Double-resonance techniques

Vibrationally mediated photodissociation (VMP) can be used to measure the vibrational spectra of small ions, such as V (OCO). Vibrationally mediated photodissociation is a double resonance technique in which a molecule first absorbs an IR photon. Vibrationally excited molecules are then selectively photodissociated following absorption of a second photon in the UV or visible [114—120]. With neutral molecules, VMP experiments are usually used to measure the spectroscopy of regions of the excited-state potential energy surface that are not Franck-Condon accessible from the ground state and to see how different vibrations affect the photodissociation dynamics. In order for VMP to work, there must be some wavelength at which vibrationally excited molecules have an electronic transition and photodissociate, while vibrationally unexcited molecules do not. In practice, this means that the ion has to have a... 

Radiofrequency spectroscopy (NMR) was introduced in 1946 [158,159]. The development of the NMR method over the last 30 years has been characterised by evolution in magnet design and cryotechnology, the introduction of computer-based operating systems and pulsed Fourier transform methods, which permit the performance of new types of experiment that control production, acquisition and processing of the experimental data. New pulse sequences, double-resonance techniques and gradient spectroscopy allow different experiments and have opened up the area of multidimensional NMR and NMRI. 

Double resonance techniques, on the other hand, of which the earliest was ENDOR (described in Chapter 1), have greatly benefited from advances in signal processing technology, of the sort now employed, for example, in wireless communication systems. 

NMR data including those observed by double resonance techniques, allowed assignment of the relative stereochemistry of 14. This was accomplished after comparing the corresponding NMR data with those obtained from the hydrogenated products of / -eudesmene. 

The detection of Brpnsted acid sites, SiO(H)Al, is the most recent achievement of 170 NMR of zeolites [119-121]. High magnetic fields and double resonance techniques have allowed the observation of this important species in zeolite HY [120]. Chemical shifts of 21 and 24 ppm have been reported for zeolite HY for the Brpnsted sites in the supercage and sodalite cage, respectively [119]. Quadrupole interaction parameters are Cq = 6.0 and 6.2 MHz and r] = 1.0 and 0.9, respectively. Signal enhancement by 1H-170 cross-polarization has also permitted the detection of the acid sites in zeolite ZSM-5 [119], where they exist with lower abundance than in HY. 

In frozen solution EPR spectra of copper proteins, ligand and copper hf interactions are only partially resolved, or not at all. It is therefore striking that only few ENDOR studies on copper proteins have been published so far17,199-201). Nevertheless, these few ENDOR results already demonstrate the power of the double resonance technique to probe the coordination environment of the copper-containing sites in these types of proteins. 

In this monograph it has been demonstrated that ENDOR is a very powerful tool to study transition metal ions in organic, inorganic and bioinorganic single crystals, polycrystalline samples and frozen solutions. Due to the high resolution of this double resonance technique, many problems in magnetic resonance, which cannot be solved with ordinary EPR, become accessible. 

To resolve hf and nuclear quadrupole interactions which are not accessible in the EPR spectra, George Feher introduced in 1956 a double resonance technique, in which the spin system is simultaneously irradiated by a microwave (MW) and a radio frequency (rf) field3. This electron nuclear double resonance (ENDOR) spectroscopy has widely been applied in physics, chemistry and biology during the last 25 years. Several monographs2,4 and review articles7 11 dealing with experimental and theoretical aspects of ENDOR have been published. 

Infrared, Raman, microwave, and double resonance techniques turn out to offer nicely complementary tools, which usually can and have to be complemented by quantum chemical calculations. In both experiment and theory, progress over the last 10 years has been enormous. The relationship between theory and experiment is symbiotic, as the elementary systems represent benchmarks for rigorous quantum treatments of clear-cut observables. Even the simplest cases such as methanol dimer still present challenges, which can only be met by high-level electron correlation and nuclear motion approaches in many dimensions. On the experimental side, infrared spectroscopy is most powerful for the O—H stretching dynamics, whereas double resonance techniques offer selectivity and Raman scattering profits from other selection rules. A few challenges for accurate theoretical treatments in this field are listed in Table I. 

The optical-optical double resonance technique was applied to determine which mechanism is predominant in the pumping of TPPZn (27, 28). Because of the large difference in the lifetimes of and T ... 

More recently, single and double resonance techniques on both " N and N compounds are being employed for accurate shift and coupling constant measurements including relative signs. Use of enrichment with N has grown because of its favorable spectral characteristics in proton magnetic resonance studies. 

The broadening effects of may be removed not only by N substitution but also by proton- N decoupling produced by double resonance techniques. The information concerning 7(N—H) is, however, lost in a decoupling experiment. 

As well as the PH3 ion, the ions PH, PH and P, caused by fragmentation are observed. Furthermore the signal for the phosphonium ion, formed according to Eq. (5), is seen. Many other, heavier ions are the products of ion molecule reactions in PH3. These have the general formulae P2H (n = 0 - 5), PjH (n = 0 - 2), and Pj. Analogous ions were also formed by ion molecule reactions in ammonia. The reactions listed in Table 3 were identified with the help of the ion cyclotron double resonance technique. 

The actual identification of which reagent ions are responsible for a given product ion is accomplished by the ion cyclotron double resonance technique. For example, the occurrence of the gas-phase reaction (15) can be verified by this technique. At a magnetic field of = 0.7 T, and with the detection set at 307 kHz, the ion 35C1 is detected. If a second strong RF field is... 

T), the F species can be selectively removed from the cell by the increase of its orbiting radius. If a chemical reaction is responsible for the appearance of Cl" in the mass spectrum, the removal of F will result in a decrease, or eventually in the total disappearance, of the chloride ion signal. Thus, the double resonance technique is particularly valuable in untangling a complicated reaction scheme. 

A suitable way to enhance NMR signals of nuclei with small magnetogyric ratios or low concentrations (rare spins 5) interacting with abundant spins I is the polarization transfer from the spin I to the spin S ensemble via a cross polarization (CP) experiment. This experiment is based on a double resonance technique, which can be applied in combination with MAS for the characterization of surface sites of solid catalysts and rigidly bound surface complexes. The pulse sequence used for CP experiments is demonstrated in Fig. 4. 

In practice, spin decoupling experiments are conducted in the following way. First, the spectrum is recorded under normal conditions. Then the spectrum is recorded while a second radiofrequency emitter irradiates at the resonance frequency of the nuclei that are to be decoupled (Fig. 9.22). This double resonance technique is used to identify nuclei which are coupled and which cause interpretation difficulties in the spectrum. 

Double Resonance Techniques used in 13C NMR Spectroscopy as Assignment Aids... 

Double Resonance Techniques used in 13C NMR Spectroscopy frequency ranges can be realized either by a very large rf power B2, so that... 

Solid-state NMR suffers from two persistent problems (1) low abundance and/or sensitivity of the observed nucleus, and (2) long spin-lattice relaxation times Tx. Both can be remedied with the help of a double-resonance technique known as cross-polarization (CP) (21). 

The potential signal enhancement available from these double-resonance techniques depends on the available polarization ( H or 14N), the detection frequency, and the polarization transfer efficiency. The transfer efficiency depends not only on the coupling between the isotopes and the time spent with the frequencies matched but also on the number of 14N transitions matched and the order in which the matching conditions occur [91]. Irradiating multiple transitions, as described in Section 4.2.1., will also affect the choice of 14N transitions to polarize [94,95],... 

H NMR spectroscopy and a double resonance technique were applied for solving the stereochemical problem57. It was necessary to decide if the stereoisomer 14a or 14b is the product (Figure 2). 

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