Molecular Orbitals and Orbital Symmetry

Both frontier molecular orbital and orbital symmetry approaches represent a significant divergence from classical chemical models of pericyclic reactions (the term pericyclic reactiOTi belongs to Woodward and Hoffmann). One of the... 

The bonding n molecular orbital pair (with m = +1 and -1) is of Tty symmetry whereas the corresponding antibonding orbital is of Tig symmetry. Examples of such molecular orbital symmetries are shown above. 

Chemiluminescence is defined as the production of light by chemical reactions. This light is cold , which means that it is not caused by vibrations of atoms and/or molecules involved in the reaction but by direct transformation of chemical into electronic energy. For earlier discussions of this problem, see 7 9h Recent approaches towards a general theory of chemiluminescence are based on the relatively simple electron-transfer reactions occurring in aromatic radical-ion chemiluminescence reactions 10> and on considerations of molecular orbital symmetry as applied to 1.2-dioxetane derivatives, which very probably play a key role in a large number of organic chemiluminescence reactions 11>. 

Roald Hoffmann, a former coworker of R.B. Woodward and Nobel Prize as well for his contribution to the frontier orbital theory (the famous Woodward-Hoffmann rules concerning the conservation of molecular orbital symmetry), has also emphasised the artistic aspects of organic synthesis "The making of molecules puts chemistry very close to the arts. We create the objects that we or others then study or appreciate. That s exactly what writers, visual artists and composers do" [15a]. Nevertheless, Hoffmann also recognises the logic content of synthesis that "has inspired people to write computer programs to emulate the mind of a synthetic chemist, to suggest new syntheses". 

The results of the alkylbenzene series may also be readily explained in terms of ir complex adsorption. In this series, the molecular orbital symmetry of individual members remains constant while the ionization potential, electron affinity, and steric factors vary. Increased methyl substitution lowers the ionization potential and consequently favors IT complex adsorption. However, this is opposed by the accompanying increase in steric hindrance as a result of multiple methyl substitution, and decrease in electron affinity (36). From previous data (Tables II and III) it appears that steric hindrance and the decreased electron affinity supersede the advantageous effects of a decreased ionization potential. The results of Rader and Smith, when interpreted in terms of tt complex adsorption, show clearly the effects of steric hindrance, in that relative adsorption strength decreases with increasing size, number, and symmetry of substituents. 

The interpretation of chemical reactivity in terms of molecular orbital symmetry. The central principle is that orbital symmetry is conserved in concerted reactions. An orbital must retain a certain symmetry element (for example, a reflection plane) during the course of a molecular reorganization in concerted reactions. It should be emphasized that orbital-symmetry rules (also referred to as Woodward-Hoffmann rules) apply only to concerted reactions. The rules are very useful in characterizing which types of reactions are likely to occur under thermal or photochemical conditions. Examples of reactions governed by orbital symmetry restrictions include cycloaddition reactions and pericyclic reactions. 

R. G. Pearson, Molecular Orbital Symmetry Rules, Chemical and Engineering News, Sept. 1970, p. 66. 

In absence of a catalyst, simple olefins are essentially fixed in their bonding configurations reaction paths to interconversions through molecular collisions, fusions, and disassociations are apparently closed because of orbital symmetry restrictions, as proposed by Hoffman and Woodward 8°). Mango 8 has postulated that in the presence of certain transition metal catalysts, these orbital symmetry restraints are lifted, allowing bonds to flow freely and molecular systems to interchange. Thus, the conservation of molecular orbital symmetry is a key function of the catalyst. 

Spectroscopic analysis revealed that the thermally initiated [3 + 2] polycycloaddition produced 1,4- and 1,5-substituted triazole isomers in an approximately 1 1 ratio. This ratio appears to be statistic and dependant on the bulkiness of the organic moieties. For example, hfr-r-P[30(4)-20] with butyl spacers contained slightly more 1,4-triazole isomers than did hb-r-P[30(6)-20] with hexyl spacers. This becomes clearer if we look at the proposed transition states a and b of the [3 + 2]-dipolar cycloaddition (Scheme 16). Because of their molecular orbital symmetry, the acetylene and azide functional groups arrange in two parallel planes, a so-called two-plane orientation complex [48], which facilitates a concerted ring formation. If the monomer fragment or the polymer branch ( ) attached to the functional groups are bulky, steric repulsion will come into play and transition state a will be... 

Criteria and guidelines useful in network elucidation and supplementing the rules derived in this chapter include considerations of steric effects, molecularities of postulated reaction steps, and thermodynamic constraints as well as Tolman s 16- or 18-electron rule for reactions involving transition-metal complexes and the Woodward-Hoffmann exclusion rules based on the principle of conservation of molecular orbital symmetry. Auxiliary techniques that can be brought to bear include, among others, determinations of isomer distribution, isotope techniques, and spectrophotometry. 

The same holds of course also for exponents. Here, however, the optimization is too difficult. Since we are forced to use for molecular calculations exponents optimum for atoms, the following questions may be asked. To what degree do atomic orbitals change when molecules are formed What sequence, combination and extent of optimization of the orbital exponents X for molecules is necessary and useful How many STO symmetry basis functions are needed to adequately represent each molecular orbital symmetry type What kind of functions (Is, 2s, 3s, 2p,. .. 3d, is needed and what number of each type should... 

The discussion of other MCSCF wavefunctions will be preceded by a brief discussion of the full Cl expansion. The full Cl expansion for a molecular system consists of all possible orbital occupations and all possible spin couplings consistent with the overall molecular spatial and spin symmetry. If the orbital basis set is complete (i.e. an arbitrary function of the three spatial coordinates may be represented exactly with the basis), the CSF expansion space is also complete and is called complete Cl. Only finite, and therefore incomplete, basis sets are considered in this discussion. If the reductions due to spatial symmetry are ignored, the number of expansion terms is given by the WeyF dimension formula... 

This proliferation of reaction paths may be subdivided into those which involve the transfer of only one electron (paths 1 and 3) and those in which a second electron jump also occurs (paths 2, 4, 5 and 6). Molecular orbital symmetry favours45 the first electron jump in collisions where the halogen molecule has a roughly collinear orientation with respect to the K2 dimer, but places no requirement on the orientation of the K2 dimer itself. Examination54 of the electronic structure of the incipient KJ XY- complex, suggests that a second electron jump is inhibited when the orientation of the K2 dimer is broadside to the halogen molecule axis and enhanced when the K2 axis is collinear. In particular, chemi-ionization seems most favoured when the KKX Y complex is most nearly in a linear configuration. Thus, the reaction dynamics may be summarized as illustrated in Fig. 10. 

According to the conservation of orbital symmetry theory, whether a compound will undergo a pericyclic reaction under particular conditions and what product will be formed both depend on molecular orbital symmetry. To understand pericyclic reactions, therefore, we must now review molecular orbital theory. We will then be able to understand how the symmetry of a molecular orbital controls both the conditions under which a pericyclic reaction takes place and the configuration of the product that is formed. 

As a prelude to further analysis, it is useful to review one important property of molecular orbitals. As noted in Chapter 1, symmetry-correct molecular orbitals must be either symmetric or antisymmetric with respect to the full symmetry of the basis set of atomic orbitals that are used to construct the molecular orbitals. In the analysis of orbital symmetries, we will need to consider only the number of molecular S5nnmetry elements that are sufficient to distinguish between allowed and forbidden pathways. Also, it is not necessary to consider here the minor perturbation of molecular orbital symmetry that results from isotopic or alkyl substitution. In other words, to a first approximation the basis set orbitals of any conjugated diene are considered to be the same as those for 1,3-butadiene. Figtue 11.13 shows the... 

Pericyclic Reactions and Molecular Orbital Symmetry Chapter 1 3... 

The most important observation in the smdy of pericyclic reactions is the existence of conservation of molecular orbital symmetry throughout the transformation, meaning thereby that the symmetric orbitals are converted into symmetric orbitals whereas antisymmetric orbitals are converted into antisymmetric orbitals. In this approach, symmetry properties of various molecular orbitals of the bonds that are involved in the bond breaking and formation process during the reaction are considered and identified with respect to C2 and m elements of symmetry. These properties remain preserved throughout the course of reaction. Then a correlation diagram is drawn in which the molecular orbital levels of like symmetry of the reactant are related to that of the product by drawing lines. 

The effects of molecular structure on the rates of energy transfer are manifested in many ways. Additional pathways are available in collisions with molecules, such as the influence of ionic potential surfaces [35], the availability of near resonant electronic-to-vibrational and rotational energy transfer pathways [36], and the introduction of nonadiabatic transitions due to the breaking of the molecular orbital symmetry [37]. For the studies considered here, we might also add the competition between reaction and the desired energy transfer process, the possibility of energy transfer processes in the entrance or exit channels, selective changes in... 

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