Internal conversion (chemistry)
- Internal conversion (chemistry)
Internal conversion is a transition in from a higher to a lower electronic state in a molecule. It is sometimes called "radiationless de-excitation", because no photons are emitted. It differs from intersystem crossing in that, while both are radiationless methods of de-excitation, the molecular spin state for internal conversion remains the same, whereas it changes for intersystem crossing. The energy of the electronically excited state is given off to vibrational modes of the molecule. The excitation energy is transformed into heat.
A classic example of this process is the quinine sulfate fluorescence, which can be quenched by the use of various halide salts. What happens is that the excited molecule can de-excite by increasing the thermal energy of the surrounding solvated ions.
A general and quantitative discussion of intramolecular radiationless transitions is the subject of an article by M. Bixon and J. Jortner ("J. Chem. Phys.", 48 (2) 715-726 (1968)).
Several natural molecules perform a very fast internal conversion. This ability to transform the excitation energy of photon into heat can be a crucial property for photoprotection. Fast internal conversion reduces the excited statelifetime, and thereby prevents bimolecular reactions (oxidative stress and free radicals).DNA has an extremely short lifetime due to a mindbogglingly ultrafast internal conversion.[cite web |url= http://www.chemistry.ohio-state.edu/~kohler/dna.html |title=ultrafast internal conversion of DNA |accessdate=2008-02-13 |format= |work= ] ]Melanin is also a molecule with extremely fast internal conversion. [ cite journal |author=Meredith, Paul; Riesz, Jennifer |title=Radiative Relaxation Quantum Yields for Synthetic Eumelanin |url= |journal= Photochemistry and photobiology |volume=79 |issue=2 |pages=211–216 |year=2004 |pmid= |issn= |doi=10.1562/0031-8655(2004)079<0211:RCRQYF>2.0.CO;2] This is what makes it a good photoprotective substance.]Both Melanin and DNA have internal conversion rates that are many orders of magnitude faster than any man-made molecule.
In applications that make use of bimolecular electron transfer the internal conversion is undesirable. For example it is advantageous to have a long lived excited states in Grätzel cells (Dye-sensitized solar cells).Bimolecular electron transfer always produces a reactive chemical species (it produces free radicals)
References
Wikimedia Foundation.
2010.
Look at other dictionaries:
Internal conversion — This article is about the nuclear process. For the chemical process, see Internal conversion (chemistry). Internal conversion is a radioactive decay process where an excited nucleus interacts with an electron in one of the lower atomic orbitals,… … Wikipedia
Conversion — may refer to: Contents 1 Economy and Finance 2 Law 3 Marketing 4 Religion 5 Sport … Wikipedia
chemistry — /kem euh stree/, n., pl. chemistries. 1. the science that deals with the composition and properties of substances and various elementary forms of matter. Cf. element (def. 2). 2. chemical properties, reactions, phenomena, etc.: the chemistry of… … Universalium
Z-matrix (chemistry) — In chemistry, the Z matrix is a way to represent a system built of atoms. A Z matrix is also known as an internal coordinate representation. It provides a description of each atom in a molecule in terms of its atomic number, bond length, bond… … Wikipedia
Cluster chemistry — In chemistry, a cluster is an ensemble of bound atoms intermediate in size between a molecule and a bulk solid. Clusters exist of diverse stoichiometries and nuclearities. For example, carbon and boron atoms form fullerene and borane clusters,… … Wikipedia
Defining equation (physical chemistry) — For the detailed nature of defining equations see Physical quantity Main article: Physical chemistry In physical chemistry, there are numerous quantities associated with chemical compounds and reactions; notably in terms of amounts of substance,… … Wikipedia
radiation — radiational, adj. /ray dee ay sheuhn/, n. 1. Physics. a. the process in which energy is emitted as particles or waves. b. the complete process in which energy is emitted by one body, transmitted through an intervening medium or space, and… … Universalium
photochemical reaction — Chemical reaction initiated by absorption of energy in the form of visible (light), ultraviolet, or infrared radiation. Primary photochemical processes occur as an immediate result, and secondary processes may follow. The most important example… … Universalium
Radioactive decay — For particle decay in a more general context, see Particle decay. For more information on hazards of various kinds of radiation from decay, see Ionizing radiation. Radioactive redirects here. For other uses, see Radioactive (disambiguation).… … Wikipedia
radioactivity — /ray dee oh ak tiv i tee/, n. Physics, Chem. the phenomenon, exhibited by and being a property of certain elements, of spontaneously emitting radiation resulting from changes in the nuclei of atoms of the element. Also called activity. [1895… … Universalium
