectroscopic measurements were also performed. The observed 7c2,si value of 86 Hz was similar to that of a typical Si(sp )-C(sp ) bond, while the /c 2, si value of 30 Hz was much smaller than that of a Si(sp )-C(sp ) bond (64-70 Hz). 

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). 

Figure 9. Heteronuclear multiple bond correlation (HMBC) spectrum of Lomefloxacin mesylate.

The other common inverse-detection method, heteronuclear multiple quan-turn coherence (HMQC) relies on multiple-quantum coherence transitions during the pulse sequence. Due to the multiple-quantum coherence transitions it is more laborious to theoretically follow the course of magnetization, and the cross peak will be broader in the Fi dimension due to the /hh evolution. Unlike HSQC, HMQC can also be optimized for Jch couplings. This heteronuclear multiple bond correlation experiment, or HMBC, ° ° has lower sensitivity than HMQC/HSQC experiments, and the Jch correlations can appear as artefacts in the spectrum. However, the cross peak volume should follow the concentration of analyte, so with proper method validation HMQC and HMBC should also be applicable for quantification. 

When we collect a 2-D NMR spectrum, both the second frequency dimension data (fj or Fj) and the first frequency dimension data (f2 or F2) may be phase sensitive. (Note that fj and f2 appear to be reversed but this naming convention derives from the order of their time domain precedents, tj and t2, in the NMR pulse sequence.) Zero-and first-order phasing of the second dimension of a 2-D NMR data set is required in many cases. Some experiments, most notably the gradient-selected heteronuclear multiple bond correlation (gHMBC) experiment, use the absolute value of the signal and hence do not require phasing. 

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