Energy electronic, transition state

No simple electrophilic substitution, for example nitrosation, nitration, sulfonation or halogenation of a C—H bond, has so far been recorded in the pteridine series. The strong 7T-electron deficiency of this nitrogen heterocycle opposes such electrophilic attack, which would require a high-energy transition state of low stability. 

Examine atomic charges and the electrostatic potentit map for the lower-energy transition state. Which atom appear to be most electron rich in each Is the negativ charge concentrated on a single atom in the transition stat or delocalized Add this charge information (either or 5- ) to the molecular structure for the transition stat which you drew previously. 

Geometries were fully optimized at the HF/6-31G level of theory, and single point energies were evaluated at the MP2/6-31G level to indude the effects of electron correlation. Transition states were characterized by harmonic frequency analysis. 

FIGURE 7.9 The electronic energy transitions for gaseous sodium atoms. There are two slightly different transitions between the same two levels because of the effect of two spin states that differ slightly in energy. 

A different type of stereoelectronic control has been found in the breakdown in solution of tetrahedral addition intermediates that arise in ester and amide hydrolysis and other reactions of carboxyl and carbonyl groups. In the case of an intermediate such as structure 8.47, in which there are two atoms with non-bonded electrons (generally O or N), the lowest-energy transition state for breakdown is a conformation in which nonbonded electrons of each are anti to the group being expelled (structures 8.48).50... 

Activated monosubstituted benzenes The major doubt concerning a general linear free-energy relationship for aromatic substitution is contained in the question whether resonance contributions to an electron-deficient transition state are sufficiently... 

Figure 16.13. Energy-level scheme for a diatomic molecule, showing the rotational energy transition (r), the vibrational energy transition (v) in the ground state (No), and electronic energy transition (e) from S0 to the excited state (,S i).

In summary, our photophysical studies indicate that the thermally activated relaxation pathways of (2E)Cr(III) very likely involve 2E-to- (intermediate) surface crossing. These (intermediates) can be associated with some, not necessarily the lowest energy, transition state (or transition states) for ground state substitution. The Arrhenius activation barriers for thermally activated relaxation are remarkably similar from complex to complex, but they can be altered in systems with highly strained ligands. Some of this work indicates that the steric and electronic perturbations of the ligands dictate the choice among possible relaxation channels. 

One approach of use in ground state organic chemistry is a static one. This assumes that one can predict the reactivity of a molecule from a description of the starting material itself. Thus, very electron rich centers are subject to electrophilic attack, electron poor sites in the molecules are expected to be susceptible to facile nucleophilic attack, weak bonds are subject to scission, etc. This approach is imperfect, in that a reaction course is really determined by a preference for the lowest energy transition state. Nevertheless, this starting state reasoning is quite useful, since most often it is, indeed, the predicted site of attack which affords the preferred transition state. 

Figure 9 Potential energy curves for a mixed-valence iron(III,II) molecule, with various total spins S, and with coupling parameters such that the ordering of spin states is reversed in the electron-transfer transition state. (Reprinted with permission from Ref. 81, 1990 American Chemical Society)...

Fig. 10. Arrhenius energy profile showing the energy barrier between reactant(s), Ag, and prod-uct(s), By. Activation energies and E are required to elevate reactant(s) to transition state, Bj, or electronically excited transition state, Bj. Only molecules that can pass through electronically excited transition state Bj will exhibit chemiluminescence.

If the energy is supplied by a chemical reaction, then, for a given wavelength (X), the combined enthalpy and activation energy for the formation of the electronically excited transition state (Fig. 10) will be given by... 

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