Single-molecule statistical physics

Unimolecular dissociation is one of the simplest types of irreversible chemical reactions It takes place in a single isolated molecule with an internal energy that exceeds the first dissociation threshold see Fig. 1(a) for a schematic overview. Nevertheless, the underlying atomic-level reaction mechanisms are very complex. Their theoretical description requires all the power of modern quantum chemical methods, statistical physics and nonlinear dynamics, and even then the full rigor can be achieved just for small, mostly triatomic molecules. Experimental studies have to be likewise advanced Up to three laser pulses are combined in a modern experiment to elucidate all details of the dissociation process. 

Kawakatsu, T. (2004) Statistical Physics of Polymers An Introduction. Heidelberg Springer. Kiriy A, Gorodyska G, Minko S, Jaeger W, StSpinek P, Stamm M. Cascade of coil-globule conformational transitions of single flexible polyelectrolyte molecules in poor solvent. J Am Chem Soc 2002 124 (45) 13454-13462. 

Chemical studies usually deal with a solute which can be a single molecule or a molecular complex or transition state in a chemical reaction. In such systems, the role of the solvent is mainly a physical perturbation which can be simulated at a lower theoretical level than that required for the study of the subsystem of chemical interest. The success of continuum models confirms this statement. In order to describe the solution at the molecular level and to perform full statistical mechanics computations on a model of macroscopic sample, one may set up some computationally efficient approaches by limiting the quantum chemical study to the solute and using one of the usual classical force-fields to represent the solvent molecules. The computation of the statistical averages can be done by means of either Monte Carlo or molecular dynamics algorithms. The so-called QM/MM models are now widely used in such chemical studies. 

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