Internal field effect

Another method used to vary the AG° of the recombination reaction without chemical modification of the centers, consists of placing the system in an electric field whose orientation and intensity are well defined [141]. However, the energy level shifts induced by the field also change the electronic factors, so that the interpretation of the experimental results is not straightforward. Bixon and Jortner have proposed using electric field effects to elucidate the nature of the primary electron step in bacterial photosystems [142], a problem that will be discussed in Sect. 3.5. One basic difficulty encountered in this method is the evaluation of the internal field effectively seen by the redox centers in the membrane. 

Here V(<(), t) = pF(t) cos 4> is the potential arising from an external applied electric field F(f). Here, just as with the translational diffusion equation treated in Ref. 7, we consider subdiffusion, 0 < ct < 1 phenomena only. Here, the internal field effects are ignored, which means that the effects of long-range torques due to the interaction between the average moments and the Maxwell fields are not taken into account. Such effects may be discounted for dilute systems in first approximation. Thus, the results obtained here are relevant to situations where dipole-dipole interactions have been eliminated by extrapolation of data to infinite dilution. 

Reis et al. report theoretical studies of the urea250 and benzene251 crystals. Their calculations start from MP2 ab initio data for the frequency-dependent molecular response functions and include crystal internal field effects via a rigorous local-field theory. The permanent dipolar fields of the interacting molecules are also taken into account using an SCF procedure. The experimental linear susceptibility of urea is accurately reproduced while differences between theory and experiment remain for /2). Hydrogen bonding effects, which prove to be small, have been estimated from a calculation of the response functions of a linear dimer of urea. Various optoelectronic response functions have been calculated. For benzene the experimental first order susceptibility is accurately reproduced and results for third order effects are predicted. Overall results and their comparison with studies of liquid benzene show that for compact nonpolar molecules environmental effects on the susceptibilities are small. 

In the simplified treatment given, the effect of reflection at the surfaces of the sample and internal-field effects have not been taken into account. Although these must be taken into account in more accurate treatments, their effects are generally small. It has also been assumed that absorption peaks due to different modes do not overlap. If they do, they must be resolved to find their separate absorbances. There are various methods available for doing this. 

The partitioning of acrylonitrile between particles and the aqueous phase of polybutadiene latex was measured by Raman spectroscopy using rabber latex as substrate. Overlapping peaks corresponding to acrylonitrile in the polymer and aqueous phases were resolved with the use of a peak fit. Peak intensities yielded the partition values after corrections for internal field effects in the two phases. 23 refs. 

The fact that the electric field at the dipole is not exactly the same as the applied one results in shielding effects. These internal field effects were treated historically by Lorentz (1879), Clausius (1879), and Massotti (1847). A more general approach was developed by Onsager (1938) introducing the reaction field. Within this approach Eq. 12.10 is modified to... 

All the terms on the right depend on 012/0 3 or on experimentally measured values. We only need to solve Eqs. (1-4) simultaneously. We can also obtain a /a from the oriented sample if we use 72 in the analysis. However, it is believed that in most cases, and a2/a2 in the isotropic material and the oriented material are the same (i.e., the internal field effects on the polarizability are small). When the internal field effects are sufficiently small, it is more convenient to obtain a /a as a function of a2/c 3 in the isotropic case and use this result for the oriented samples. 

When the internal field effects are significant, a fifth equation must be found to determine the five unknowns. Citra et al. [58] showed that there is one additional unique experiment that can provide the final unknown. By recording the spectra at an angle y from the fiber axis (after correcting for birefringence effects), the scattering intensity for backscattering is (in their nomenclature)... 

The leaving group also affects the amount of internal versus terminal alkene that is formed. The poorer the leaving group, the more El cb-like is the transition state. This trend is illustrated for the case of the 2-butyl system by the data in Table 6.6. Positively charged leaving groups, such as in dimethylsulfonium and trimethylammonium salts, may favor a more El cb-like transition state because their inductive and field effects increase the acidity of the p protons. 

Lockhart DJ, Kim PS (1992) Internal Stark effect measurement of the electric field at the amino terminus of an alpha helix. Science 257 947-951... 

Based on the fundamental dipole moment concepts of mesomeric moment and interaction moment, models to explain the enhanced optical nonlinearities of polarized conjugated molecules have been devised. The equivalent internal field (EIF) model of Oudar and Chemla relates the j8 of a molecule to an equivalent electric field ER due to substituent R which biases the hyperpolarizabilities (28). In the case of donor-acceptor systems anomalously large nonlinearities result as a consequence of contributions from intramolecular charge-transfer interaction (related to /xjnt) and expressions to quantify this contribution have been obtained (29). Related treatments dealing with this problem have appeared one due to Levine and Bethea bearing directly on the EIF model (30), another due to Levine using spectroscopically derived substituent perturbations rather than dipole moment based data (31.) and yet another more empirical treatment by Dulcic and Sauteret involving reinforcement of substituent effects (32). 

Manuelli, A. Knobloch, A. Bernds, A. Clemens, W. 2002. Applicability of coating techniques for the production of organic field effect transistors. 2nd International IEEE Conference on Polymers and Adhesives in Microelectronics and Photonics, POLYTRONIC 2002. pp. 201-204. 

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