Triple-resonance methods

A method to observe microwave resonances in antiprotonic helium has been proposed and is being prepared for the coming antiproton decelerator (AD) ring at CERN [36]. It is called 2-laser-microwave triple resonance method, which has the following steps. 

Fig. 4. Two-laser microwave triple resonance experiment explained at the example of the already observed (n, L) = (37,35) — (38,34) transition. Left Simulated delayed annihilation time spectra of the laser/microwave triple resonance method. Right Simulated laser and microwave resonance profiles...

H-X-Y Triple-resonance methods for correlating protons and a heteratom (Y) whilst using a second heteroatom (X) to either relay the correlations or edit the correlation spectrum... 

Three-dimensional (3D) NMR is an upcoming area that holds a lot of promise. Peter Rinaldi, who is an acknowledged leader in 2D and 3D NMR, has written a timely and authoritative review in a special Invited Paper (48). In addition, he and his coworkers have used 3D NMR and triple resonance methods to study poly(dimethylsiloxanes) (50), and fluoropolymers (49). 

The role of peptides in the development of solid-state NMR of aligned samples has been reviewed by Sinha et al In particular, the role of synthetic peptides in the development of triple-resonance methods has been described. Recent developments of pulse sequences and NMR probes for triple-resonance NMR of aligned samples have also been presented. 

Measurements on solids in which copper is in a symmetric environment supplement the solution information. The inert complex ion [Cu P(OMe)3 4] has also been studied by H- P, Cu triple resonance methods. ... 

Cf1 - 27A1 or mC H "B triple resonance as well as H 27AI double resonance methods have been employed to measure the local neighborhood between B or A1 nuclei in the zeolite framework and the C (or H) nuclei in the SDA [203]. A set of various boro- or aluminosilicates has been investigated, and Fig. 5 illustrates one of the examples. 

ESR methods unambiguously establishes the presence of species bearing unpaired electrons (ion-radicals and radicals). The ESR spectrum quantitatively characterizes the distribution of electron density within the paramagnetic particle by a hyperfine structure of ESR spectra. This establishes the nature and electronic configuration of the particle. A review by Davies (2001) is highly recommended as a guide to current practice for ESR spectroscopic studies (this quotation is from the title of the review). The ESR method dominates in ion-radical studies. Its modern modifications, namely, ENDOR and electron-nuclear-nuclear triple resonance (TRIPLE) and special methods to observe ion-radicals by swiftness or stealth are described in special literatures (Moebius and Biehl 1979, Kurreck et al. 1988, Werst and Trifunac 1998). 

Considering that the literature on the development of experimental methods and important fields of application of X/Y correlations in inorganic, organoelement and organometallic chemistry up to 1997 has been covered in earlier reviews,11 we will focus here on recent improvements of experimental techniques and novel applications for compound characterisation. Despite the recently increasing interest in the application of X/Y correlation spectroscopy in solids,12,13 this review will cover only solution NMR techniques. Likewise, a survey of specialised triple-resonance NMR experiments devoted to the characterisation of bio-molecules, and their application, is considered beyond the scope of this article. 

It ought to be pointed out that the methods of transfer of population and also of polarization between the states are being developed further. One might mention, in particular, the cw all-optical triple resonance (AOTR) method evolved by Stwalley s group [282] and tried out on Na2-The method consists of the use of three tunable single-frequency continuous wave lasers allowing flexible movement of both population and polarization from the thermally populated level to any other level of the ground state. 

In this review we shall discuss only those methods that have been found useful in organosilicon studies chemical approaches (isotopic substitution and use of a shift reagent) are mentioned in Section X. We are concerned here with spectroscopic experiments that are loosely and arbitrarily divided into three categories correlations, triple resonance and selective experiments. 

In most cases, structural characterization of carbosilane dendrimers is accomplished by multinuclear one-dimensional NMR spectroscopy (1H, 13C and 29Si). However, as larger dendrimers are characterized standard spectroscopic methods become less useful due to the overlap of signals. This problem has been elegantly circumvented as described in a recent paper by Tessier, Rinaldi and coworkers56. In this paper the researchers described the use of 1 H/13C/29Si triple resonance, 3D and pulse field gradient NMR techniques to... 

Another method that is important for structure assignment is the electron-nuclear-nuclear triple resonance (TRIPLE) spectroscopy (Endeward et al., 1998 Makinen et al., 1998), which is an extension of the ENDOR method. In the general TRIPLE experiment, transitions of different nuclei are driven simultaneously. One ENDOR transition is irradiated saturating rf power at a constant frequency, while the entire ENDOR frequency range is swept to obtain the TRIPLE spectrum. 

Many different types of 3D and 4D experiments can be devised. We described briefly one class, in which two different kinds of 2D experiments are joined, primarily to use the third dimension to clarify the presentation of spectral data. With the ability of most modern NMR spectrometers to provide precise frequency and phase control for several radio frequencies simultaneously and to apply them efficiently in the probe, a class of 3D and 4D triple resonance experiments has become feasible. These experiments usually use two HSQC and/or HMQC sequences to transfer magnetization in the path I— S— T— S— I. I is almost always H 5 and T are 13C and 15N in proteins, the type of molecule in which these experiments are most often utilized. In addition,31P may be involved in nucleic acids, another frequent subject for these methods, and many other nuclides can be used in other applications. 

Many workers have in fact used density matrix methods for the calculation of line shapes and intensities in multiple resonance experiments, and two excellent reviews of the background theory are available. (49, 50) In addition there is also a simple guide (51) to the actual use of the method which is capable of predicting the results of quite elaborate experiments. Major applications have included the calculation of the complete double resonance spectrum from an AX spin system which gives 12 transitions in all (52) an extremely detailed study of the relaxation behaviour of the AX2 systems provided by 1,1,2-trichloroethane and 2,2-dichloroethanol (53) the effects of gating and of selective and non-selective pulses on AB and AX spin systems and the importance of the time evolution of the off-diagonal elements of the density matrix in repetitively pulsed FT NMR and spin-echo work (54) the use of double resonance to sort out relaxation mechanisms and transient responses (55) the calculation of general multiple resonance spectra (56) and triple resonance studies of relaxation in AB and AX spin systems. (57)... 

Triple resonance 3D-NMR has been used to study polymers containing three NMR active nuclei, and parallel methods for studying hydrocarbon-based polymers are now being used. H- C-X (X = E, Si, and NMR correlation experiments help to simplify and unambiguously assign both main chain and chain-end resonances in a variety of fluoropolymers, polyorga-nosilanes, and phosphorus-containing polystyrenes. 

Heteronuclear Triple Resonance Experiments. - A very small number of new triple-resonance experiments was reported. A version of the 3D HNCA experiment was presented that suppresses coherence transfer through /(N,C ). The method is based on the scalar couplings of sequential and to the same which is not the case for the intraresidue and... 

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