Resonance push-pull effect

It has been postulated that the stability of free radicals is enhanced by the presence at the radical center of both an electron-donating and an electron-with-drawing group.This is called the push-pull or captodative effect (see also pp. 159). The effect arises from increased resonance, for example ... 

This description of quantum mechanical methods for computing (hyper)polarizabilities demonstrates why, nowada, the determination of hyperpolarizabilities of systems containing hundreds of atoms can, at best, be achieved by adopting, for obvious computational reasons, semi-empirical schemes. In this study, the evaluation of the static and dynamic polarizabilities and first hyperpolaiizabilities was carried out at die Time-Dependent Hartree-Fock (TDOT) [39] level with the AMI [50] Hamiltonian. The dipole moments were also evaluated using the AMI scheme. The reliability of the semi-empirical AMI calculations was addressed in two ways. For small and medium-size push-pull polyenes, the TDHF/AMl approach was compared to Hartree-Fock and post Hartree-Fock [51] calculations of die static and dynamic longitudinal first hyperpolarizability. Except near resonance, the TDHF/AMl scheme was shown to perform appreciably better than the ab initio TDHF scheme. Then, the static electronic first hyperpolaiizabilities of the MNA molecule and dimer have been calculated [15] with various ab initio schemes and compared to the AMI results. In particular, the inclusion of electron correlation at the MP2 level leads to an increase of Paaa by about 50% with respect to the CPHF approach, similar to the effect calculated by Sim et al. [52] for the longitudinal p tensor component of p-nitroaniline. The use of AMI Hamiltonian predicts a p aa value that is smaller than the correlated MP2/6-31G result but larger than any of the CPHF ones, which results fi-om the implicit treatment of correlation effects, characteristic of die semi-empirical methods. This comparison confirms that a part of die electron... 

Phosphinocarbene or 2 -phosphaacetylene 4, which is in resonance with an ylide form and with a form containing phosphoms carbon triple bond, is a distillable red oil. Electronic and more importantly steric effects make these two compounds so stable. Carbene 4 adds to various electron-deficient olefins such as styrene and substituted styrenes. Bertrand et al. have made excellent use of the push-pull motif to produce the isolable carbenes 5 and 6, which are stable at low temperature in solutions of electron-donor solvents (THF (tetrahydrofuran), diethyl ether, toluene) but dimerizes in pentane solution. Some persistent carbenes are used as ancillary ligands in organometallic chemistry and in catalysis, for example, the ruthenium-based Grubbs catalyst and palladium-based catalysts for cross-coupling reactions. 

These electron donors and acceptors generate a polarization by an inductive and/or field effect rather than by resonance effects, and hence the internal CT transitions are at shorter wavelengths than those of the analogous donor-acceptor push-pull systems. Therefore, the zwitterions are more transparent. The hyperpolarizabilities of these zwitterions are in the same range as those of the typical donor—n-system—acceptor compounds. 

This approximation is based on the assumption that frequency dispersion and electron correlation effects can be treated independently, which has been shown to be suitable for push-pull ir-conjugated compounds in off-resonant conditions [53]. Response calculations can also be performed at correlated levels to provide frequency-dependent quantities [54]. Though they have been mostly applied to small compounds, applications to NLO phores have also been reported [55]. 

The effects of the radiation pressure force and the gradient force on an atom are essentially different. The radiation pressure force (5.10) always accelerates the atom in the direction of the wave vector k. The gradient force (5.11) pulls the atom into the laser beam or pushes it out of the beam, depending on the sign of the Doppler shift detuning Z - k v. Both the radiation pressure force and the gradient force have a resonance at a velocity such that k v = Z, when the detuning A is compensated by the Doppler shift k v. 

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