Atomic orbitals reactions

Election nuclear dynamics theory is a direct nonadiababc dynamics approach to molecular processes and uses an electi onic basis of atomic orbitals attached to dynamical centers, whose positions and momenta are dynamical variables. Although computationally intensive, this approach is general and has a systematic hierarchy of approximations when applied in an ab initio fashion. It can also be applied with semiempirical treatment of electronic degrees of freedom [4]. It is important to recognize that the reactants in this approach are not forced to follow a certain reaction path but for a given set of initial conditions the entire system evolves in time in a completely dynamical manner dictated by the inteiparbcle interactions. 

There are two mechanisms by which a phase change on the ground-state surface can take place. One, the orbital overlap mechanism, was extensively discussed by both MO [55] and VB [47] formulations, and involves the creation of a negative overlap between two adjacent atomic orbitals during the reaction (or an odd number of negative overlaps). This case was temied a phase dislocation by other workers [43,45,46]. A reaction in which this happens is... 

Many of the reactions in which acetylene participates, as well as many properties of acetylene, can be understood in terms of the stmcture and bonding of acetylene. Acetylene is a linear molecule in which two of the atomic orbitals on the carbon are sp hybridized and two are involved in 7T bonds. The lengths and energies of the C—H O bonds and C=C<7 + 27t bonds are as follows ... 

The problems which the orbital approximation raises in chemical education have been discussed elsewhere by the author (Scerri [1989], [1991]). Briefly, chemistry textbooks often fail to stress the approximate nature of atomic orbitals and imply that the solution to all difficult chemical problems ultimately lies in quantum mechanics. There has been an increassing tendency for chemical education to be biased towards theories, particularly quantum mechanics. Textbooks show a growing tendency to begin with the establishment of theoretical concepts such as atomic orbitals. Only recently has a reaction begun to take place, with a call for more qualitatively based courses and texts (Zuckermann [1986]). A careful consideration of the orbital model would therefore have consequences for chemical education and would clarify the status of various approximate theories purporting to be based on quantum mechanics. 

It is expected from simple Ihermochemical considerations that adjacent n-, 0-or lone pair orbitals should have a significant influence over the facility of atom transfer reactions. Thus, the finding that /-buloxy radicals show a marked preference for abstracting hydrogens a to ether oxygens (Figure 1.9) is not... 

Aryl vinyl sulphones, reactions of 646 Aryl vinyl sulphoxides 620 optical resolution of 287 reactions of 354, 355, 360, 361, 621 Asscher-Vofsi reaction 189 Asymmetric induction 625 Asymmetric oxidation 72-78 Asymmetric reduction 78, 79 Asymmetric synthesis 824-846 Atomic orbitals 2, 3 Azetidinones 790, 791 ot-Azidoaldehydes, synthesis of 811 Azidosulphones, photolysis of 883, 884 Azosulphones, photolysis of 879 Azoxysulphones, photolysis of 879 1-Azulyl sulphoxides, synthesis of 265... 

Atomic orbitals interact with each other to give bond orbitals (Sect. 1), which mutually interact to give molecular orbitals (Sect. 2). Here we will examine interactions of molecular orbitals, especially those of frontier orbitals important for chemical reactions. 

Evaluated from ionization potentials of dimethyl derivatives. STO-3G. picis the component of p-atomic orbital at the reaction center. 

With both (la) and (2a) above, lateral overlap of the p atomic orbitals on adjacent carbon atoms could lead to the formation of two localised n bonds as shown, and the compounds would thus be expected to resemble ethene, only twice as it were This is indeed found to be the case with (2), but (1) is found to behave differently in terms of its slightly greater stability (referred to above), in spectroscopic behaviour (see below), and in undergoing addition reactions more readily than does an isolated diene (p. 194). On looking more closely, however, it is seen that with (la), but not with (2a), lateral overlap could take place between all four p atomic orbitals on adjacent carbon atoms. Such overlap will result in the formation of four molecular orbitals (Fig. 1.2), two bonding ( and 2) and two anti-bonding (i//3 and 4)—the overlap of n atomic orbitals always gives rise to n molecular orbitals ... 

Summary Ab initio calculated bond dissociation energies of silicon compounds will be discussed by means of atomic ionization energies and atomic orbital overlap. Ring strain energies of C- as well as Si-rings are estimated by homodesmotic reactions. The hybridization concept is critically examined in the case of silicon compounds. From the most important results a set of basic rules will be presented. 

The molecular orbitals derived from overlap of six p atomic orbitals as found in trienes are shown in Figure 8.7. Since there are six electrons to accommodate, as two paired electrons per orbital, the HOMO is n3. Thus, for 2,4,6-octatriene the disrotatory mode of reaction gives the trans isomer (Scheme 8.5). 

In order to make things a bit more concrete at this point we display in Figure 1 a sterically favorable configuration for a reactive ion pair (Tl). Only the 3d atomic orbitals most directly involved in the electron exchange are shown. The theoretical model developed here is based on a so-called "outer-sphere" mechanism, in which the inner-sphere reactants preserve their integrity in the course of the exchange reaction (aside from bond distortions associated with the activation step). The... 

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