Homonuclear modulation

Figure 19 Schematic effect of the STAR operator on 2JCH and 3,/CH couplings. The vicinal component of magnetization in the long-range response that is two-bond coupled to a protonated carbon experiences modulation, which serves as a pseudo-evolution for this coupling. In contrast, the vicinal component of magnetization in the long-range response that is three-bond coupled to a protonated carbon does not exhibit a F, skew. Homonuclear modulation during the evolution period f, is still present, as the full experiment is not a constant-time experiment.

Fig. 14. IMPEACH-MBC (IMproved PErformance ACcordion-optimized Heteionucicar multiple bond correlation) pulse sequence developed by Martin and co-workers. The experiment is a further modification of the ACCORD-HMBC experiment that utilizes a constant time variable delay in lieu of a simple variable delay. The con.stant time variable delay introduces the interval, D/2 — 180° "C — D/2, which precedes the variable delay interval, vd. As the evolution lime ti is incremented, the interval vd is decremented in the usual fashion. However, at the same time, the Dll (ct A) intervals are incremented in a manner to keep the overall duration of the period D + vd a constant time interval. Hence, homonuclear modulation, which plagues ACCORD-HMBC experiments, is suppressed by the constant time of the intcrv al D + vd. In contrast, evolving heteronuclear couplings arc refocused at time D by the 180° - C pulse located at DU. These couplings then evolve during the variable interval vd to be sampled in the usual, accordion manner. By using this approach, the constant time variable delay pulse sequence element is of constant duration for homonuclear components of magnetization while serving as a variable delay for heteronuclear components.

Some of the most important 2D experiments involve chemical shift correlations between either the same type of nuclei (e.g., H/ H homonu-clear shift correlation) or between nuclei of different types (e.g., H/ C heteronuclear shift correlation). Such experiments depend on the modulation of the nucleus under observation by the chemical shift frequency of other nuclei. Thus, if H nuclei are being observed and they are being modulated by the chemical shifts of other H nuclei in the molecule, then homonuclear shift correlation spectra are obtained. In contrast, if C nuclei are being modulated by H chemical shift frequencies, then heteronuclear shift correlation spectra result. One way to accomplish such modulation is by transfer of polarization from one nucleus to the other nucleus. Thus the magnitude and sign of the polarization of one nucleus are modulated at its chemical shift frequency, and its polarization transferred to another nucleus, before being recorded in the form of a 2D spectrum. Such polarization between nuclei can be accomplished by the simultaneous appli-... 

A more useful type of 2D NMR spectroscopy is shift-correlated spectroscopy (COSY), in which both axes describe the chemical shifts of the coupled nuclei, and the cross-peaks obtained tell us which nuclei are coupled to which other nuclei. The coupled nuclei may be of the same type—e.g., protons coupled to protons, as in homonuclear 2D shift-correlated experiments—or of different types—e.g., protons coupled to C nuclei, as in heteronuclear 2D shift-correlated spectroscopy. Thus, in contrast to /-resolved spectroscopy, in which the nuclei were being modulated (i.e., undergoing... 

The HOHAHA spectrum (100 ms) of podophyllotoxin is presented. The HOHAHA, or TOCSY (total correlation spectroscopy), spectrum (100 ms) shows coupling interactions of all protons within a spin network, irrespective of whether they are directly coupled to one another or not. As in COSY spectra, peaks on the diagonal are ignored as they arise due to magnetization that is not modulated by coupling interactions. Podophyllotoxin has only one large spin system, extending from the C-1 proton to the C4 and 015 protons. Identify all homonuclear correlations of protons within this spin system based on the crosspeaks in the spectrum. 

The homonuclear dipolar recoupling technique of radio frequency-driven recoupling (RFDR) involves a hard n pulse per rotor period and its recoupling mechanism is based on the modulation of chemical shift difference [35-37]. When the delta-pulse approximation is relaxed so that the pulse width of the 7t pulse is about one-third of the rotor period, the so-called finite-pulse RFDR (fpRFDR) could selectively reintroduce the homonuclear dipole-dipole interaction under fast MAS conditions [38], Because the recoupling mechanism of fpRFDR does not require the presence of chemical shift difference, it can be applied to study samples with a singly labeled site. For the study of amyloid fibrils, the technique of fpRFDR is usually applied in a constant-time framework (see below). 

Assume a 180° PIP is applied at the centre of the 13CO in the middle of the 13C evolution time for homonuclear decoupling and at the same time a compensating PIP is applied on the other side of the l3C (Fig. 11) to minimize the disturbance to the 13C that may have a transverse or longitudinal magnetization. The two simultaneous PIPs become an amplitude modulated pulse described by 2/i cos (27iAft)Ix, where f is the pulse strength of each 180°... 

Proton-proton homonuclear decoupling has been performed by the ESLG decoupling sequence [46]. Quadrature detection in coj was achieved by using the time proportional phase increment method (TPPI) [47]. During the acquisition period, two pulse phase modulation (TPPM) heteronuclear decouphng ]48] was applied (Figure 7.6). 

Performing the CPMG experiment on a spin system with homonuclear coupling introduces the complication of echo modulation by the scalar couplings [4, 5, 7, 36]. This makes transverse proton relaxation measurements... 

These two examples show that Av is very much smaller for homonuclear double resonance, suggesting a simple way to generate v2. Instead of using a separate rf oscillator, we can use part of the output from the v, channel electronically modulated with an audio-frequency signal to generate output at v2. This technique makes it easier to control the exact value of Av and to focus v, and v2 exactly where we want them. 

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