Polarization-induced pairing

ANGLE AVERAGING IS USED TO ACCOUNT FOR AN INDUCED DIPOLE IN POLAR-NONPOLAR PAIR. 

The photochemist is rather familiar with the photoexcited triplet states and the associated intersystem crossing processes. It is well documented that the photoexcited triplet state plays an important role in organic photochemistry. It is thus conceivable that the electron spin polarization of the photoexcited triplet can be further transferred to a radical pair formed by the reactions of the triplet with a suitable substrate. Such a photoexcited triplet mechanism was first proposed by Wong and Wan in 1972 (135) to account for the "initial polarization" observed in the naphthosemiquinone radical formed in the photoreduction of the parent quinone in isopropanol. It was further considered that the triplet mechanism might also lead to CIDNP if such initially polarized radicals react rapidly to give products with nuclear spin polarization induced via the Overhauser mechanism. 

The excess collision-induced pair polarizability of molecules A and B results as a self-consistent solution of a set of equations for local fields polarizing molecules A and B and the multipolar moments induced in molecules A and B. In a first approximation, the long range field-induced change in the pair polarizability, related to the linear multipolar polarizabilities of molecules, is [27] ... 

It is obvious from this theoretical approach that the energetic stabilization of ion-pairs induced by interaction vith the electric field E becomes increasingly important as the size of the ions and their dipole moments (jj) increase. The more polar ion-pairs are more stabilized by E, clearly increasing from tight to loose ion-pairs, i.e. vith their dissociation and polarity. 

Coulombic forces from net electrostatic charge and polarization-induced forces (e.g., van der Waals forces) play a prominent role even for particles larger than the micron scale, helow which Brownian motion dominates. A dimensionless form for all interparticle forces is not possible but, assuming pairwise forces, it is common to use a ratio of a characteristic value of the interparticle force near contact of a pair (Fq) with the representative shearing force at particle scale r ya to form the dimensionless interparticle force F = FQ/ r ya ). 

Closs G L and Trifunac A D 1969 Chemically Induced nuclear spin polarization as a tool for determination of spin multiplicities of radical-pair precursors J. Am. Chem. Soc. 91 4554-5... 

Blattler C, Jent F and Paul H 1990 A novel radical-triplet pair mechanism for chemically induced electron polarization (CIDEP) of free radicals in solution Chem. Phys. Lett. 166 375-80... 

The electrostatic energy is calculated using the distributed multipolar expansion introduced by Stone [39,40], with the expansion carried out through octopoles. The expansion centers are taken to be the atom centers and the bond midpoints. So, for water, there are five expansion points (three at the atom centers and two at the O-H bond midpoints), while in benzene there are 24 expansion points. The induction or polarization term is represented by the interaction of the induced dipole on one fragment with the static multipolar field on another fragment, expressed in terms of the distributed localized molecular orbital (LMO) dipole polarizabilities. That is, the number of polarizability points is equal to the number of bonds and lone pairs in the molecule. One can opt to include inner shells as well, but this is usually not useful. The induced dipoles are iterated to self-consistency, so some many body effects are included. 

Observation of spin-polarized products resulting from these radical pairs by the method of chemically induced dynamic nuclear polarization (CIDNP)<67) was accomplished by photolysis in the probe of an NMR spectrometer using perfluoromethylcyclohexane as solvent. The results obtained were consistent with nuclear spin polarization steps involving radical pairs formed from dissociated radicals and also directly from excited states, although the former could not be detected in carbon tetrachloride, probably due to radical scavenging by the solvent. It was not possible to determine the fraction of the reaction proceeding by singlet and triplet radical pairs.<68)... 

The wide variety of absorption and emission wavelengths has allowed the use of different coumarin derivatives as FRET pairs [37], Contrary to most fluorophores, coumarin is uncharged which makes it intrinsically membrane-permeant. To induce water solubility polar groups are frequently introduced to the basic structure [35],... 

The mere exposure of diphenyl-polyenes (DPP) to medium pore acidic ZSM-5 was found to induce spontaneous ionization with radical cation formation and subsequent charge transfer to stabilize electron-hole pair. Diffuse reflectance UV-visible absorption and EPR spectroscopies provide evidence of the sorption process and point out charge separation with ultra stable electron hole pair formation. The tight fit between DPP and zeolite pore size combined with efficient polarizing effect of proton and aluminium electron trapping sites appear to be the most important factors responsible for the stabilization of charge separated state that hinder efficiently the charge recombination. 

In this case, the agreement of the values calculated by the two methods is good, but it is not always so. One reason is that the simple vector approach ignores the effects of unshared pairs of electrons. Also, highly polar bonds can induce additional charge separation in bonds that might not otherwise... 

The chemically induced dynamic nuclear polarization (DNP) opened perspective to study products formed from free radicals [102], The basis of this study is the difference in NMR spectra of normal molecules and those formed from free radicals and radical pairs. The molecules formed from radicals have an abnormal NMR spectrum with lines of emission and abnormal absorption [102]. DNP spectra help to obtain the following mechanistic information ... 

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