Theory, detection systems bonding

Detection of the dA N1 and dC N3 adducts may not in one sense be particularly important for DNA based on their central position within the helical conformation and hydrogen bonding network.37,38 Still, the deoxynucleoside studies helped to focus attention on the reversibility of alkylation by QM and provided insight into the reactions of duplex DNA described below in Section 9.3. Reaction at the deoxynucleoside level also provided an essential system for developing a theoretical treatment of QM reaction.50-52 Computations based on density functional theory well rationalized the published results on d A and correctly anticipated the results on dG and dC reviewed above and described in more detail in Chapter 2 (Freccero). 

The aims of this study were to observe isotropically shifted signals for metal ions like copper(II), which usually give signals broadened beyond detection, and to relate the observed shifts and relaxation times to those of the uncoupled ions in order to understand the phenomena in theoretical terms. This approach allows the power of the NMR technique to fully exploit paramagnetic species and obtain information on spin delocalization, chemical bonding and so on. It is likely that the theory also applies to coupled metal ion-H adical systems like those proposed for derivatives of peroxidases (compound I), which contain iron(IV) and a heme radical (44). 

Abstract In this chapter we discuss the influence of ir-electron delocalization on the properties of H-bonds. Hence the so-called resonance-assisted hydrogen bonds (RAHBs) are characterized since such systems are mainly classified in the literature as those where TT-electron delocalization plays a very important role. Both the intramolecular and intermolecular RAHBs are described. RAHBs are often indicated as very strong interactions thus, their possible covalent nature is also discussed. Examples of the representative crystal structures as well as the results of the ab initio and DFT calculations are presented. Additionally the RAHB systems, and the other complexes where rr-electron delocalization effects are detectable, are characterized with the use of the QTAIM (Quantum Theory Atoms in Molecules ) method. The decomposition scheme of the interaction energy is applied to expand the knowledge of the nature of the RAHBs. 

Thus, a unitary approach linking the quantum mechanical formalisms at the chemical bonding level has been extensively researched. It was recently established that for an adequate treatment in quantum space of the polyatomic combinations, the electronic density p(r) and not the historical wave function v /(rj,...,r ) stays as the main variable for a system with N electrons. It is so because quite contrary to the wave function, the electronic density is an experimentally detectable quantity defined in the real three-dimensional space, and not within a 3N Hilbert abstract space. It is also directly related with the total number of electrons in the concerned system through the functional relation Jp(r)dT = N. Therefore, the electronic density receives the central role within the newest quantum paradigm of matter, the Density Functional Theory (Walter Kohn as its father, the Nobel laureate in Chemistry for this theory in 1998). 

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