Timescale in chemistry

Fig. 0.3 Typical condensed phase molecular timescales in chemistry and biology. (Adapted from G. R. Fleming and P. G. Wolynes, Physics Today, p. 36, May 1990).

Prather, M. J., Timescales in Atmospheric Chemistry CH,Br, the Ocean, and Ozone Depletion Potentials, Global Biogeochem. Cycles, 11, 393-400 (1997). 

Michael BD, Harrop HA, Held KD (1981b) Timescale and mechanisms of the oxygen effect in irradiated bacteria. In Rodgers MAJ, Powers EL (eds) Oxygen and oxy-radicals in chemistry and biology. Academic Press, New York, pp 285-292... 

Bond making and bond breaking are key events in chemistry. Until recently, direct experimental observation of these events - as they unfolds - was not possible. This was mainly due to the underlying ultrafast dynamics on the femtosecond timescale a timescale which already could be inferred from early theoretical works in molecular reaction dynamics (see, e.g., [1]). 

Over a century later, chemical kinetics remains a field of very considerable activity and development indeed nine Nobel prizes in Chemistry have been awarded in this subject area. The most recent (1999) was to A. H. Zewail whose work revealed for the first time what actually happens at the moment in which chemical bonds in a reactant molecule break and new ones form to create products. This gives rise to a new area femtochemistry. The prefix femto (abbreviation f ) represents the factor 10 i and indicates the timescale, which is measured in femtoseconds, of the new experiments. As some measure of how short a femtosecond is, while you read these words light is taking about 2 million femtoseconds (2 x 10 fs) to travel from the page to your eye and a further 1 000 fs to pass through the lens to the retina. 

Many problems of polymer physics and chemistry focusing on processes in certain space and timescales can incorporate processes characterized by shorter or longer space and timescales. In such cases, simulation can reasonably rely on multiscale schemes, that is, schemes in which the system is modeled differently for different subsystems. It should be emphasized that multiscale simulation is not just a complex simulation - it is a nrrmerical strat y that involves two or more physical processes or phenomena that ate coupled and often require disparate methods of solution. While substantial progress has occurred in mrrltiscale simulation over the past few years, this area remains extremely challenging, with issues that are still largely unresolved. 

One of the most exciting possibilities of ultrafast laser techniques is to follow the course of fundamental chemical reactions on the relevant timescale at which they occur. Previously, it was only possible to know the individual states of molecules A and B before reacting and the final state of the compound molecule AB. In contrast, the details of the chemical reaction can now be followed on a femtosecond scale with information on how chemical bonds are formed and broken. In particular, the existence of transition states has been demonstrated. This new field of science is frequently referred to as femtochemistry [9.191-9.204], for which A. Zewail was awarded a Nobel prize in chemistry (1999). 

Relationship Between Chemistry and Timescale in a Protometabolic System... 

Photogenerated electron transfer reactions in chemistry typically occur on a timescale significantly greater than the decay timescale of coherent processes. Electron transfer reactions fail this criterion in poly(phenylen-evinylene) based polymers, for which the photogenerated coherent wave-function persists for 25 fs (in solution), and electron transfer occurs in =45 fs in the solid BHJ material. In P3HT, the coherent wavefunction persists for 100 fs in solution, while the electron transfer timescale is <100 fs in the BHJ material. It is therefore necessary to develop testable hypotheses regarding the mechanism of electron transfer in situations where a coherent photoexcited state is involved in the ultrafast electron transfer in bulk heterojunction solar cells. 

A final note must be made about a common problem that has plagued many kinetic treatments of reactive intermediate chemistry at low temperatures. Most observations of QMT in reactive intermediates have been in solid matrices at cryogenic temperatures. Routinely, reactive intermediates are prepared for spectroscopy by photolyses of precursors imbedded in glassy organic or noble gas (or N2) solids. The low temperatures and inert surroundings generally inhibit inter- and intramolecular reactions sufficiently to allow spectroscopic measurements on conventional and convenient timescales. It is under such conditions, where overbarrier reactions are diminished, that QMT effects become most pronounced. 

In many chemical and even biological systems the use of an ab initio quantum dynamics method is either advantageous or mandatory. In particular, photochemical reactions may be most amenable to these methods because the dynamics of interest is often completed on a short (subpicosecond) timescale. The AIMS method has been developed to enable a realistic modeling of photochemical reactions, and in this review we have tried to provide a concise description of the method. We have highlighted (a) the obstacles that should be overcome whenever an ab initio quantum chemistry method is coupled to a quantum propagation method, (b) the wavefunction ansatz and fundamental... 

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