Transition states antiaromatic

We term the in-phase combination an aromatic transition state (ATS) and the out-of-phase combination an antiaromatic transition state (AATS). An ATS is obtained when an odd number of electron pairs are re-paired in the reaction, and an AATS, when an even number is re-paired. In the context of reactions, a system in which an odd number of electrons (3, 5,...) are exchanged is treated in the same way—one of the electron pairs may contain a single electron. Thus, a three-electron system reacts as a four-electron one, a five-electron system as a six-electron one, and so on. 

The potential surfaces of the ground and excited states in the vicinity of the conical intersection were calculated point by point, along the trajectory leading from the antiaromatic transition state to the benzene and H2 products. In this calculation, the HH distance was varied, and all other coordinates were optimized to obtain the minimum energy of the system in the excited electronic state ( Ai). The energy of the ground state was calculated at the geometry optimized for the excited state. In the calculation of the conical intersection... 

There is another usefiil viewpoint of concerted reactions that is based on the idea that transition states can be classified as aromatic or antiaromatic, just as is the case for ground-state molecules. A stabilized aromatic transition state will lead to a low activation energy, i.e., an allowed reaction. An antiaromatic transition state will result in a high energy barrier and correspond to a forbidden process. The analysis of concerted reactions by this process consists of examining the array of orbitals that would be present in the transition state and classifying the system as aromatic or antiaromatic. 

The aromatic-antiaromatic transition state rules are. another formulation of the Woodward-Hoffmann type rules (Table 8.3). 

Aromatic transition state = allowed reaction —, antiaromatic transition state — forbidden reaction. 

Applying these rules in pericyclic reactions it has been shown and a generalization given that thermal reactiom occur via aromatic transition states while photochemical reactions proceed via antiaromatic transition state. A cyclic transition state is considered to be aromatic or isoconjugate with the corresponding aromatic system if the member of conjugated atoms and that of the n... 

Pericyclic reactions that pass through aromatic transition states are allowed in the ground state those that pass through antiaromatic transition states are ground-state forbidden.36... 

Delocalization energy (this is related to aromaticity and antiaromaticity) Transition state structures and energies (see the hedge below)... 

An allowed pericyclic reaction has an aromatic transition state whereas a forbidden reaction has an antiaromatic transition state (p. 40). However, the aromaticity or... 

While the initial formulation of homoaromaticity pre-dated the introduction of orbital symmetry by some eight years the two concepts are inextricably linkedThis is most evident when pericyclic reactions are considered from the perspective of aromatic or antiaromatic transitions states and the Huckel/Mobius concept. The inter-relationship can be demonstrated by the electrocyclic reaction shown in Scheme 1. ... 

Castro and Kamey conjectured that the bond shift that takes 47 into 58 should proceed through a Mobius antiaromatic transition state. They located a Mbbius TS 57, shown in Figure 3.13. The value of NICS(O) is +19.0 and the computed chemical shifts of the inner hydrogens are 26.4 and 26.7 ppm, all indicative of its Mobius antiaromatic nature. 

Cyclooctatetraene offers another example of fluxional behavior. In an operation distinct from boat-boat ring reversal depicted in 5-14, the locations of the single and double bonds are switched. Anet and co-workers used the same molecule to examine the bond-switching process, whose antiaromatic transition state is 5-25b. (The transition state to ring reversal... 

The single-parameter X-model is now extended to a parametric description of complex reactions with an arbitrary number of reaction parameters. Let p( 3) be the number of reaction partn s (reactants, products or intermediates) the reaction lattice is then isomorphic to the lattice Pip + 1) 2 with a diagram of a higher dimensional cube (6.32). Accordin y, the dynamic sublattice is isomorphic to P(p) = 2 and thus contains at least one element of the non-roechanistic dimension A (see Ch. "Generalized reaction lattice"). Ck>nsequently, the choice of the reaction path is no longer unique - in contrast to the sin e-parameter X model for pericyclic reactions with a well defined reaction path (via an aromatic or antiaromatic transition state.). The formal algebraic description of... 

The terms aromatic and antiaromatic have been extended to describe the stabilization or destabilization of TRANSITION STATES of PERICYCLIC REACTIONS. The hypothetical reference structure is here less clearly defined, and use of the term is based on application of the Huckel (4n+2) rule and on consideration of the topology of orbital overlap in the transition state. Reactions of molecules in the ground state involving antiaromatic transition states proceed, if at all, much less easily than those involving aromatic transition states. 

In msmy cases the information derived from these loops may be sufficient to estimate a starting geometry for a computational location of the conical intersection. However, more detailed information is available. If phase inversion between B and C (antiaromatic transition state with wave function I B-C>) is assumed, and if the position vectors of the three structures A, B, and C with wave functions A>, B>, and I C>, respectively, are denoted by r, rg, and r, the phasepreserving coordinate Q, = 2r - b - d for in-phase) connecting A with B - C and the phase-inverting coordinate Qo = - fc (O for out-of-phase) connecting B with C are... 

Application of this method to pericyclic reactions led to the generalization that thermal reactions take place via aromatic or stable transition states whereas photochemical reactions proceed via antiaromatic or unstable transition states. This is the case because a controlling factor in photochemical processes is conversion of excited state reactants into ground state products. Thus, the photochemical reactions convert the reactants into the antiaromatic transition states that correspond to forbidden thermal pericyclic reactions and so lead to corresponding products. 

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