Reactive spinning

Elastomeric fibres based upon both polyester-urethane and polyether-urethane structures followed. The early work by Bayer led to the use of highly polar solvents such as dimethyl formamide. Formation of fibres by reactive spinning, where the isocyanate-ended polymer is extruded into an aqueous solution of a chain-coupling agent, was described in 1949 [75] and by dry-spinning a solution of the chain-coupled polymer in 1951 [76], However, Bayer did not immediately use their technology to produce commercial fibres [77],... 

F. Ogliaro, N. Harris, S. Cohen, M. Filatov, S. R de Visser, and S. Shaik, A Model Rebound Mechanism of Hydroxylation by Cytochrome P450. Stepwise and Effectively Concerted Pathways, and Their Reactivity Patterns, J. Am. Chem. Soc. 2000,122, 8977. Calculations explain puzzling aspects of cytochrome P450 hydroxylation reactions in terms of two, different, reactive spin states of the enzyme. 

The wide electrochemical window in aqueous and protic media is the most intriguing feature of polycrystalline BDD material. Additionally, the specific formation of oxyl radicals is singular. In aqueous or alcoholic solutions such very reactive spin centers can be created in high concentration [56]. This opens unique reaction pathways which have previously been considered as inaccessible by... 

Critical to the observation of magnetic field effects in solution is the ability of the RP to interconvert between triplet (nonreactive) and singlet (reactive) spin states. Having established the various interactions present in the RP, we can now consider how such a mixing process might take place. 

Polyurethane fibers (elastanes) can be made via either of the following two ways. In the first technique, the PU is first synthesized using the prepolymer method, and then the PU is spun from solution (approximately 30% PU) to form a filament or yarn. The yarn can either be collected as one long strand or be cut as it is collected. The second method, called reactive spinning, involves synthesizing the polyurethane and forming the filament all at the same time using a reactive bath to spin the fibers. ... 

Lockhart, T.P. Bergman, R.G. Lvidence for the reactive spin state of... 

Paramagnetic centers can be directly attached to the site of interest via covalent bonds. Such methodology is particularly well developed for proteins, where the amino acid residue of interest can be converted to a cysteine residue by site-directed mutagenesis, and a thiol-reactive spin label can be attached under mild conditions. If a peptide or protein is synthesized, the nonnative spin-labeled amino acid TOAC can be incorporated directly into the chain.In other synthetic systems, nitroxides can be conveniently attached via ether, ester, or amide bonds, an approach that is also viable for labeling ceramic and metal oxide surfaces. The complexity of supramolecular systems that can be investigated by this approach is exemplified by a study on the interaction between spin-labeled starburst dendrimers and DNA. ... 

ABSTRACT. This introductory paper presents a brief version of those areas of molecular reaction dynamics relevant to the subject of the Workshop. These include the following influence of reagent s electronic, vibrational and rotational states upon reactivity spin-orbit effects in elementary reactions, and dynamical aspects of stereochemistry, with special emphasis on orientation and alignment effects in chemical reactions. 

Conceptual DPT perturbational perspective on chemical reactivity Spin-resolved conceptual DFT introductory remarks and different representations Key spin-polarized reactivity indices Global quantities Local reactivity indicators Non-local reactivity indicators Generalized philicity concepts... 

Wliile the earliest TR-CIDNP work focused on radical pairs, biradicals soon became a focus of study. Biradicals are of interest because the exchange interaction between the unpaired electrons is present tliroiighoiit the biradical lifetime and, consequently, the spin physics and chemical reactivity of biradicals are markedly different from radical pairs. Work by Morozova et al [28] on polymethylene biradicals is a fiirther example of how this method can be used to separate net and multiplet effects based on time scale [28]. Figure Bl.16.11 shows how the cyclic precursor, 2,12-dihydroxy-2,12-dimethylcyclododecanone, cleaves upon 308 mn irradiation to fonn an acyl-ketyl biradical, which will be referred to as the primary biradical since it is fonned directly from the cyclic precursor. The acyl-ketyl primary biradical decarbonylates rapidly k Q > 5 x... 

Turro N J, Buchachenko A L and Tarasov V F 1995 How spin stereochemistry severely complicates the formation of a carbon-carbon bond between two reactive radicals in a supercage Acc. Chem. Res. 28 69-80... 

Thennal dissociation is not suitable for the generation of beams of oxygen atoms, and RF [18] and microwave [19] discharges have been employed in this case. The first excited electronic state, 0( D), has a different spin multiplicity than the ground 0( P) state and is electronically metastable. The collision dynamics of this very reactive state have also been studied in crossed-beam reactions with a RF discharge source which has been... 

MMVB is a hybrid force field, which uses MM to treat the unreactive molecular framework, combined with a valence bond (VB) approach to treat the reactive part. The MM part uses the MM2 force field [58], which is well adapted for organic molecules. The VB part uses a parametrized Heisenberg spin Hamiltonian, which can be illustrated by considering a two orbital, two electron description of a sigma bond described by the VB determinants... 

The results of the derivation (which is reproduced in Appendix A) are summarized in Figure 7. This figure applies to both reactive and resonance stabilized (such as benzene) systems. The compounds A and B are the reactant and product in a pericyclic reaction, or the two equivalent Kekule structures in an aromatic system. The parameter t, is the reaction coordinate in a pericyclic reaction or the coordinate interchanging two Kekule structures in aromatic (and antiaromatic) systems. The avoided crossing model [26-28] predicts that the two eigenfunctions of the two-state system may be fomred by in-phase and out-of-phase combinations of the noninteracting basic states A) and B). State A) differs from B) by the spin-pairing scheme. 

You can also plot ihe electrostatic polenlial. the total charge density. or the total spin density determined during a semi-enipincal or ah initio calculation. This information is useful in determining reactivity and correlating calculalional results with experimental data. Th ese examples illustrate uses of lb ese plots ... 

Recall that the symmetry labels e and o refer to the symmetries of the orbitals under reflection through the one Cy plane that is preserved throughout the proposed disrotatory closing. Low-energy configurations (assuming one is interested in the thermal or low-lying photochemically excited-state reactivity of this system) for the reactant molecule and their overall space and spin symmetry are as follows ... 

Electron spin resonance (esr) (6,44) has had more limited use in coal studies. A rough estimate of the free-radical concentration or unsatisfied chemical bonds in the coal stmcture has been obtained as a function of coal rank and heat treatment. For example, the concentration increases from 2 X 10 radicals/g at 80 wt % carbon to a sharp peak of about 50 x 10 radicals/g at 95 wt % carbon content and drops almost to zero at 97 wt % carbon. The concentration of these radicals is less than that of the common functional groups such as hydroxyl. However, radical existence seems to be intrinsic to the coal molecule and may affect the reactivity of the coal as well as its absorption of ultraviolet radiation. Measurements from room... 

This reactivity pattern can be rationalized in terms of a diabatic model which is based upon the principle of spin re-coupling in valence (VB) bond theory [86]. In this analysis the total wavefunction is represented as a combination of two electronic configurations arising from the reactant (reaction coordinate. At the outset of the reaction, is lower in energy than [Pg.141]

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