Zwitterionic ground state

The papaver alkaloid Narcotoline (265) can be converted into the yellow colored Cotarnoline (267) on hydrolysis which adopts a zwitterionic ground state (56MI1, 57MI1) (Scheme 87). The corresponding dihydro derivative was also identified in nature. The UV spectrum of the zwitterionic Tarkonine (268), which forms red crystals from acetone, is shifted bathochromically in comparison to the chloride. Tarkonine is a very weak base (pA b = 9.58), which is well in accord with the formation of an inner salt, and Cotarnoline (267) is a stronger base than Tarkonine (268) (pA b = 9.15). The pK values are larger than 13 (66MI3). The methylated derivative of Narcotoline is Narcotine (266). 

Figure 1.12 Reichardt s betaine dye in its zwitterionic ground state (a) and first excited state (b). The ground state has a larger dipole moment (15 D) than the excited state (6 D). Measurement of the energy of the transition between these two states (n - n ) is the basis for the Ej scale of solvent polarity...

Figure 1.14 Increasing solvent polarity stabilizes the zwitterionic ground state Or) of Rei-chardt s dye relative to the first excited state (n ), leading to an increase in the transition energy...

In general, in cases where the carbanion can be stabilized by catalytic intervention, as in the initial conversion of a 2-ketoacid to a cyanohydrin, the transition state leading to its formation will be stabilized. In addition, the stability of the carbanion generated by loss of carbon dioxide also depends on its molecular environment. The rate of decarboxylation of pyridine-2-carboxylic acid is enhanced in a nonpolar environment as the zwitterionic ground state is destabilized and the less polar transition state is stabilized.5... 

IV, and V.(13) The absorption spectra of III and IV (Figure 5b) are similar to FBB and its analogues, however the spectrum of V is anomalous. The spectrum of V resembles that of the anion VI, and suggest that this dye has a zwitterionic ground state. No improvement in the figure of merit of these dyes can reasonably be correlated with their structure. 

Fig. TI.5 Schematic figure showing how /30 (solid line) varies with bond-order alternation (BOA) for a donor-acceptor polyene from a neutral ground-state structure (left) to a fully zwitterionic ground-state structure (right), along with the variation of the constituent terms according to the two-state Eq. (16) ...

For a series of 10 donor-acceptor substituted /ranj-tetramethylenes, calculations have shown a trend paralleling the empirical diradical/zwitterionic test proposed by Hall. Three of the ten cases, (104), (105) and (106), were predicted to have zwitterionic ground states. 

The mechanistic details of this reaction have been examined by several workers. Absorption of a photon produces the enamine excited singlet state EA, which undergoes inter system crossing to the excited triplet state EA. Cyclization leads to an excited zwitterion ZW, which relaxes to the zwitterionic ground state (Scheme 8) . A suprafacial hydride shift produces the observed product. The presence of the zwitterionic intermediate has been detected by laser flash photolysis . In studies on the related sulfide ring closure, the zwitterion was trapped by a dipolarophile ... 

Reichert [83] proposed an alternative model for the absorption process involving transition from a zwitterionic ground state to the excited state (Eqn. 91). 

Compound 2 (/ i = i 2= H) shows an opposite behavior. Its zwitterionic ground state has a calculated dipole moment of 22.6 D, which is reduced in its quinoid, excited state to a value of 13.7 D. An increase in solvent polarity destabilizes the charge-transfer transition of this compound. shifting the value of its longest wavelength band hypsochromically, from 620 nm in nonpolar CHCI3 to 442 nni in water (negative solvatochromism) (Fig. 3). 

FIGURE 6.3 Molecular first hyperpolarizability, /3, values calculated by semi-empirical INDO methods are shown as a fimction of twist angle for a twisted chromophore (see upper part of the figure). The maximum predicted value of p is large and negative, suggesting a zwitterionic ground state. 

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