Perturbation, external heavy atom

If kiac and kd are much larger in 5-bromoacenaphthylene than in acenaphthylene and both rate constants are relatively large in the former, the quantum yield of reaction would be expected to be less sensitive to small increases in klsc, due to external heavy-atom perturbation, and more sensitive to increases in kd resulting from external heavy atoms. 

In contrast to aromatic hydrocarbons, heavy-atom substitution onto carbonyl and heterocyclic molecules appears to have little effect on radiative and nonradiative intercombinational transitions. Wagner(138) has shown that as determined by the type II photoelimination, aliphatic ketones (n -> it excited states) are not sensitive to external heavy-atom perturbation. As seen previously in our discussion of type II photoelimination, aliphatic ketones undergo this cleavage from both the excited singlet and triplet states (in... 

Since external heavy-atom perturbation is expected to be considerably less important than internal heavy-atom perturbation, one can conclude that use of a heavy-atom solvent should have a relatively small effect on inter-combinational processes involving n->n transitions. 

The intensity of 7, 1 So transitions remains unaffected by external heavy-atom perturbations. This difference in behavior forms an additional tool for... 

The effect is observed when the heavy atom is substituted in the molecule (intramolecular effect) as also when the molecule collides with a heavy atom containing perturber (intermolecular or external effect). The dramatic enhancement of S —T transition due to intramolecular and intermolecular heavy atom perturbations are respectively shown in Figure 3.8 for chloronaphthalenes in ethyl iodide and other perturbants. Molecular oxygen has a perturbing effect on S — T absorption spectra of organic molecules in solution. Under a pressure of 100 atm of Os, well defined... 

H-abstraction intramolecular, 378 Half-value concentration, 181 Half-band width, 69 Half quenching concentration, 173 Hamiltonian operator, 65 perturbing, 67 Hammet equation, 110 He-Ne laser, 318 Heavy atom perturbation, 70 external, 145 intermolecular, 71 intramolecular, 71... 

Batch mode can be performed in membranes and in powdered solids. The simplest procedures use the solid phase as a medium to support the analytes and carry out the measurement. Among membranes, a variety of cellulosic supports have been used, and the nature of the analyte plays an important role in producing luminescence. Polar or ionic molecules give off a strong adsorption interaction due to hydrogen bonding and induce intense phosphorescent signals, while nonpolar compounds produce weak dispersion interactions and small shifts in fluorescence spectra and weak phosphorescence. External heavy-atom perturbation can be used to induce... 

Patemo-Buchi reactions, 238-255 Pauli s exclusion principle, 19,24,31,40 Perturbation, external heavy atom, 145 Perturbation theory time dependent, 53 Phantom triplet, 229 Phase-shift method, 309, 311 a-phosphorescence, 129 -phosphorescence, 157 Phosphorescence... 

The internal and external heavy atom effects, IHA and EHA, have attracted a considerable attention in the community of molecular spectroscopists. This is part of an old problem of understanding environmental effects from solvents or solid matrices on S-T absorption or on phosphorescence of solute molecules. For higher temperature studies the triplet decay is quenched either by collision or by vibrational interaction with the matrix or the solvent. The molecules subject to studies in this respect have mostly been aromatic molecules perturbed by molecular oxygen, nitric oxide or other paramagnetic molecules, molecules either with heavy atoms and/or forming charge transfer complexes. 

The dynamics of excitons in isotopically mixed naphthalene crystals has been reviewed and the effects of orientation of metal ion perturbers in naphthalene-crown ether metal ion complexes on the external heavy atom examined . ... 

If the perturbing atom is attached directly to the molecule we speak of an Internal Heavy Atom Effect, whereas if the atom is part of a solvent molecule, we speak of an External Heavy Atom Effect. We shall be mainly concerned with the interned heavy-atom effect. 

A part of the relativistic corrections due to a heavy neighboring atom can be approximated by third-order perturbation theory with nonrelativistic functions. The contribution to shielding comes from the cross term involving the spin-orbit coupling, I S interaction, and the external field-orbit interaction. In nonrelativistic terms, the shielding mechanism is that the external field induces an orbital angular momentum on the electrons of the heavy atom (Br or I), this produces a polarization in the electron spin by spin-orbit coupling, and the spin polarization is transferred to the resonant nucleus by a Fermi contact and by a nuclear spin-electron spin dipolar... 

Koser, G.F. and Liu, V.-S., Heavy-atom effect on the photodimerization of acenaphthylene substituent analysis on the efficiency of external aromatic perturbers, /. Org. Chem., 43, 478,1978. Muszkat, K.A. and Sharafi-Ozeri, S., Electronic overlap population as reactivity measure. The photodimerization of acenaphthylene, Chem. Phys. Lett., 38, 346, 1976. 

Most discrete MTP implementations are similar in many respects, e.g., limited expansion up to order 2-4, spherical harmonic description, interaction calculation in the atoms local frames. Hence, what distinguishes these force fields and implementations from each other is primarily in how they treat the other interaction terms. Most importantly, static multipoles only consist of a first-order perturbation of the electrostatic operator. Describing second-order effects leads to polarizability—the charge density s ability to respond to an external electric field—a critical aspect of certain systems (e.g., dielectric changes) [62-64]. Here, possible implementations are ordered in terms of increased overall accuracy (and thus computational investment and larger parametrization effort). Given the heavy requirements of such refined force fields, it is important to point out that "more is not always better," and each system of interest will call for a fine balance of accuracy and statistical sampling. 

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