Frequency labelling

Fig. 2. The mSR frequency spectrum of GaAs at 10 K in an external field of 1.15 T applied along a <110) direction. The upper two frequencies result from Mu that has an isotropic hyperfine interaction. The starred frequencies are from Mu that have hyperfine interactions axially symmetric about (111) axes. The angles in brackets refer to the direction of the external field with respect to the Mu symmetry axes. The frequency labelled vis due to a diamagnetic center. From Kiefl et al. (1985).

Thus, the magnetization is transferred from the amide proton to the attached nitrogen and then simultaneously to the intra- and interresidual 13C spins and sequential 13C spin. The 13C chemical shift is labelled during /, and 13C frequency during t2. The desired coherence is transferred back to the amide proton in the identical but reverse coherence transfer pathway. The 15N chemical shift is frequency labelled during t3, and implemented into the 13C 15N back-INEPT step. The sensitivity of the HNCOmCA-TROSY experiment is excellent and nearly similar to HNCA-TROSY except for the inherent sensitivity loss by a factor of /2, arising from additional quadrature detection needed for 13C frequency discrimination in the fourth dimension. The excellent sensitivity is due to a very efficient coherence transfer pathway,... 

By frequency labelling the 13C (z — 1) spin during additional incremented time delay, and implementing this into the 13C — 13C INEPT step in the HN(CO)CA-TROSY experiment, a four-dimensional HNCOCA-TROSY experiment25 is obtained (Fig. 9b), which alleviates inevitable resonance... 

Figure 2(B) shows an equivalent pulse scheme used for studying exchange processes in the rotating frame [22, 36-38]. As in the above experiment, the first 90° pulse, followed by the incremental delay t, creates a frequency-labeled transverse magnetization that is projected onto the rotating-frame axis (x or y) by a spin-locking pulse of duration Xm. While the magnetization is spin locked, exchange will occur. As in the previous case, the data...

The preparation period consists of the creation of a non-equilibrium state and, possibly, of the frequency labeling in 2D experiments. Usually, the preparation period should be designed in such a way that in the created non-equilibrium state, the population differences or coherences under consideration deviate as much as possible from the equilibrium values. During the relaxation period, the coherences or populations evolve towards an equilibrium (or a steady-state) condition. The behavior of the spin system during this period can be manipulated in order to isolate one specific type of process. The detection period can contain also the mixing period of the 2D experiments. The purpose of the detection period is to create a signal which truthfully reflects the state of the spin system at the end of the relaxation period. As always in NMR, sensitivity is a matter of prime concern. 

It has been mentioned in Section 7.3, and it was implicit all over Chapter 7, that a finite time is required to achieve selective saturation or inversion of a signal by a soft pulse, during which time polarization starts to be exchanged, causing non-linearity of the response (see also Section 9.3). It should be stressed that this is not the case in all common 2D experiments based on non-selective pulses, which have durations of the order of microseconds instead of milliseconds, as required for selectivity. Selectivity in 2D experiments is intrinsic because of the double frequency labeling along f and /2. 

Fig. 4.1. Schematic diagram of the second harmonic generation experimental apparatus with the sample in the reflection geometry. The polarization analyzers are set to transmit p-polarized light at the frequency labeled in the figure. The (co/2co) filters transmit the (fundamental/harmonic) light while blocking the (harmonic/fundamental) light. For phase measurements, a quartz plate is mounted on a translation stage for movement towards the sample at a distance L.

Fig. 5. Distribution of C C stretching frequencies for L M-C=CPh complexes. Data are taken from Table VI. The frequency labels correspond to the centers of the bins.

Fig. 14. Schematic of selective excitation and ID exchange spectroscopy, (a) Typical pulse sequence with a soft selective pulse centered at pulsation a>s with a frequency dispersion AcoP <3C Aoj much smaller than the typical linewidth. After an evolution time te smaller or of the order of the spin-lattice relaxation time, a reading sequence of hard pulses that covers uniformly the whole broad line is applied, (b) Effect of a selective excitation on a homogeneously broaden line, (c) Selective frequency labeling of an inhomogeneously broaden line at the irradiation pulsation cos of the first soft pulse. For a soft n pulse, the magnetizations of all the spins that can exchange energy at this pulsation are reversed. By following the difference spectra between the spectra acquired at different evolution times te and the fully relaxed spectrum AS(te) — S(t -> oo) — S(te), limits or evaluation of the correlation time tc of the motion can be achieved.

The DQF-COSY sequence (Fig. 5.40) differs from the basic COSY experiment by the addition of a third pulse and the use of a modified phase-cycle or gradient sequence to provide the desired selection. Thus, following tj frequency labelling, the second 90° pulse generates multiple-quantum coherence which is not observed in the COSY-90 sequence since it remains invisible to the detector. This may, however, be reconverted into single-quantum coherence by the application of the third pulse, and hence subsequently detected. The required phase-cycle or gradient combination selects only signals that existed as double-quantum coherence between the last two pulses, whilst all other routes are cancelled, hence the term double-quantum filtered COSY. 

Only the /lz term leads to cross-peaks by chemical exchange, so the other term will be ignored (in an experiment this is achieved by appropriate coherence pathway selection). The effect of the first part of the sequence is to generate, at the start of the mixing time, Tmix, some z-magnetization on spin 1 whose size depends, via the cosine term, on tl and the frequency, Qv with which the spin 1 evolves during /). The magnetization is said to be frequency labelled. 

Peaks are sometimes seen in two-dimensional spectra at co-ordinates Tj = 0 and F2 = frequencies corresponding to the usual peaks in the spectrum. The interpretation of the appearance of these peaks is that they arise from magnetization which has not evolved during q and so has not acquired a frequency label. 

In the simple-spring model, the crystal is in contact with an immobile object. The model can be extended to cover situations where the object takes part in the oscillation to some extent. A typical object of this kind would be a small (< 10 im) sphere [40]. Figure 2c depicts the physical situation and the equivalent circuit representation. Note that the motion occurs into the lateral direction even though the spring is drawn vertically. In the following, we assume a spring constant independent of frequency, labeled its. From Fig. 2c, we infer the load to be ... 

The observant reader will have by now realized that the above experiment, which Derome (1987) calls frequency labeling, provides no additional information beyond the simple H spectrum of chloroform. Actually,... 

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