Non-local electronic perturbations

Non-local electronic perturbations of metal-carbonyl bonds... 

Key words Metal-carbonyl bonds, non-local electronic perturbations, carbonyl-hemes, chemisorbed carbon monoxide... 

In this communication I will compare the electronic structures of metal surfaces with that of metalloporphyrins and discuss the possibility of isolate non-local electronic perturbations for CO terminally bound to metal surfaces from the vibrational spectra of carbonyl-hemes. 

Abstract Metal-carbonyl bonds, as found for carbon monoxide either liganded to metalloporphyrins or chemisorbed onto metal surfaces, are discussed. These two classes of systems are compared with emphasis on short range vs. long range perturbations of the vibrational bands, and Vcq. It is concluded that metalloporphyrins serve as models for terminal bondi on metal surfaces and that data obtained for carbonyl-hemes enable the isolation of non-local electronic effects, which are much discussed in connection with alkali promoters coadsorbed with CO on metal catalysts. 

Quantum mechanics has made important contributions to the development of theoretical chemistry, e.g. the concept of quantum mechanical resonance in the interpretation of the perturbation in the excited states of polyelectronic systems, the concept of exchange in the formation of a covalent bond, the concept of non-localized bonds (though, in my view, unsatisfactory and only arising from a neglect of electronic repulsions), the concept of dispersion forces etc., but it is noteworthy that all these ideas owe their success and justification to their ability to account qualitatively for previously unexplained experimental facts rather than to their quantitative mathematical aspect. 

In this paper, the multiphonon relaxation of a local vibrational mode and the non-radiative electronic transitions in molecular systems and in solids are considered using this non-perturbative theory. Results of model calculations are presented. According to the obtained results, the rate of these processes exhibits a critical behavior it sharply increases near specific (critical) value(s) of the interaction. Also the usual increase of the non-radiative transition rate with temperature is reversed at certain value of the non-diagonal interaction and temperature. For a weak interaction, the results coincide with those of the perturbation theory. 

To sum up, we have developed a general non-perturbative method that allows one to calculate the rate of relaxation processes in conditions when perturbation theory is not applicable. Theories describing non-radiative electronic transitions and multiphonon relaxation of a local mode, caused by a high-order anharmonic interaction have been developed on the basis of this method. In the weak coupling limit the obtained results agree with the predictions of the standard perturbation theory. 

When irradiated at room temperature the non-locally compensated centers trap electrons to yield Ce + in 0 symmetry, while the holes become self-trapped producing perturbed centers (24), thus ... 

In solution, even centrosymmetric molecules can have large dipole moments in the excited state, as was first demonstrated in the case of 9,9 -bianthryl (7) (Beens and Weller, 1968). The symmetrical compound 8 is an example in which electronic excitation is localized almost entirely in one of the polar aromatic end groups, due to solvent-induced local site perturbation (Liptay et al., 1988a). In solution, polyenes may also show unsymmetrical charge distributions with non vanishing dipole moments (Liptay et al., 1988b see also Section 2.1.2). 

One of the most striking properties of lanthanide metals and compounds is the relative insensitivity of electrons in the unfilled 4f shell to the local environment, compared to non-f electron shells with similar atomic binding energies. Whereas the 5d and 6s electrons form itinerant electron bands in the metallic solids, the 4f electrons remain localised with negligible overlap with neighbouring ions. For the maximum in the 4f radial charge distribution lies within those of the closed 5s and 5p shells, so the 4f shell is well shielded from external perturbations on the atomic potential, such as the crystal field. 

Within Green function theory, many-electron effects can be introduced through a non-local and energy-dependent self-energy operator [15]. Since the self-energy is hard to calculate, various approximations are introduced and among the simplest ones is the so-called GW approximation, which is derived from many-body perturbation theory. Although the GW approximation offers in principle a sophisticated account of the electron correlation effects, practical realizations are commonly also based on the LDA method. 

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