Neutral-ionic transition

Double pump experiments on an organic charge transfer complex TTF-CA by Iwai and coworkers demonstrated a new class of coherent control on a strongly correlated electron-lattice system [44]. While the amplitude of the coherent oscillation increased linearly with pump fluence for single pump experiments, the amplitude in the double pump experiments with a fixed pulse interval At = T exhibited a strongly super-linear fluence dependence (Fig. 3.16). The striking difference between the single- and double-pulse results indicated a cooperative nature of the photo-induced neutral-ionic transition. 

Neutral - ionic transition in organic charge - transfer salts... 

The neutral-ionic transition (NIT) at t = 0 occurs abruptly[94] when the Madelung energy M of the ionic lattice exceeds the energy I — A to transfer an electron form D to A. Long-range Coulomb interactions are treated self-consistently as part of A in the modified Hubbard model[95],... 

Figure 8 The singlet-triplet gap, Est, near the neutral-ionic transition of (36) in the restricted basis as a function of T = A — U/2. Open and closed symbols refer to boundary conditions with and without crossovers, respectively, and the stars as joint N —> oo extrapolations based on both[97].

The elucidation of neutral-ionic transitions is an important current topic. Several phenazine complexes are likely candidates. The diamagnetic-paramagnetic interface... 

As well as viscosity, other factors to be aware of include the purity of the ionic liquids. The presence of residual halide ions in neutral ionic liquids can poison transition metal catalysts, while different levels of proton impurities in chloroalumi-... 

This is a reaction in which neutral molecules react to give a dipolar or ionic transition state, and some rate acceleration from the added neutral salt is to be expected53, since the added salt will increase the polarity or effective dielectric constant of the medium. Some of the rate increases due to added neutral salts are attributable to this cause, but it is doubtful that they are all thus explained. The set of data for constant initial chloride and initial salt concentrations and variable initial amine concentrations affords some insight into this aspect of the problem. 

Fig. 23.9 Observed excitation spectra 6sl9d —> 9dn d in neutral Ba (b) circular, (c) linear polarization of laser beams and (d) computed spectra. The sharp resonances A, B and C correspond respectively to 6sia — 9d5/2,6s1/2 — 9d3/2 and 5d — lOg ionic transitions used as standard calibrations with the Fabry-Perot fringes (a) (from ref. 31).

Interestingly, several of these materials are rather close to the neutral-ionic boundary and they can be driven from the (quasi) ionic to the (quasi) neutral phases, in reversible phase transitions, by means of temperature or/and pressure variations. Such variations are thought to act primarily on the Magdelung energy parameter [72,73]. The first observation of such neutral-to-ionic transition in any kind of materials was curiously in the form of a distinct color change of the samples when pressure was applied above a certain threshold [72]. 

Cady, H. H., Larson, A. C. and Cromer, D. T. (1963). The crystal of a-HMX and a refinement of the structure of /3-HMX. Acta Crystallogr., 16, 617-23. [279t] Cailleau, H., Luty, T., Le Cointe, M. and Lemee-Cailleau, M. -L. (1997). Cooperative mechanism at the neutral-to-ionic transition Biuletyn Instytutu Chemii Fizycnej I Teoretycznej Politechniki Wroclawskiej, 5, 19-34. [196, 197f, 197]... 

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]... 

Jacobsen, C. S. and Torrance, J. B. (1983). Behavior of charge-transfer absorption upon passing through the neutral-ionic phase transition. J. Chem. Phys., 78, 112-5. [195, 196]... 

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]... 

Le Cointe, M., Lemee-Cailleau, M. H., Cailleau, H. and Toudic, B. (1996). Structural aspects of the neutral-to-ionic transition in mixed stack charge-transfer complexes. 

As well as viscosity, other factors to be aware of include the purity of the ionic liquids. The presence of residual halide ions in neutral ionic liquids can poison transition metal catalysts and different levels of proton impurities in chloroalumi-nate(iii) ionic liquids can alter the product distribution of the reaction. The reduced temperatures required for many polymerization reactions in ionic liquids, together with the reduced solubility of oxygen in ionic liquids compared to conventional solvents, means that two of the most common quenching methods are reduced in effectiveness. When detailed studies are being carried out, in particular kinetic studies, it is necessary to completely stop further reaction so that accurate data are obtained. 

Atom, ionic (electronic transition) Molecule, neutral (electronic transition)... 

---