Excitation selective

2 Line-scan methods using selective excitation 

Selective excitation denotes the manipulation of the NMR signal within restricted frequency regions of the magnetization response. It is useful in spectroscopy to select 

Most types of selective excitation can be modified for simultaneous excitation of n slices or volume elements. Such an approach is advantageous when a limited number of slices or volume elements, but not the entire 3D object, needs to be investigated. By suitable coding of the volume information in n experiments an improvement in signal-to-noise ratio of can be gained [Boll, Miill]. Compared to 3D volume imaging, multislice and multi-volume techniques (cf. Section 9.1) suffer from the lack of achieving well-defined boundaries. 

Apart from preparation of magnetization in slices and lines, selective excitation can also be used for point selection. Here the objective normally is not to scan an image in a pointwise fashion, but rather to localize a selected volume element to acquire a spectroscopic response from it [Auel] (cf. Section 9.1). 

With the hard pulses that we have encountered, the time variation of excitation is simply a square pulse, in which the rf power is turned on and off as fast as possible, and the amplitude is constant during the pulse. However, the use of a simple square pulse leads to a far from ideal frequency excitation profile for a soft pulse, because the Fourier transform of the square pulse is a sine function with sidelobes, as we saw in Fig. 3.8. Over the millisecond range used for a selective (soft) pulse, it 

FIGURE 9.5 Trajectories of magnetizations from 0 to 900 Hz off resonance, excited by a rectangular pulse of width 1 ms with yB /2ir = 250 Hz, which is a 90° pulse on resonance. A magnetization at 1000 Hz from resonance would complete almost a full circle and give no signal. From Freeman.106 

FIGURE 9.6 Illustration of the E-BURP selective excitation pulse, (a) Depiction of the time excitation function. (b) Frequency domain excitation profiles for absorption (solid line) and dispersion 

In some spectrometers it is not feasible to generate BURP or related tailored soft pulses. A useful alternative is the DANTE pulse sequence (delays alternating with nutation for tailored excitation). Normal high excitation power is used, but rather than apply a single 90° pulse, the DANTE method uses a sequence of small angle pulses that sum to 90° but with a short time between pulses during which nuclei 

FIGURE 9.7 Trajectories for the on-resonance magnetization and several off-resonance magnetizations from a DANTE pulse sequence consisting of 18 pulses along the x axis, each of 5°. From Free- 

Schmidt, Pulse response in the presence of quadru-polar splitting, in Pulsed Magnetic and Optical Resonance, Proceedings of the Ampere International Summer School II, Basko polje, 2-13 September 1971, edited by R. Blinc (University of Ljubljana, Ljubljana, Yugoslavia, 1972), pp. 75-83. 

In section II.A.2. we discussed how the rf pulse acts on the nuclear spins. We now discuss this topic further and apply the results to several examples having to do with selective excitation. There are many reasons for wanting to perform selective excitation in NMR. Some of them are to simplify complex spectra, to do selective population transfer in order to get the sign of spin-spin coupling, and to study cross relaxation. An important application of what we might call selective de-excitation is a notch in the irradiation pattern to suppress an unwanted resonance such as the solvent peak in a complex proton spectrum. Morris and Freeman (1978) have reviewed many aspects of such experiments. 

So far, we have only talked of applications where the word selective means with respect to locations of NMR lines in spectra. The selectivity can also apply to other properties 

Another example of separating long Tg components from short Tg components is given by Rabenstein, et al. (1979). They named the sequence inversion-recovery spin-echo (IRSE) because it is literally just that. Instead of the n/2 pulse to monitor the magnetization recovering after the initial 7t pulse, a n-n/2 echo is used. The separation of the two pulses in the spin echo sub-sequence is adjusted so that the short T2 component does not contribute significantly to the echo. This sequence was successfully used to isolate the sharp NMR lines of small molecules from the broad lines of proteins in solutions. 

As already mentioned, the overwhelmingly common excitation uses an approximately square pulse and such a pulse will excite the nuclei in only approximately a uniform fashion. 

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A covalent bond (or particular nomial mode) in the van der Waals molecule (e.g. the I2 bond in l2-He) can be selectively excited, and what is usually observed experimentally is that the unimolecular dissociation rate constant is orders of magnitude smaller than the RRKM prediction. This is thought to result from weak coupling between the excited high-frequency intramolecular mode and the low-frequency van der Waals intemiolecular modes [83]. This coupling may be highly mode specific. Exciting the two different HE stretch modes in the (HF)2 dimer with one quantum results in lifetimes which differ by a factor of 24 [84]. Other van der Waals molecules studied include (NO)2 [85], NO-HF [ ], and (C2i J )2 [87]. 

First, it is possible to excite a chromophore corresponding to the active site, and detennine which modes interact with it. Second, by using UV excitation, the amino acids with phenyl rings (tryptophan and tyrosine, and a small contribution from phenylalanine) can be selectively excited [4], The frequency shifts in the resonance Raman spectrum associated with them provide infomiation on their enviromnent. 

Figure B2.5.18 compares this inter molecular selectivity with intra molecular or mode selectivity. In an IR plus UV, two-photon process, it is possible to break either of the two bonds selectively in the same ITOD molecule. Depending on whether the OFI or the OD stretching vibration is excited, the products are either IT -t OD or FIO + D [24]- hr large molecules, mirmnolecular selectivity competes with fast miramolecular (i.e. unimolecular) vibrational energy redistribution (IVR) processes, which destroy the selectivity. In laser experiments with D-difluorobutane [82], it was estimated that, in spite of frequency selective excitation of the...

The development of tunable, narrow-bandwidtli dye laser sources in tire early 1970s gave spectroscopists a new tool for selectively exciting small subsets of molecules witliin inhomogeneously broadened ensembles in tire solid state. The teclmique of fluorescence line-narrowing [1, 2 and 3] takes advantage of tire fact tliat relatively rigid chromophoric... 

Fluorescence and phosphorescence are types of luminescence, ie, emission attributed to selective excitation by previously absorbed radiation, chemical reaction, etc, rather than to the temperature of the emitter. Laser-iaduced and x-ray fluorescence are important analytical techniques (see... 

Fluorometry and Phosphorimetry. Modem spectrofluorometers can record both fluorescence and excitation spectra. Excitation is furnished by a broad-band xenon arc lamp foUowed by a grating monochromator. The selected excitation frequency, is focused on the sample the emission is coUected at usuaUy 90° from the probe beam and passed through a second monochromator to a photomultiplier detector. Scan control of both monochromators yields either the fluorescence spectmm, ie, emission intensity as a function of wavelength X for a fixed X, or the excitation spectmm, ie, emission intensity at a fixed X as a function of X. Fluorescence and phosphorescence can be distinguished from the temporal decay of the emission. 

The photolysis of 1,2-benzisoxazole in the absence of air in acetonitrile gave salicylonitrile and benzoxazole (67AHC(8)277). When air-saturated acetonitrile was employed, 2,2 -dimeriz-ation to (38) occurred, accompanied by benzoxazole. Photolysis of the 2,2 -dimer (38) and benzoxazole did not alter the ratio, thus indicating that neither one arose from the other. Selective excitation also ruled out dimer formation from benzoxazole under the reaction conditions (Scheme 9). This dimerization is similar to that observed for benzimidazole, except that in that series no 2,2 -dimerization was observed (74TL375). 

PL investigations of site-selective excitation, according to which the films show additional absorptions at lower frequency than those seen in dilute solution [45] ... 

Pain production is the most common injury inflicted on man. This noxious stimulus is perceived almost instantly after skin - tentacle contact. A subpopulation (30 - 40%) of visceral sensory C fibers denoting noscioception have been shown to be selectively excited experimentally in nerve ganglia preparations by a component of... 

A homonuclear spin-system may be excited with radiofrequency (r.f.) pulses that are so Intense (in the order of p.s), compared to the frequency width of the spectrum, that all resonances are excited essentially uniformly. This is a nonselective excitation. A homonuclear spin-system may also be excited with a relatively weak, r.f. pulse (in the order of ms), in the sense that all components of a given multiplet are inverted at time zero, whereas the other resonances in the spectrum remain essentially unperturbed this is a selective excitation. The r.f. pulse may be single-selective, that is, there is an inversion of one multiplet in the spectrum, or double-selective, triple-selective, and so on, where two, three, or more separate multiplets in the spectrum are inverted simultaneously while the remaining resonances remain unperturbed. 

Figure 1.16 Time domain representation and frequency excitation function of a soft pulse. The soft pulse selectively excites a narrow region of a spectral range and leads to a strong offset-dependent amplitude of the excitation function.

Gaussian pulses are frequently applied as soft pulses in modern ID, 2D, and 3D NMR experiments. The power in such pulses is adjusted in milliwatts. Hard" pulses, on the other hand, are short-duration pulses (duration in microseconds), with their power adjusted in the 1-100 W range. Figures 1.15 and 1.16 illustrate schematically the excitation profiles of hard and soft pulses, respectively. Readers wishing to know more about the use of shaped pulses for frequency-selective excitation in modern NMR experiments are referred to an excellent review on the subject (Kessler et ai, 1991). 

Figure 7.1 Selective excitation of only one multiplet by a selective pulse transforms a 2D experiment into a ID technique. A selective pulse generates the transverse magnetization. The result is a trace of the corresponding 2D spectrum. (Reprinted from Mag. Reson. Chem. 29, H. Kessler ei al., 527, copyright (1991), with permission from John Wiley and Sons Limited, Baffins Lane, Chichester, Sussex P019 lUD, England.)...

The SELINCOR experiment is a selective ID inverse heteronuclear shift-correlation experiment i.e., ID H,C-COSYinverse experiment) (Berger, 1989). The last C pulse of the HMQC experiment is in this case substituted by a selective 90° Gaussian pulse. Thus the soft pulse is used for coherence transfer and not for excitation at the beginning of the sequence, as is usual for other pulse sequences. The BIRD pulse and the A-i delay are optimized to suppress protons bound to nuclei As is adjusted to correspond to the direct H,C couplings. The soft pulse at the end of the pulse sequence (Fig. 7.8) serves to transfer the heteronuclear double-quantum coherence into the antiphase magnetization of the protons attached to the selectively excited C nuclei. 

Figure 30. Selective excitation from 11 > to 2 > by the adiabatic rapid passage (ARP) in the case of three-level model. Upper part time variation of the population. Middle part-time variation of laser frequency. Bottom part-envelope of the laser pulse. Taken from Ref. [42].

Watanabe, K., Takagi, N. and Matsumoto, Y. (2005) Mode-selective excitation... 

The same flow-aligning side-chain liquid crystalline polymer has been studied [43] in extensional flow using a rheo-NMR method in which selective excitation of... 

The ability to selectively excite a particular ion (or group of ions) by irradiating the cell with the appropriate radiofrequencies provides a level of flexibility unparalleled in any other mass spectrometer. The amplitude and duration of the applied RF pulse determine the ultimate radius of the ion trajectories. Thus, by simply turning on the appropriate radiofrequency, ions of a single m/z may be ejected from the cyclotron. In this way, a gas-phase separation of analyte from matrix is achieved. At a fixed radius of the ion trajectories the signal is proportional to the number of orbiting ions. Quantitation therefore requires precise RF control. 

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