Double-resonance methods

A major limitation of CW double resonance methods is the sensitivity of the intensities of the transitions to the relative rates of spin relaxation processes. For that reason the peak intensities often convey little quantitative information about the numbers of spins involved and, in extreme cases, may be undetectable. This limitation can be especially severe for liquid samples where several relaxation pathways may have about the same rates. The situation is somewhat better in solids, especially at low temperatures, where some pathways are effectively frozen out. Fortunately, fewer limitations occur when pulsed radio and microwave fields are employed. In that case one can better adapt the excitation and detection timing to the rates of relaxation that are intrinsic to the sample.50 There are now several versions of pulsed ENDOR and other double resonance methods. Some of these methods also make it possible to separate in the time domain overlapping transitions that have different relaxation behavior, thereby improving the resolution of the spectrum. 

The combination of higher fields and pulsed, double resonance methods is now making it possible to use ESR as a tool to determine distances within macromolecules. This is a valuable supplement to the very widespread use of multi-dimensional NMR in structural biology.33... 

From a study of the microwave spectrum of 2-methylselenophene, the second-order Stark effect in the ground state was determined.11 The technique used was double radiofrequency-microwave resonance. For the identification by the double resonance method transitions of chiefly the A-state were chosen. From these observations the components of the dipole moment of 2-methylselenophene and the total dipole moment were determined. 

Numerous new developments and applications of solid state NMR techniques have emerged. Multidimensional NMR methods are able to probe connectivity patterns of zeolite framework structures and solve ambiguities in line assignments [27], high-resolution techniques for quadrupolar nuclei have been developed [31-34], and powerful double-resonance methods permit the study of spatial... 

Additional information can be obtained by employing double resonance methods which probe the dipolar interaction of 29Si and 27A1 within the zeolite framework [93, 94]. The underlying NMR methods will be considered in more detail in Sects. 3 and 4 of this chapter. 

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. 

E. W. Randall, in Some Double Resonance Methods in Imaging Experiments, Proceedings of International School of Physics (Enrico Fermi), Nuclear Magnetic Double Resonance , (Ed. B. Maraviglia), North-Holland, Amsterdam, 1993, pp. 423-448. 

With this procedure, as with the double-resonance methods in atomic physics, Zeeman and Stark splittings, hyperfine structures and A doublings in molecules can be measured with high precision, even if the observed level splittings are far less than the optical dopp-ler width. From the width of the rf resonance and from the time response of the pumped systems, orientation relaxation rates can be evaluated for individual v J") levels. Other possible applications of this promising technique have been outlined by Zare 30) Experiments to test some of these proposals are currently under investigation and their results will be reported elsewhere. 

Milov, A. D., Salikhov, K. M., and Shchirov, M. D. (1981). Application of the double resonance method to electron spin echo in a study of the spatial distribution of paramagnetic centers in sohds. Soviet. Phys. Solid State 23, 565—569. 

Many factors and considerations are germane to future research in this area. On the technical side, achieving cluster size selection stands as one of the most important and sought-after goals. It would be most desirable to achieve this while maintaining sufficiently high densities for studies of photoinitiated reactions to be carried out with product state resolution and/or ultrafast time resolution. The two methods that, in our opinion, are most viable are molecular beam deflection, as pioneered by Buck (1994) and coworkers, and laser-based double-resonance methods. Less direct approaches are deemed inferior. 

J. Brossel and F. Bitter, A new "double resonance" method for investigating atomic energy levels. Application to Hg 3PX, Phys. Rev. 86 308 (1952). 

A rotational spectrum of FeO in its X5 A ground state was described by Endo, Saito and Hirota [253]. Transitions involving relatively low J values were observed for three fine-structure states, with 12 = 4, 3 and 2. Subsequently a much more comprehensive rotational spectrum was measured by Krockertskothen, Knockel and Tiemann [254] using double resonance methods we will discuss these results in the next chapter. More... 

Figure 11.1. (a) Principles of the microwave/optical double resonance method, (b) Change of polarisation of fluorescent light resulting from AM= 1 radiofrequency transitions. 

The studies of BaO were important pioneering experiments showing the power of microwave/optical double resonance methods. We shall describe a number of significant applications of these methods later in this chapter. 

As can be seen, in each case the outermost pair of electrons occupies an s orbital, so that the molecules are normally divalent. A number of alkaline earth diatomic molecules have been studied by optical spectroscopy and double resonance methods. We have already described the very early microwave investigations of BaO in both its ground X 1 + and excited A 1 X states, and now proceed to discuss other molecules in this series. We describe these in their chronological order. 

As we have seen elsewhere in this book, the techniques of high-resolution rotational spectroscopy are beginning to be applied to transition metal molecules, and this is particularly true of double resonance methods. We now review some of the work which has been described, again in a mainly chronological order. 

The inherent sensitivity of double resonance methods has led to studies of transition metal and rare earth compounds which would be difficult with more conventional methods. In chapter 10 we described the study of rotational transitions of the FeO molecule by Endo, Saito and Hirota [46] using a conventional ffee-space absorption cell, and by Allen, Ziurys and Brown [47] who used a high-temperature oven to study the reaction between iron vapour and nitrous oxide. We also mentioned, however, that a comprehensive study had been made by Krockertskothen, Knockel and Tiemann [48] using a double resonance technique which we now describe in detail. Allen, Ziurys and Brown [47] combined the data from different experiments to produce the best analysis. 

TiN has an X 2S+ ground state and rotational transitions in the v = 0 level have been measured and analysed [69, 70] pure millimetre wave and microwave/optical double resonance methods were used, over a frequency range from 37 to 446 GHz. 14N hyperfrne structure was observed for the two lowest rotational transitions, and the spectrum analysed using the conventional effective Hamiltonian, again expressed in cartesian form ... 

For 20 years center stage has been occupied by two-dimensional (and now three-and four-dimensional) NMR techniques. 2D NMR and its offshoots offer two distinct advantages (1) relief from overcrowding of resonance lines, as the spectral information is spread out in a plane or a cube rather than along a single frequency dimension, and (2) opportunity to correlate pairs of resonances. In the latter respect 2D NMR has features in common with various double resonance methods, but as we shall see, 2D NMR is far more efficient and versatile. Hundreds of different 2D NMR techniques have been proposed in the literature, but most of these experiments can be considered as variations on a rather small number of basic approaches. Once we develop familiarity with the basic principles, it will be relatively easy to understand most variations of the standard 2D experiments. 

In the assignment of resonance agnal, the procedures are based prinrarily on the chemical shift values. In additicxi, when multiplets exist, the double resonance method and decoupling by the deuteration of the adjacent hydrogens are used. The disap-pearence of a resonance signal by deuteration is also helpful for tl assigmnent. 

It is often the case that nuclei present at the surface of catalysts are also present in the bulk, such as oxygen atoms in metal oxides. Therefore, a number of innovative techniques have been developed in order to selechvely probe surface species. Frequently this entails the use of double-resonance methods, involving, for example, polarizahon transfer between two nuclei. 

A new heteronuclear dipolar interaction recoupling scheme suitable for the characterization of heteronuclear dipolar couplings for multiple-spin systems under very fast MAS condition has been proposed The new technique has been shown to be superior to the rotational echo double resonance method and other recoupling schemes as far as the interference of homonuclear dipolar interaction is concerned. The potential of the technique is illustrated for fluorapatite sample at a spinning frequency of 25 kHz. 

---