Excitation, electronic experimental investigations

What developments are likely in the future Now that many reactions are known, the investigation of reaction mechanisms of disilenes is just beginning. Much is likely to be learned about details of reaction pathways. Further studies, both experimental and theoretical, should lead to a more complete understanding of the chemical bonding in disilenes. Vibrational spectra and electronic excited states of disilenes have received very little... 

For many years, investigations on the electronic structure of organic radical cations in general, and of polyenes in particular, were dominated by PE spectroscopy which represented by far the most copious source of data on this subject. Consequently, attention was focussed mainly on those excited states of radical ions which can be formed by direct photoionization. However, promotion of electrons into virtual MOs of radical cations is also possible, but as the corresponding excited states cannot be attained by a one-photon process from the neutral molecule they do not manifest themselves in PE spectra. On the other hand, they can be reached by electronic excitation of the radical cations, provided that the corresponding transitions are allowed by electric-dipole selection rules. As will be shown in Section III.C, the description of such states requires an extension of the simple models used in Section n, but before going into this, we would like to discuss them in a qualitative way and give a brief account of experimental techniques used to study them. 

The physical chemist of today has a wide variety of methods at his disposal for the experimental investigation of electronic structure and all of them have been used in attempts at obtaining evidence of the participation of outer d-orbitals in bonding. One such group of methods is constituted by the various techniques of radiofrequency spectroscopy, which have the advantage that they yield information about the molecule in its ground state. In this they have a distinct superiority over, say, electronic absorption spectra where it is necessary to consider both ground and excited states. Moreover much of the data derived from radiofrequency spectroscopic methods concerns essentially just one part of the molecule so that attention can be concentrated on those atoms of interest in whatever study happens to be under way. 

The extensive series of known quadruply bound molybdenum dimers has also been the subject of detailed experimental investigations to unravel the electronic properties characterizing excited states in these Mo2+ derivatives. Consideration of the Mo2Xj spectra is presented first since the Mo2(02CR)4 compounds exhibit significantly different features in their electronic spectra. 

The C3N radical is an important astro-chemical compound. Its chemical properties are not well understood. Not even the structure of the system is known. An experimental group is investigating the spectral properties of the radical. They have difficulties in locating the lowest electronically excited states and would like to be aided by theoretical calculations. The quantum chemical problem is then to compute the electronic structure for the two lowest states of the radical. 

The other way to study the "conductivity of protein molecules towards electron tunneling is to investigate the quenching of luminescence of electron-excited simple molecules by redox sites of proteins [95,96]. Experiments of this sort on reduced blue copper proteins have involved electron-excited Ru(II)(bpy)3, Cr(III)(phen)3, and Co(III)(phen)3 as oxidants. The kinetics of these reactions exhibit saturation at protein concentrations of 10 3 M, suggesting that, at high protein concentrations, the excited reagent is bound to reduced protein in an electron transfer precursor complex. Extensive data have been obtained for the reaction of reduced bean plastocyanin Pl(Cu(I)) with Cr(III)(phen)3. To analyze quenching experimental data, a mechanistic model that includes both 1 1 and 2 1 [Pl(Cu(I))/ Cr(III)(phen)3] complexes was considered [96]... 

Recent advances in experimental techniques, particularly photoionization methods, have made it relatively easy to prepare reactant ions in well-defined states of internal excitation (electronic, vibrational, and even rotational). This has made possible extensive studies of the effects of internal energy on the cross sections of ion-neutral interactions, which have contributed significantly to our understanding of the general areas of reaction kinetics and dynamics. Other important theoretical implications derive from investigations of the role of internally excited states in ion-neutral processes, such as the effect of electronically excited states in nonadiabatic transitions between two potential-energy surfaces for the simplest ion-molecule interaction, H+(H2,H)H2+, which has been discussed by Preston and Tully.2 This role has no counterpart in analogous neutral-neutral interactions. 

The fact remains that the formation of electronically excited products in photo-induced e.t. reactions remains a hypothesis to this day. Experimental evidence is very difficult to obtain, and it does not appear that any specific investigation has been oriented in this direction so far. The presence of electronically excited molecular ions could in principle be ascertained by the observation of their luminescence, but this is known to be generally so weak that it is beyond the detection limits of most, if not all, currently available instrumental techniques. The conclusion concerning the intermediacy of electronically excited products in photo-induced e.t. must therefore be that it is unproven, though not unreasonable. 

A few chapters of the current volume describe different state-of-the-art experimental techniques used to unravel photophysical and photochemical properties of complex molecular systems. These chapters are especially tailored for the scholarly description of electronic excited state properties of nucleic acid bases and related species predicting different tautomeric distributions and possible nonra-diative deactivation processes. It is interesting to note that guanine provides particularly challenging case to discuss. Recent theoretical and experimental investigations show the existence of relatively significantly less stable imino tautomers in the... 

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