Electronic spectroscopy transition metal compounds

During the last few years the versatility of ENDOR spectroscopy has been improved by a number of new techniques which make use either of special types of pumping fields (CP-ENDOR, PM-ENDOR), of more than one rf field (DOUBLE ENDOR, multiple quantum transitions, nuclear spin decoupling) or a different display of the spectrum (EI-EPR). In addition to these techniques, alternative methods have been developed (electron spin echo and electron spin echo ENDOR) which are able to supplement or to replace the ENDOR experiment under certain conditions. The utility of all these various advanced techniques, particularly in studies of transition metal compounds, has recently been demonstrated. 

The development of electronic structure theories for metal complexes has always been closely linked with electron spectroscopy of transition metal compounds. We shall in the following describe both DFT and wave function methods that have been used in the study of excited states. We shall also discuss their application to the tetroxo systems. 

One suspects that the TT-allyl structure that is available in these conjugated systems is required for the necessary reactivity. Such is not the case with transition metal compounds that can coordinate with the tt electrons of the isolated double bond. More elucidation is needed of the actual bond structure of the growing chain and might be obtained from NMR spectroscopy (44-47) or similar techniques. 

Van der Laan G, Kirkman IW (1992) The 2p absorption spectra of 3d transition metal compounds in tetrahedral and octahedral symmetiy. J Phys Cond Matter 4 4189-4204 vanAken PA, Liebscher B, Styrsa VJ (1998) Quantitative detemtination of iron oxidation states in minerals using Fe 12 3-edge electron energy-loss near-edge stracture spectroscopy. Phys Chem Minerals 25 323-327... 

Applications of the complete active space (CAS) SCF method and multiconfigurational second-order perturbation theory (CASPT2) in electronic spectroscopy are reviewed. The CASSCF/CASPT2 method was developed five to seven years ago and the first applications in spectroscopy were performed in 1991. Since then, about 100 molecular systems have been studied. Most of the applications have been to organic molecules and to transition metal compounds. The overall accuracy of the approach is better than 0.3 eV for excitation energies except in a few cases, where the CASSCF reference function does not characterize the electronic state with sufficient accuracy. 

The CASPT2 approach has been used in a large variety of applications ranging from molecular structure determinations in ground and excited states [15] to calculations of binding energies of transition metal compounds [28, 29]. There is, however, little doubt that the most spectacular success of the new approach has been in electronic spectroscopy and photochemistry. Up until now, a lot of effort has been put into the refinement of computational methods for molecular ground states and it... 

Excitation energies were computed with errors less than 0.2 eV in both cases. This was a higher accuracy than any of the earlier calculations on Ni and benzene had achieved. The CASPT2 method has during the last three years been applied to a large number of electronic spectra. Some of the earlier applications in organic chemistry have already been reviewed [13]. Here we discuss additional applications in organic systems but also the electronic spectroscopy of transition metal compounds, where the demands on the approach are different. 

The Amsterdam Density Functional package (ADF) is software for first-principles electronic structure calculations (quantum chemistry). ADF is often used in the research areas of catalysis, inorganic and heavy-element chemistry, biochemistry, and various types of spectroscopy. ADF is based on density functional theory (DFT) (see Chapter 2.39), which has dominated quantum chemistry applications since the early 1990s. DFT gives superior accuracy to Hartree-Fock theory and semi-empirical approaches, especially for transition-metal compounds. In contrast to conventional correlated post-Hartree-Fock methods, it enables accurate treatment of systems with several hundreds of atoms (or several thousands with QM/MM)." ... 

Electron spin resonance (ESR) spectroscopy is also known as electron paramagnetic resonance (EPR). spectroscopy or electron magnetic resonance (EMR) spectroscopy. The main requirement for observation of an ESR response is the presence of unpaired electrons. Organic and inorganic free radicals and many transition metal compounds fulfil this condition, as do electronic triplet state molecules and biradicals, semicon-ductor impurities, electrons in unfilled conduction bands, and electrons trapped in radiation-damaged sites and crystal defect sites. 

This distribution of orbitals is common to a large family of transition metal compounds. The symmetry maybe disrupted further if the six atoms coordinated to the metal are not equivalent, but it is still generally possible to group the resulting orbitals into approximate tjg and Cg sets. These symmetry properties have wonderful implications for the spectroscopy of these compounds, because, despite the rich complexity of the electronic states in transition metals, there are some general patterns that emerge for this family of compounds, patterns that we can exploit to analyze the nature of the metal and its bonds. 

The purpose of this chapter is to provide an overview of a rather wide array of experimental techniques that can tell us about the electronic structure of molecules. Some of these techniques, such as photoelectron (PE) spectroscopy, which is based on Einstein s photoelectric effect, are generally applied to gas-phase molecules. They can give high-resolution spectra, providing information about molecular vibrations and even, in a few cases, rotations. At the other end of the scale, UV/vis spectroscopy, particularly as applied to transition-metal complexes in solution, involves broad bands, and although it is an important and widely-used method, the information it gives is limited. Emission spectroscopy of transition-metal compounds has also become important. 

The pulse Fourier transform approach to magnetic resonance spectroscopy has been extensively developed and successfully applied to systems of one-half spin and their mutual interactions. But resonance spectroscopy of spin systems with the higher half- and integer spin quantum numbers is commonplace, for example, in the case of alkali metal nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) of transition metal compounds involving multi-quantum transitions. Similarly, magnetic resonance at zero field entails the observation of multi-quantum transitions. 

Some Aspects of the Electron Paramagnetic Resonance Spectroscopy of d-Transition Metal Compounds by F. E. 

There are two main thrusts for the study of polynuclear d-transition metal compounds (i) molecular magnetic and/or electronic materials, and (ii) as models for polynuclear metalloenzyme sites. EPR spectroscopy is of course an invaluable tool in both areas. 

A,x = 4.6mT and Ayy =5.2 mT. Reproduced with permission of the Royal Society of Chemistry from Mabbs FE (1993) Some aspects of the electron paramagnetic resonance spectroscopy of d-transition metal compounds. Chemical Society Reviews 22 313-324. 

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