Structure resonance modulation spectroscopy

The structure of furazan (1) which was first reported in 1965 <65JCP(43)166> has been redetermined using double resonance modulation microwave spectroscopy <88ZN(A)597>. The bond lengths and bond angles for the heterocyclic ring are given in Table 1 where they are compared with those obtained from MO calculations. 

Microwave spectroscopy is a powerful tool for the determination of molecular structure. Thiazoles and thiadiazoles have been studied by this technique, but it was not until 1976 that a paper on the microwave spectrum of 1,2,3-thiadiazole appeared. Bond distances and angles for 1,2,3-thiadiazole (7) are listed in Table 4 (76MI42400). The success of this project is owed in part to the development of double resonance modulated (DRM) microwave spectroscopy which allows for quick analysis of an individual spectrum. 

X-Ray diffraction measurements have not been reported on 1,2,4-thiadiazole. Double resonance modulation microwave spectroscopy has recently been used to study the structure of four isomeric thiadiazoles (76ZN(A)1681). The bond lengths and bond angles reported for 1,2,4-thiadiazole are shown in Scheme 2. 

A determination of the complete structure of the four thiadiazoles by double resonance modulation microwave spectroscopy has provided precise information of the bond distances and angles of each isomer. The results show that closest comparability exists between 1,2,4- and 1,2,5-thiadiazole on the one hand, and between the 1,2,3- and 1,3,4-isomers on the other.271... 

Electron spin resonance (ESR) spectroscopy is a very powerful and sensitive method for the characterization of the electronic structures of materials with unpaired electrons. There is a variety of ESR techniques, each with its own advantages. In continuous wave ESR (CW-ESR), the sample is subjected to a continuous beam of microwave irradiation of fixed frequency and the magnetic field is swept. Different microwave frequencies may be used and they are denoted as S-band (3.5 GHz),X-band (9.25 GHz), K-band (20 GHz), Q-band (35 GHz) and W-band (95 GHz). Other techniques, such as electron nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM) spectroscopies, record in essence the NMR spectra of paramagnetic species. 

Not all the information can be obtained by the basic CW experiment that is considered by many chemists as all there is to EPR. Elucidating geometric structure or small spin densities requires the separation of small hyperfine couplings or dipole-dipole couplings between electron spins from larger interactions. This can be achieved by double resonance experiments, such as electron nuclear double resonance (ENDOR) [8,9] and electron electron double resonance (ELDOR) spectroscopy and further pulse-EPR techniques [10] such as electron spin echo envelope modulation (ESEEM). Pulse-EPR techniques may also provide more information on dynamic processes than simple CW experiments and may access longer time scales. 

Mossbauer spectra have been reported for some tellurium heterocycles e.g., (8). The complete structure of 1,2,5-thiadiazole (9) has been determined by double-resonance modulation (DRM) microwave spectroscopy. ... 

Double resonance modulation microwave spectroscopy, MA structure proof, 5, 6... 

Coherent Raman detected EPR spectroscopy is a new experimental method which combines optical and EPR transitions. It was applied for the first time to a bioinolecule, azurin. The information obtained allows to test electronic and structural models of the Cu site to be probed.23 In a subsequent report the same group of authors has applied microwave modulated circular dichroism to unravel individual resonance lines in overlapping spectra and to assign them to specific electronic transitions.24... 

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