Symmetry forbidden chemical reactions

Hoffmann solved the problem introducing the notion of symmetry-allowed and symmetry-forbidden chemical reactions. What really mattered was the symmetry of the tt molecular orbitals involved. These are the orbitals, which certainly have to change their character when the reaction proceeds one of them changes from nr to cr. In the case of the cis-butadiene, we have two it orbitals (Fig. 14.2 Ib) [Pg.943]

Orbital symmetry rules for predicting thermally and photo-chemically driven symmetry-allowed and symmetry-forbidden concerted reactions. 

The important concept is that it is possible to make certain spin and symmetry based selection rules for the prediction of allowed and forbidden chemical reactions. Also important is the so-called non-crossing rule introduced by Neumann and Wigner, and independently by Teller which states that two orbitals of the same symmetry cannot intersect in an orbital energy correlation diagram. Correlation diagrams can provide valuable information about the allowed transition state of a chemical reaction and therefore about possible outcomes of a chemical reaction. 

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... 

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. 

The question of stepwise vs. concerted processes in the catalysis of symmetry-forbidden reactions becomes more important when the postulated forbidden-to-allowed process itself is questioned. Certainly, stepwise processes in catalysis are common, and have been known to intervene in catal5dic reactions for some time. So to establish that an overall chemical transformation A B proceeds through distinct steps e.g., A - X - - B) is not, in itself, chemically significant. To suggest, however, that a chemical transformation A - B can proceed smoothly on the coordination sphere of a transition metal, and essentially nowhere else, introduces the possibihty of novel chemistry, not encountered in or outside of catalysis. The mere observation that a symmetry-forbidden reaction A - B proceeds in the presence of a metal, does not, in itself, mean that the symmetry-restricted transformation A B proceeded in a concerted manner on the coordination sphere of that metal. Here, the question of stepwise vs. concerted takes on new significance, for it bears on the possible existence of a special kind of chemical transformation. 

Berson, Jerome A. 1972. "Orbital-Symmetry-Forbidden Reactions." Accounts of Chemical Research 5 406 14. 

Symmetry and stability analysis. The semi-empirical unrestricted Hartree-Fock (UHF) method was used for symmetry and stability analysis of chemical reactions at early stage of our theoretical studies.1,2 The BS MOs for CT diradicals are also expanded in terms of composite donor and acceptor MOs to obtain the Mulliken CT theoretical explanations of their electronic structures. Instability in chemical bonds followed by the BS ab initio calculations is one of the useful approaches for elucidating electronic structures of active reaction intermediates and transition structures.2 The concept is also useful to characterize chemical reaction mechanisms in combination with the Woodward-Hoffman (WH) orbital symmetry criterion,3 as illustrated in Figure 1. According to the Woodward-Hoffmann rule,3 there are two types of organic reactions orbital-symmetry allowed and forbidden. On the other hand, the orbital instability condition is the other criterion for distinguishing between nonradical and diradical cases.2 The combination of the two criteria provides four different cases (i) allowed nonradical (AN), (ii) allowed radical (AR), (iii) forbidden nonradical (FN), and (iv) forbidden radical (FR). The charge and spin density populations obtained by the ab initio BS MO calculations are responsible for the above classifications as shown in Fig. 1. 

Hence, from the standpoint of invariance, any chemical reaction including polymerization is characterized by a set of observables (e.g., the elements of symmetry with operators commuting with the Hamiltonian of the reaction) A. The processes during which all the combination of properties of the reacting system (formally characterized by a set of eigenvalues a ) remains invariant are allowed in the sense of symmetry. This is the nature of both physical and chemical kinetic selection rules including the Woodward-Hoffmann principle. Hence, the specific feature of all selection rules is the fact that they allow much less than forbid . In other words, each of them exhibits a kind of veto right on the occurrence of chemical reactions. At the same time, the processes allowed with respect to symmetry may be forbidden by thermodynamic or steric factors. 

Therefore, interaction of the half-filled TTg orbitals of O2 with the filled H u orbitals of N2 is symmetry forbidden. Electron density could flow from the half-filled TTg HOMO of O2 to the empty TTg orbitals on N2, but that is contrary to the nature of the atoms based on their electronegativities. Therefore, transfer of electron density from O2 to N2 is excluded for chemical reasons. As a result, the reaction... 

Summary Reactions of free methyl cations with diisobutylamine (1), isobutylaminotrimethylsilane (2) and hexamethyldisilazane (3) were studied by the radiochemical method. It was shown that in all cases studied, the proton transfer is a predominant channel. Probabilities for the reaction to enter this channel are 0.87 for 1, 0.67 for 2 and 0.93 for 3. The last value contradicts the well-known dependence of the lowering of the proton affinity of amines upon the CHa/SiHa ratio. However, quantum chemical calculations have shown that the interaction of the silazane HOMO with the CHa LUMO is symmetry forbidden. This fact may result in the lowering of the methyl cation affinity for silazanes and preference for the proton transfer channel. 

It turns out through our extensive studies that the electron wavepacket description of chemical reactions offers an interesting and novel way of comprehending chemical phenomena, in which the shift of nuclear configuration induces a qualitative change in the electronic states through nonadiabatic interactions. This situation is typically observed in chemical reactions that are non-concerted reactions or Woodward-Hoffmann symmetry forbidden... 

The inclusion of the electron correlation energy in the theoretical analysis of chemical reactions becomes a must when electron bond pairs of starting structures are destroyed during reaction. This concerns in the first place the reactions of bond breaking since a calculation in terms of the HF approximation leads to incorrect dissociated states, such as F2 F" + F rather than 2F. Also the reactions proceeding via biradical type structures (see Sect. 1.6 and 8.3) and those forbidden by the orbital symmetry conservation rules fall within the category of such transformations. A typical example is provided by the electrocyclic-type reaction ... 

Thermal dimerization of ethylene to cyclobutane is forbidden by orbital symmetry (Sect 3.5 in Chapter Elements of a Chemical Orbital Theory by Inagaki in this volume). The activation barrier is high E =44 kcal mof ) [9]. Cyclobutane cannot be prepared on a preparative scale by the dimerization of ethylenes despite a favorable reaction enthalpy (AH = -19 kcal mol" ). Thermal reactions between alkenes usually proceed via diradical intermediates [10-12]. The process of the diradical formation is the most favored by the HOMO-LUMO interaction (Scheme 25b in chapter Elements of a Chemical Orbital Theory ). The intervention of the diradical intermediates impfies loss of stereochemical integrity. This is a characteric feature of the thermal reactions between alkenes in the delocalization band of the mechanistic spectrum. 

Symmetry has for many years played a vital role in the elucidation of molecular structure although apart from some special situations it was not thought to have a dominant influence on the structure or chemical properties of molecules. In recent years however it has played a large part in the interpretation of many organic reactions through the work of Woodward and Hoffman, and the concept of symmetry allowed or forbidden reaction is now an important part of mechanistic organic chemistry. 

The HOMO of the excited ethylene molecule has the same symmetry as the LUMO of a ground-state ethylene. An excited molecule can react with a ground-state molecule to give cyclobutane (Figure 15-22). The [2 + 2] cycloaddition is therefore photo chemically allowed but thermally forbidden. In most cases, photochemically allowed reactions are thermally forbidden, and thermally allowed reactions are photochemically forbidden. 

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