Pulse interferograms

If a sample contains equivalent nuclei A (13C) subject to spin-spin coupling with nuclei X ( H), the transverse magnetization arises from two or more Larmor frequencies, depending on the multiplicity. The corresponding magnetization vectors periodically rephase and dephase with the field vector B, as in the off-resonance case with one Larmor frequency (Section 2.4.1). The FID signal is thus modulated by the frequency of the coupling constant JAX [7,13] as illustrated in Fig. 2.5 (a) for hexadeuteriodimethyl sulfoxide. 

Similarly, in a sample containing two nonequivalent nuclei Ax and A2, the transverse magnetization results from two components due to two Larmor frequencies. In this case, the FID signal is modulated by the chemical shift difference of Larmor frequencies, Ay = Vj — v2. This modulation is illustrated in Fig. 2.6(a) by the FID signal of a sample of 1 -13C-n -glucose after mutarotation. The product mixture of mutarotation contains a-and /i-glucopyranose with differently shielded glycosidic carbons separated in the 13C NMR spectrum by 87.5 Hz. 

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Figure 1.7. Pulse interferogram and FT NMR spectrum of glycerol, DOCH2)2CHOD, [020, 25 °C, 100 MHz]...

If NMR spectra are computed by Fourier transformation of pulse interferograms (Chapter 2), complex quantities are used during computation. Real and imaginary components v (o>) and i u(io) of the NMR spectrum are obtained as a result. Magnitude or power spectra P io) can be computed from the real and imaginary parts as follows ... 

In most pulsed NMR experiments, the rf field is applied off-resonance. Modulated pulse interferograms (Fig. 2.4(e), 2.5(a), 2.6(a), and 2.7(a)) arise because the vectors of transverse magnetization do not precess with a constant phase shift of itj2 relative to the vector Bj. This is demonstrated in Fig. 2.8. The transverse magnetization is then a resultant of two components, t>(f) with a phase shift of n/2 relative to B, and u(r) in phase with Bt ... 

When several pulse interferograms must be accumulated in order to improve the signal noise ratio, x is the minimum repetition time between two pulses or the minimum pulse interval. 

Fig. 2.14. 22.63 MHz PFT 13C NMR spectra of 3-methyl-5,6,7,8-tetrahydroquinoline 200 mg/mL deuteriochloroform proton decoupled 512 accumulated pulse interferograms pulse width 10 ps pulse interval 0,4 s ...

Fig. 2.16. Phase corrected 22.63 MHz PFT 1 C NMR spectrum of mutarotated D-galactose 100 mg/mL deuterium oxide 30 C proton decoupled 512 accumulated pulse interferograms ...

An obvious way to avoid signal noise attenuation for slowly relaxing carbons is to add a delay time to the pulse interval. This relaxation delay should be at least in the order of 3 7]. However, accumulation of pulse interferograms becomes time consuming by this method. 

Fig. 3.8. 22.63 MHz PFT t3C lH NMR spectrum of glycylalaninc 0.3 mol/L in deuterium oxide pll = 3.4 25 C 2048 accumulated pulse interferograms pulse width 5 ps pulse interval 0,4 s magnitude internal reference D = 1,4-dioxane ...

Fig. 5.5. PFT 13C f1 - NMR spectrum of D-glucose, 20 MHz, 1 mol/L in D20, temperature 30 °C, 90 °C pulses accumulation of 200 pulse interferograms (a) series of spectra recorded 15, 30, 45, 60, 75 and 90 min. after sample preparation (b) spectrum of the same sample, recorded 12hrs. after sample preparation.

Fig. 5.6. PFT 1 1H NMR spectra of D-ribose, 22.63 MHz. 1 g/2 mL D20, temperature 30 C, accumulation of 2000 pulse interferograms (6 K data points), 90 pulses, pulse interval 6 s, 2500 Hz, the numbers of the signals refer to the numbering of the C atoms. A quantitative evaluation of the spectrum gave 62% of /j-ribopyranosc (P (i), 20.3% of a-ribopyranose (Pa). 11.6% of /1-ribo-furanose (Ffi) and 6.1 % of a-ribofuranose (Fa) [132b],...

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