TAILIEUCHUNG - Ideas of Quantum Chemistry P92

Ideas of Quantum Chemistry P92 shows how quantum mechanics is applied to chemistry to give it a theoretical foundation. The structure of the book (a TREE-form) emphasizes the logical relationships between various topics, facts and methods. It shows the reader which parts of the text are needed for understanding specific aspects of the subject matter. Interspersed throughout the text are short biographies of key scientists and their contributions to the development of the field. | 876 15. Information Processing - the Mission of Chemistry Claude Elwood Shannon 1916-2001 American mathematician professor at the Massachusetts Institute of Technology his professional life was associated with the Bell Laboratories. His idea now so obvious that information may be transmitted as a sequence of 0 and 1 was shocking in 1948. It was said that Shannon used to understand problems in zero time . where p stands for the probability of the event the information reports. How much information is contained in the news that in a single trial coin came down tails Well it is I - log2 2 1 bit. The news there is air in Paris is of no use in a TV news service 39 because in this case I - log21 0. Claude Shannon introduced the notion of the average information associated with all possible N results of an event in the usual way N N H Pili - 2 Pi lOg2Pi i 1 i 1 information where H is called the entropy of information because a similar formula works in entropy thermodynamics for entropy. The quantity H a measure of our ignorance is largest if all pi are equal. At a given instant we estimate the probabilities of all possible results of an event we compute Ho then reliable information arrives and the estimation changes we compute the information entropy in the new situation Hf . Then according to Shannon the measure of the information received is I Ho - Hf. Example 1. Information flow in transcription. The sequence of three DNA bases codon there are four bases possible A T G C or a codon codes for a single amino acid there are 20 possible amino acids in protein. Why three Maybe three is too many Let us see what the problem looks like from the point of view of information flow. A single codon carries the following information in bits Icodon3 log2 i 4 4 4 I 6 39Now we know why the everyday TV news is full of thefts catastrophes and unbridled crimes. Although it apparently looks upsetting in fact it represents an optimistic signal this is just incredibly rare.

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