Ionicity chloranil

The absolute values of the Madelung energies Em for naphthalene TCNE and hexamethylbenzene p-chloranil are smaller than the cost of ionizing the lattice Iu Aa, so they are predicted by Eq. (12.2.8) to be complexes of almost neutral constituents, in agreement with experiment Em for TMPD TCNQ and TMPD chloranil are larger than fD — Aa, so they are predicted by Eq. (12.2.7) to be complexes of almost fully ionic constituents, in agreement with experiment these results are for mixed-stack crystals, where the D and A species are stacked atop each other, with large intermo-lecular overlap [13]. 

J. B. Torrance, A. Girlando, J. J. Mayerle, J. I. Crowley, V. Y. Lee, P. Batail, and S. J. LaPlaca, Anomalous nature of neutral-to-ionic phase transition in tetrathia-fulvalene-Chloranil, Phys. Rev. Lett. 47 1747-1750 (1981). 

A (somewhat reduced) neutral-to-ionic first-order-like transition has been reported for another material TMB-TCNQ (TMB = 3,3, 5,5 -te-tramethylbenzidine) [79]. At room temperature p = 0.6, cr = 10-4 S/cm, and 2A = 0.2 eV. At ambient pressure, the transition is at Tc = 205 K and in this case p goes only from = 0.6 to = 0.7. At ambient temperature, the transition occurs at a pressure of 4.3 kbar. In contrast to the case of TTF-p-chloranil, the high-temperature phase is not really a neutral phase (p = 0.6) and the low-temperature phase is found to consist of both ionic and neutral molecules. Lattice dimerization also occurs in this compound below 205 K [79]. 

Fig. 6.5 Schematic representation of the structural features of the neutral and ionic phases of the complex of tetrathiafulvalene (TTF) 6-1 and chloranil (CA) 6-VI. In both cases the view is on the ab plane. In the neutral phase the molecules lie on crystallographic inversion centres at (0,0,0) and (0, 1/2, 0), while in the ionic phase those points are no longer crystallographic inversion centres. (Based on Cailleau et al. 1997, with permission.)...

Gallier, J., Toudic, B., Delugeard, Y., Cailleau, H Gourdji, M Peneau, A. andGuibe, L. (1993). Chlorine-nuclear-quadrupole-resonance study of the neutral-to-ionic transition tetrathiafulvalence-chloranil. Phys. Rev. C, 47,11688-95. [197]... 

Le Cointe, M., Lemee-Cailleau, M. H., Cailleau, H., Toudic, B., Toupet, L., Heger, G., Moussa, F., Schweiss, R, Kraft, K. H. and Karl, N. (1995a). Symmetry breaking and structural changes at the neutral-to-ionic transition tetrathiafulvalene-/ -chloranil. Rev., SectB, 51, 3374-86. [196, 197]... 

Tokura, Y, Kaneko, Y, Okamoto, H., Tanuma, S., Koda, T, Mitani, T. and Saito, G. (1985). Spectroscopic study of the neutral-to-ionic phase transition in TTF- chloranil. Mol. Cryst. Liq. Cryst., 125, 71-80. [196]... 

Ionic ring-expansions of 2-substituted thietanes in many cases probably proceed via the ion 106, which can open to a five-membered cyclic cation. The ring-expansion of 74 by such a mechanism has already been discussed in Section The conversion of 2-methylthietane to thiophene and di- and tetrahydro-thiophenes with triphenylmethyl cations, chloranil, and aluminum oxide also may occur via similar ions. 2-Methylthietane also yields 3-butenethiol when heated with alumina. ° The isomerization of hydroxy-benzodihydrothiophenes by aluminium chloride is believed to involve common thietane intermediates that subsequently undergo ring expansion. ... 

Fig. 12.8 A schematic representation of the phase transition between the neutral (D°A°) and the ionic (D A ] phases following population of a local CT excitation, as a series of cooperative CT-interactions in the CT complex crystal TTF/CA (TTF = tetrathio-fulvalene, CA = chloranile). The arrows hv refer to irradiation with light pulses to populate the local CT excitation.

Shifts in bond-stretching frequencies as a result of changes in bond strength can be anticipated in strong donor-acceptor complexes. Intensities are generally decreased, and in certain cases vibrations which are ordinarily symmetry forbidden may appear. For weak complexes, the spectra hardly differ from a superposition of the spectra of the components, a difficulty common to all the spectroscopic tools. Many specific cases have been reviewed in refs. 1 and 3. We will note only the recently described complex of phenothiazine and chloranil (XII) as it is illustrative of a common result. In a Nujol mull or KBr pellet the quinone carbonyl peak appears at 6.4 p. (1560 cm ), shifted up from its usual 6.0 p (1660 cm" ) position. This immediately identifies the complex as of the ionic type. 

D AT-Vinylcarbazole-p- chloranil Spontaneous ionic polymerization K increases with solvent polarity)... 

There is another equilibrium system that can lead to ionic conduction. When an electron-accepting compound is introduced into a solvent that is an electron donor, the reaction to form a donor-acceptor pair ensues D + A (DA). This donor-acceptor pair may subsequently dissociate to give a pair of ions (DA) D" " + A . This reaction is distinct from ion-pair reactions, since charge transfer does occur formally, but the role of the low-dielectric solvent is similar in affecting the equilibrium. A typical electron acceptor, chloranil (tetrachloro-l,4-benzoquinone), in reacting with MBBA in the dual role of solvent and electron donor, was able to alter the conductance. In another case, equal parts of the donor hydroquinone and... 

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