Free induction decay irradiated

The laser heating technique can be applied to perform temperature jumps by irradiating short laser pulses at the sample container. Ernst et al. (54) used such a temperature jump protocol to perform stop-and-go experiments. After the start of the laser pulse, the temperature inside the sample volume is raised to the reaction temperature, the conversion of the adsorbed reactants proceeds, and the H MAS NMR measurement is performed. After the laser pulse is stopped, the temperature inside the sample volume decreases to ambient temperature, and the C MAS NMR measurement is made. Subsequently, the next laser pulse is started and, in this way, the reaction is recorded as a function of the reaction time. By use of the free-induction decay and the reaction time as time domains and respectively, a two-dimensional Fourier transformation leads to a two-dimensional spectrum, which contains the NMR spectrum in the Ej-dimension and the reaction rate information in the Ts-dimension (54,55). 

A pseudo solid-like behavior of the T2 relaxation is also observed in i) high Mn fractionated linear polydimethylsiloxanes (PDMS), ii) crosslinked PDMS networks, with a single FID and the line shape follows the Weibull function (p = 1.5)88> and iii) in uncrosslinked c/.s-polyisoprenes with Mn > 30000, when the presence of entanglements produces a transient network structure. Irradiation crosslinking of polyisoprenes having smaller Mn leads to a similar effect91 . The non-Lorentzian free-induction decay can be a consequence of a) anisotropic molecular motion or b) residual dipolar interactions in the viscoelastic state. 

The principle of presaturation relies on the phenomenon that nuclei which are unable to relax, because their population in the ground state a and the excited state (3 is the same, do not contribute to the free induction decay after pulse irradiation. Prior to data acquisition, a highly selective low-power pulse irradiates the desired solvent signals for 0.5 to 2 s, thus leading to saturation of the solvent signal frequency. During data acquisition, no irradiation should occur. NOESY-type presaturation is an effective pulse sequence of presaturation. The pulse sequence consits of three 90° pulses (similar to the first increment of a NOESY experiment) ... 

Acquisition of the free induction decay of the rare spins by continued irradiation of the iH field for heteronuclear dipolar decoupling. 

Modern instruments are Fourier transform NMR, which use a constant magnetic field commonly produced by a superconducting magnet and a strong radio-frequency pulse that irradiates the sample. The free induction decay signal emission of the sample is... 

The NMR signal obtained from the resonating nuclei after the sample has been irradiated by the pulse is the so-called free-induction decay curve. This curve consists of peaks and valleys. The spectrometer samples the free-induction decay curve at set time intervals and records the data, which are in a time domain. NMR spectra, however, are normally given in terms of frequency and therefore the spectrum must be transformed by use of the Fourier transform pairs ... 

Tryptophan. A typical photo-CIDNP experiment goes as follows. A solution of the substrate in the presence of 0.2 to 0.4 mM flavin is irradiated in the NMR probe by an argon ion laser (0.6 s, 5 W light pulses) prior to the rf pulse and acquisition of the free induction decay (FID). Alternating "light" and "dark" FID s are taken, which after... 

The NQR response of a single crystal to a resonant pulse of rf irradiation is completely analogous to the spin 1/2 NMR case. One difference is that the intensity of the free induction decay (FID) response depends on the orientation of the radiofrequency field Hi with respect to the EFG principal axis system. For a spin I = 1 system, the three resonance lines v+, v. and ly can be observed only when the field Hi is oriented, respectively, along the x, y and z principal axes of the EFG tensor. As an example, for a v line, the expected value of the magnetization along the y axis is proportional to... 

The main application at low frequencies is, of course, NMR. In FT-NMR, a magnetized sample is irradiated by radio frequency (RF) radiation to manipulate the bulk magnetization. The basic signal is the free induction decay, but hundreds of sophisticated RF pulse sequences have been invented for specific purposes, including medical imaging. 

Continuous wave spectrometers have been replaced by FT-NMR spectrometers in which the sample is irradiated with a short pulse that covers the entire range of relevant rf frequencies, and a free induction decay (FID) is recorded and then converted into a spectrum. 

Pulsed NMR irradiates a high-power pulse onto a sample, and detects its response—free induction decay (FID)—as a function of time. From the rate of damping of FID, relaxation time can be obtained, which allows molecular mobility information to be obtained. In general, pulsed NMR is a broadband NMR technique that cannot provide knowledge on an individual nucleus that has been s arated by chemical shift as in high-resolution NMR. However, if the sensitive H nucleus is the subject of investigation, the relaxation time of the entire system can be obtained accurately in a short time and when components with differing relaxation times coexist, their composition can be determined. 

Fig. 11 O-DNP experiments by Bennati and co workers, (a) Magnitude of the NMR free induction decay (FID) for 25 mM D- %-TEMPOL in water with and without microwave irradiation (9.7 GHz), showing a maximum enhancement c of -170 for the first point of the FID. The Boltzmann signal required 4,096 scans whereas the DNP-enhanced signal was recorded within eight scans, (b) H NMR spectra of water containing 25 mM D- N-TEMPONE at 94 GHz. (Reproduced from [45, 120] by permission of the PCCP Owner Societies)...

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