Chemical reactions forbidden

Since the optical transitions near the HOMO-LUMO gap are symmetry-forbidden for electric dipole transitions, and their absorption strengths are consequently very low, study of the absorption edge in Ceo is difficult from both an experimental and theoretical standpoint. To add to this difficulty, Ceo is strongly photosensitive, so that unless measurements arc made under low light intensities, photo-induced chemical reactions take place, in some cases giving rise to irreversible structural changes and polymerization of the... 

Hydrogenation of olefins is a good example for demonstrating the roles of the surface atoms in catalysis. The orbital symmetry rule in chemical reactions suggests that the highest occupied molecular orbital (HOMO) of one reaction partner and the lowest unoccupied molecular orbital (LUMO) of the other should meet the symmetry requirements. In this respect, a concerted addition of an H2 molecule to the double bond of an olefin, that is, a molecular addition reaction, is a forbidden process. Adsorption of olefin on transition metal surfaces undoubtedly changes the population of electrons in the HOMO (7tu) and the LUMO (re ) as shown schematically in Fig. 1. In spite of such perturbation of the electron densities of the HOMO and the... 

Upper excited states are extremely short-lived. When the molecule is promoted to an excited singlet state beyond S1 the non-radiative deactivation by internal conversion is much faster than the spin-forbidden intersystem crossing to any triplet state. Therefore, the first excited singlet state is formed with near unit quantum yield. If an upper triplet state could be reached, it would also deactivate very rapidly to T1 and no singlet excited state would be formed. The extremely short lifetime of all upper excited states Sb(m>1) and Tb(w>1) means that luminescence emission and chemical reaction are, as a rule, not observed from such states. There are some exceptions to this rule, but there are many more mistaken reports of chemical reactions from short-lived upper excited states. Any such report... 

The probability that an excited state will react is defined according to its rate parameters simply as kTz, where kr is the rate constant for the reaction and t is the kinetic lifetime of the excited state (tt for the triplet), determined by the rates of all reactions undergone by that state. The unimolecular physical reactions of excited triplets are spin-forbidden and therefore relatively slow, so that chemical reactions are often highly efficient (krTT = 1). 

The real breakthrough in recognizing the role that symmetry plays in determining the course of chemical reactions has occurred only recently, mainly through the activities of Woodward and Hoffmann [5, 6], Fukui [7, 8], Bader [9, 10], Pearson [11], Halevi [12, 13], and others. The main idea in their work is that symmetry phenomena may play as important a role in chemical reactions as they do in the construction of molecular orbitals or in molecular spectroscopy. It is even possible to make certain symmetry based selection rules for the allowedness and forbiddenness of a chemical reaction, just as is done for spectroscopic transitions. 

The statement a chemical reaction is symmetry allowed or symmetry forbidden, should not be taken literally. When a reaction is symmetry allowed, it means that it has a low activation energy. This makes it possible for the given reaction to occur, though it does not mean that it always will. There are other factors which can impose a substantial activation barrier. Such factors may be steric repulsions, difficulties in approach, and unfavorable relative energies of orbitals. Similarly, symmetry forbidden means that the reaction, as a concerted one, would have a high activation barrier. However, various factors may make the reaction still possible for example, it may happen as a stepwise reaction through intermediates. In this case, of course, it is no longer a concerted reaction. 

Halevi s method to determine whether a chemical reaction is allowed or forbidden considers both the electronic and vibrational changes in the molecule. Of course, its high degree of rigor may render its application more complicated as compared with the methods which focus only upon changes in the electronic structure. The approaches introduced by Fukui and Woodward and Hoffmann seem to have received more widespread acceptance and utilization. 

The electronic transition to the MLCT and nn excited states are optically allowed transitions, and they have relatively large transition moments. They do not involve the population of an orbital that is antibonding with regard to the M—L bonds, in contrast to the forbidden transitions to the LF excited states. This is one of the reasons for photostability of Red) complexes. However, as discussed in the following sections, photoinduced chemical reactions have been reported in some cases, where transitions to reactive higher-energy states arise from photoexcitation with shorter wavelength irradiation or thermal activation from lowest excited state. 

DOT CLASSIFICATION Forbidden SAFETY PROFILE A powerful oxidizer. Moderately flammable due to spontaneous chemical reaction. Explosion hazard due to shock, chemical reaction, or exposure to heat. A storage hazard it may explode at room temperature. Explodes when heated to 100°C. When contaminat-ed it is very sensitive. Solution in water may explode if heated or dried. When heated to decomposition it emits highly toxic fumes of Cr and NOx. Incompat-ible with reducing materials Brp3 BrFs. 

DOT CLASSIFICATION Forbidden SAFETY PROFILE A poison. Can explode spontaneously. The solid, liquid and vapor are shock-sensitive explosives. Concentrated solutions in organic solvents may explode. Moderate fire hazard in the form of vapor by chemical reaction. A powerful oxidant. Moderately explosive when exposed to heat. The liquid explodes on contact with arsenic, sodium, silver foil, or phosphorus. Incompatible with Sb, ethyl ether, Ag, metals. When heated to decomposition it... 

DOT CLASSIFICATION Forbidden SAFETY PROFILE A severe explosion hazard when shocked, exposed to heat or flame, or by spontaneous chemical reaction. It has no known uses as an explosive because it is far too sensidve in the dry state to store or handle safely. If this material must be worked with, it should be kept wet. A convenient way of keeping it wet is with ether when it is needed in the dry state, it simply has to be taken out into the open and the ether wiU evaporate, leaving it perfecdy dry. When dry, it will explode when given the slightest touch, vibradon, or rise in temperature. Even a puff of air directed into it can cause it to detonate. It is a high explosive and is very violent. Incompadble with O3. H2S, CI2, Br2, acids. See also IODIDES. 

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