Radiofrequency

The bulk magnetization is stimulated into processional motion around Bq by a radiofrequency (RF) pulse at Vj, applied tlirough a solenoid-like coil whose axis is perpendicular to Bq. This motion amounts to a nuclear... 

Figure B 1.16.9 shows background-free, pseudo-steady-state CIDNP spectra of the photoreaction of triethylamine with (a) anthroquinone as sensitizer and (b) and (c) xanthone as sensitizer. Details of the pseudo-steady-state CIDNP method are given elsewhere [22]. In trace (a), no signals from the p protons of products 1 (recombination) or 2 (escape) are observed, indicating that the products observed result from the radical ion pair. Traces (b) and (c) illustrate a usefiil feature of pulsed CIDNP net and multiplet effects may be separated on the basis of their radiofrequency (RF) pulse tip angle dependence [21]. Net effects are shown in trace (b) while multiplet effects can...

A MBER spectrometer is shown schematically in figure C1.3.1. The teclmique relies on using two inhomogeneous electric fields, the A and B fields, to focus the beam. Since the Stark effect is different for different rotational states, the A and B fields can be set up so that a particular rotational state (with a positive Stark effect) is focused onto the detector. In MBER spectroscopy, the molecular beam is irradiated with microwave or radiofrequency radiation in the... 

There are other important properties tliat can be measured from microwave and radiofrequency spectra of complexes. In particular, tire dipole moments and nuclear quadmpole coupling constants of complexes may contain useful infonnation on tire stmcture or potential energy surface. This is most easily seen in tire case of tire dipole moment. The dipole moment of tire complex is a vector, which may have components along all tire principal inertial axes. 

Measurements of Stark splittings in microwave and radiofrequency spectra allow tliese components to be detennined. The main contribution to tire dipole moment of tire complex arises from tire pennanent dipole moment vectors of tire monomers, which project along tire axes of tire complex according to simple trigonometry (cosines). Thus, measurements of tire dipole moment convey infonnation about tire orientation of tire monomers in tire complex. It is of course necessary to take account of effects due to induced dipole moments and to consider whetlier tire effects of vibrational averaging are important. 

The early MBER spectra were of two types first, pure rotational (microwave) transitions and, secondly, radiofrequency transitions between different hyperfine levels the latter were observable only for Ar-HCl, because... 

The Ar-HCl and Ar-HF complexes became prototypes for the study of intennolecular forces. Holmgren et al [30] produced an empirical potential energy surface for Ar-HCl fitted to the microwave and radiofrequency spectra,... 

Dyke T R 1984 Microwave and radiofrequency spectra of hydrogen-bonded complexes in the vapor phase Topics in Current Chemistry 120 85-113... 

The essential features of an NMR spectrometer shown m Figure 13 5 are not hard to understand They consist of a magnet to align the nuclear spins a radiofrequency (rf) transmitter as a source of energy to excite a nucleus from its lowest energy state to the next higher one a receiver to detect the absorption of rf radiation and a recorder to print out the spectrum... 

It turns out though that there are several possible variations on this general theme We could for example keep the magnetic field constant and continuously vary the radiofrequency until it matched the energy difference between the nuclear spin states Or we could keep the rf constant and adjust the energy levels by varying the magnetic... 

Application of a radiofrequency pulse to give an excess of nuclei m the higher spin state... 

From the Heisenberg uncertainty principle as stated in Equation (1.16) estimate, in cm and Hz, the wavenumber and frequency spread of pulsed radiation with a pulse length of 30 fs, typical of a very short pulse from a visible laser, and of 6 ps, typical of pulsed radiofrequency radiation used in a pulsed Fourier transform NMR experiment. 

If the source emits radiofrequency radiation consisting of two frequencies, v and v, of the same amplitude then... 

If the radiofrequency spectmm is due to emission of radiation between pairs of states - for example nuclear spin states in NMR spectroscopy - the width of a line is a consequence of the lifetime, t, of the upper, emitting state. The lifetime and the energy spread, AE, of the upper state are related through the uncertainty principle (see Equation 1.16) by... 

For radiofrequency and microwave radiation there are detectors which can respond sufficiently quickly to the low frequencies (<100 GHz) involved and record the time domain specttum directly. For infrared, visible and ultraviolet radiation the frequencies involved are so high (>600 GHz) that this is no longer possible. Instead, an interferometer is used and the specttum is recorded in the length domain rather than the frequency domain. Because the technique has been used mostly in the far-, mid- and near-infrared regions of the spectmm the instmment used is usually called a Fourier transform infrared (FTIR) spectrometer although it can be modified to operate in the visible and ultraviolet regions. 

Interstellar molecules detected by their radiofrequency, microwave or millimetre wave spectra... 

Electromagnetic and Radiofrequency Shielding. Because bismuth is highly diamagnetic, its ahoys are quite useful in appHcations where electronic equipment must be protected from outside interference or where equipment can cause outside interference. 

The technique for measurement which is most easily interpreted is the inversion-recovery method, in which the distribution of the nuclear spins among the energy levels is inverted by means of a suitable 180° radiofrequency pulse A negative signal is observed at first, which becomes increasingly positive with time (and hence also with increasing spin-lattice relaxation) and which... 

FID Free induction decay, decay of the induction (transverse magnetisation) back to equilibrium (transverse magnetisation zero) due to spin-spin relaxation, following excitation of a nuclear spin by a radio frequency pulse, in a way which is free from the influence of the radiofrequency field this signal (time-domain) is Fourier-transformed to the FT NMR spectrum (frequency domain)... 

RFIIEMI (Radiofrequency and electromagnetic interference) which regulatory agency specifications the product must meet. 

In NMR spectroscopy the precise energy differences between such nuclear magnetic states are of interest. To measure these differences, electromagnetic waves in the radiofrequency region (1-600 MHz) are applied, and the frequency at which transitions occur between the states, is measured. At resonance the condition... 

Figure 1 shows the detailed steps of the measurement, from the perspective of a coordinate system rotating with the applied radiofrequency giq = yA)- The sample is in the magnetic field, and is placed inside an inductor of a radiofirequency circuit... 

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