multiple region selected 2D NMR correlation experiments

Carbon–fluorine two-dimensional NMR correlation spectra

“…Failure to obtain a good gHMQC spectrum at the higher frequency and the intensity anomalies in Fig. 1 at 470 MHz are probably attributable to inefficient excitation with 1808 pulses over the wide 19 F chemical shift range [9]. Further, the 300 Hz doublets from the 2 J FF coupling observed in Fig.…”

“…The gHMQC is more forgiving to pulse width calibration and gives better spectra (except for in-equivalent methylenes) for molecules with short transverse relaxation times and is thus normally used by organic and polymer chemists. Cheatham et al have noted that adiabatic 1808 pulses in the fluorine observe dimension are required to observe the entire 19 F spectrum and have described the use of selective adiabatic pulses to cover the several different 19 F resonances [9]. We have incorporated a similar concept in a modified gHSQC pulse sequence, gHSQCad, using a simple adiabatic WURST-n shape [15].…”

“…S4 is the complete spectrum. These spectra were observed on a probe with a 12 ms 908 fluorine pulse; probes with longer 908 pulses would require selective irradiation experiments [9]. However, HSQC should be the preferred detection scheme since very large numbers of increments, even with linear prediction, are needed for good resolution in the 19 F dimension of a 13 C detected experiment.…”

“…Excellent 2D spectra are obtained for correlations to CF and CF 3 groups and to CF 2 groups with identical chemical shifts for both fluorines over limited chemical ranges, but the paucity of data and the difficulty of the experiments has resulted in few examples in which 13 C spectra have been critical to determining structures of unknown fluorochemicals. Recently, Cheatham and Groce [9] have described the use of selective adiabatic pulses to decouple several different 19 F resonances over a wide chemical shift range. However, we are not aware of any published examples of correlations to CF 2 's in which the fluorines are chemically in-equivalent.…”

HSQC pulse sequences for 19F

“…Cheatham and Groce have proposed taking advantage of the fact that the fluorine spectrum is frequently rather sparse, thereby allowing the available RF energy to be concentrated at just a few frequencies, rather than having to be truly broadband. [6] And Newmark and Webb do report an HSQC sequence using just a single adiabatic pulse for refocusing, [3] but find that this results in unphaseable spectra.…”