Neutral and charge-separated resonance

Figure 2. Neutral and charge separated resonance forms for various n systems.

An analytical structure-(hyper)polarizability relationship based on a two-state description has also been derived [49]. In this model a parameter MIX is introduced that describes the mixture between the neutral and charge-separated resonance forms of donor-acceptor substituted conjugated molecules. This parameter can be directly related to BLA and can explain solvent effects on the molecular hyperpolarizabilities. NMR studies in solution (e.g. in CDCl3) can give an estimate of the BLA and therefore allow a direct correlation with the nonlinear optical experiments. A similar model introducing a resonance parameter c that can be related to the MIX parameter was also introduced to classify nonlinear optical molecular systems [50,51]. 

Figure 1 Neutral and charge-separated resonance structures of two vinylogue merocyanine dyes (Ml, M2) and a conjugated betaine dye (stilbazolium betaine) (M3).

To overcome this impasse, the only thing to do is to adopt a solvaton set reflecting a right mixture of neutral and charge-separated resonance stractures. From Tab.l it emerges that a solvaton set of such a type is the one corresponding to the subset of the net 7i-electron charges that are not very dependent on the approximations of the method. 

Figure 3.7. Definition of regions from A to E of bond length alternation with the percentage contribution of neutral (structure I) and charge separated (structure II) resonance structures to the ground state. This pattern demonstrates the evolution of the charge separated structure from the neutral from passing through polar structure with almost equal bond lengths (region C). (From Ref. [319] with permission of the American Association for the Advancement of Science.)...

The picosecond pulsed (pp), nanosecond pulsed (np), and cw experiments all seek to obtain RR spectra which accurately reflect the vibrational degrees of freedom assignable to the neutral, ground-states of Bchl (P and accessory) and Bpheo prior to photo-excitation and charge separation. These spectra are important elements in the analysis of time-resolved Raman spectra (e.g., picosecond time-resolved resonance Raman, PTR, experiments) on transient species for two reasons ... 

Whereas the pATa for the a-protons of aldehydes and ketones is in the region 17-19, for esters such as ethyl acetate it is about 25. This difference must relate to the presence of the second oxygen in the ester, since resonance stabilization in the enolate anion should be the same. To explain this difference, overlap of the non-carbonyl oxygen lone pair is invoked. Because this introduces charge separation, it is a form of resonance stabilization that can occur only in the neutral ester, not in the enolate anion. It thus stabilizes the neutral ester, reduces carbonyl character, and there is less tendency to lose a proton from the a-carbon to produce the enolate. Note that this is not a new concept we used the same reasoning to explain why amides were not basic like amines (see Section 4.5.4). 

We describe the electronic structure of pp chromophores based on an old, but extremely powerful model, originally proposed by Mulliken [67] to describe DA complexes in solution. The model is based on the assumption that the low-energy physics of pp chromophores is dominated by the resonance between the neutral and the charge separated (zwitterionic) structures. Two basis states, DA) and D+A ), separated by an energy 2z and mixed by a matrix element —yfit, completely define the electronic Hamiltonian. The solution of this problem is trivial and was already discussed by several authors (see, e.g. [68] and reference therein). For future reference we explicitly write the ground and excited states ... 

Beak and Siegel proposed the formation of a zwitterionic intermediate upon decarboxylation, which could provide rate acceleration through dipole-stabilization (see Fig. 6, structure 1) [7]. A related mechanism was put forth 20 years later when Lee and Houk proposed protonation at 04 instead of 02 (Fig. 6, structure 2) [27]. Quantum mechanical calculations on model systems (R=H in Fig. 6) were used by Lee and Houk to calculate and compare reaction energetics for these two pathways. It was shown that the activation enthalpy of the parent reaction was reduced from 44 kcal/mol in the gas phase to 22 kcal/mol with 02 protonation. The barrier was further reduced upon 04 protonation, to a value of 15.5 kcal/mol, indicating that 04 protonation is a viable strategy for catalysis. The preference for 04 protonation can be explained, at least in part, by examining the reasonable resonance structures of the decarboxylated intermediates (see Fig. 6). While every resonance structure for the 02 protonated intermediate involves charge separation, 04 protonation yields a stabilized neutral carbene. 

Figure 3. Two limiting charge transfer resonance forms of a donor-acceptor polyene molecule the neutral form a) and the charge separated form b).

The other possibility is represented by compounds in which the structure of HOMO and LUMO states is reversed as compared with conventional chromophores, i.e. the ground state is quinoid neutral while the first excited state is charge-separated and aromatic (zwitterionic chromophores) in this case the resonance energy is gained upon excitation a low Egg and a high A/u, are expected for these molecules and this should correspond to hi quadratic nonlinearity (Fig. 2.6). 

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