Molecular structure complex, coherence

Despite the apparent simplicity of their molecular structure, the metabolism of these agents is chemically so complex, and their routes of bioactivation and inactivation so intimately intertwined, that a detailed and coherent picture of their behavior in the body is not available. The presentation to follow considers first their in vivo de-esterification, and only then the nonen-zymatic and enzymatic mechanisms postulated to be involved. 

H. Coherence in Electron Diffraction Complex Molecular Structures... 

To gain a proper understanding of the behaviour of a complex system we must first appreciate the structure and properties of the elementary units of which it is composed. In the study of ice this means that we must begin with a study of the water molecule, for it is from the individuality of the structure of that molecule that most of the unusual properties of ice and water arise. Without such a relation back to the fundamentals of molecular structure, the study of a particular material becomes simply a catalogue of its properties—useful, no doubt, but not very illuminating. In this book we shall try, at all stages, to show this relation so that a coherent picture emerges. Similar pictures can be built up for all solids the outlines, it is true, have many variations but they all follow in the same sort of way from the basic elements of which they are built. 

The fact that such an experimental window for coherent control in liquids does actually exist was verified in experiments on the selective multiphoton excitation of two distinct electronically and structurally complex dye molecules in solution (Brixner et al. 2001(b)). In these experiments, despite the failure of single-parameter variation (wavelength, intensity or linear chirp control), adaptive femtosecond pulse shaping revealed that complex laser fields could achieve chemically selective molecular excitation. These results prove, first, that the phase coherence of complex molecules persists for more than 100 fs in a solvent environment. Second, this is direct proof that it is the nontrivial coherent manipulation of the excited state and not of the frequency-dependent two-photon cross sections that is responsible for the coherent control of the population of the excited molecular state. 

In fact, semiempirical models have had a fundamental role in theoretical quantum chemistry until high speed/large memory computers became widely accessible, because the quantum chemical description of realistic molecular systems is an intrinsically complex problem. Moreover, coherent quantum dynamics processes that take place in large molecular structures at ambient conditions are still not amenable to be treated by the current ab-initio theoretical methods, unless in supercomputer facilities. It is, thus, important to keep developing reliable and efficient semiempirical time-dependent quantum chemistry methods. 

Since many of these developments reach into the molecular domain, the understanding of nano-structured functional materials equally necessitates fundamental aspects of molecular physics, chemistry, and biology. The elementary energy and charge transfer processes bear much similarity to the molecular phenomena that have been revealed in unprecedented detail by ultrafast optical spectroscopies. Indeed, these spectroscopies, which were initially developed and applied for the study of small molecular species, have already evolved into an invaluable tool to monitor ultrafast dynamics in complex biological and materials systems. The molecular-level phenomena in question are often of intrinsically quantum mechanical character, and involve tunneling, non-Born-Oppenheimer effects, and quantum-mechanical phase coherence. Many of the advances that were made over recent years in the understanding of complex molecular systems can therefore be transposed and extended to the study of... 

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