A Nonadiabatic Excited State Molecular Mechanics/Extended Hückel Ehrenfest Method

2016 ◽  
Vol 120 (48) ◽  
pp. 27688-27698 ◽  
Author(s):  
Robson da Silva Oliboni ◽  
Graziele Bortolini ◽  
Alberto Torres ◽  
Luis G. C. Rego
Keyword(s):  
Excited State ◽  
Molecular Mechanics ◽  
Extended Hückel

The importance of conformation in the reactivity of radical cations. Changing configuration at saturated carbon centres

1992 ◽  
Vol 70 (1) ◽  
pp. 272-279 ◽  
Author(s):  
Allyson L. Perrott ◽  
Donald R. Arnold
Keyword(s):  
Electron Transfer ◽  
Excited State ◽  
Molecular Mechanics ◽  
Radical Cation ◽  
Trans Isomer ◽  
Radical Cations ◽  
Singlet Excited State ◽  
Photostationary State ◽  
Configurational Isomerization ◽  
Cis Isomer

Irradiation of an acetonitrile solution of cis 1-methyl-2-phenylcyclopentane (1bcis); 1,4-dicyanobenzene (2), an electron-accepting photosensitizer; and 2,4,6-collidine (3), a nonnucleophilic base, leads to configurational isomerization of the cyclopentane; the photostationary state lies > 99% in favour of the trans isomer. The mechanism proposed for this reaction involves formation of the radical cation of 1bcis by photoinduced electron transfer to the singlet excited state of 2, deprotonation of the radical cation assisted by the base 3, reduction of the resulting benzylic radical by the radical anion [Formula: see text], and reprotonation of the benzylic anion to give both the cis and the trans isomers of 1b. The photostationary state is controlled by the relative rates of deprotonation of the radical cations of 1bcis and trans; these rates are dependent upon the extent of overlap of the SOMO of the radical cation, which is largely associated with the phenyl ring, and the benzylic carbon–hydrogen bond. Molecular mechanics calculations (MM3 and MMP2) are used to calculate the preferred conformations of the isomers. The required orbital overlap is 31% effective with the global minimum conformation of the cis isomer and essentially ineffective for the low-lying conformations of the trans isomer. This proposed mechanism is supported by Stem–Volmer quenching studies, which indicate that both isomers quench the singlet excited state of 2 at the diffusion-controlled rate, and by deuterium incorporation studies. When irradiation of the cis isomer is carried out in acetonitrile–methanol-O-d as solvent, isomerization is accompanied by deuterium exchange at the benzylic position; the trans isomer is stable under these conditions. Keywords: photosensitized electron transfer, radical cation, deprotonation, configurational isomerization, conformation, molecular mechanics (MM3).

Computational Photobiology and Beyond

2010 ◽  
Vol 63 (3) ◽  
pp. 413 ◽  
Author(s):  
Igor Schapiro ◽  
Mikhail N. Ryazantsev ◽  
Wan Jian Ding ◽  
Mark M. Huntress ◽  
Federico Melaccio ◽  
...  
Keyword(s):  
Quantum Mechanics ◽  
Excited State ◽  
Ab Initio ◽  
Molecular Mechanics ◽  
Chemical Treatment ◽  
Quantum Chemical ◽  
Organic Molecules ◽  
Molecular Devices ◽  
Computational Studies ◽  
Computational Investigation

In this paper we review the results of a group of computational studies of the spectroscopy and photochemistry of light-responsive proteins. We focus on the use of quantum mechanics/molecular mechanics protocols based on a multiconfigurational quantum chemical treatment. More specifically, we discuss the use, limitations, and application of the ab initio CASPT2//CASSCF protocol that, presently, constitutes the method of choice for the investigation of excited state organic molecules, most notably, biological chromophores and fluorophores. At the end of this Review we will also see how the computational investigation of the visual photoreceptor rhodopsin is providing the basis for the design of light-driven artificial molecular devices.

Photochemistry of the o-nitrobenzyl system in solution: effects of distance and geometrical constraint on the hydrogen transfer mechanism in the excited state

1991 ◽  
Vol 69 (8) ◽  
pp. 1193-1200 ◽  
Author(s):  
D. Gravel ◽  
R. Giasson ◽  
D. Blanchet ◽  
R. W. Yip ◽  
D. K. Sharma
Keyword(s):  
Excited State ◽  
Triplet State ◽  
Molecular Mechanics ◽  
Absorption Spectroscopy ◽  
Structural Parameters ◽  
Hydrogen Abstraction ◽  
Hydrogen Atom Transfer ◽  
Structural Constraints ◽  
Excited Triplet ◽  
Flash Absorption Spectroscopy

Two rigid nitrobenzene derivatives, 5-nitro-1,2,3,4-tetrahydro-1,4-methanonaphthalene (6), and 5-nitro-1,2,3,4-tetra-hydro-1,4-ethanonaphthalene (7), have been synthesized and studied by picosecond flash absorption spectroscopy, steady-state irradiation, and MMX molecular mechanics calculations. Transients with lifetimes of 770 and 410 ps have been detected and assigned to the excited triplet states of 6 and 7, respectively. Consistent with prediction, the o-quinonoid intermediate is not detected in the time domain between the laser excitation and the end of decay of the triplet state. Intramolecular abstraction of the bridgehead benzylic hydrogen by the cited triplet state proceeds with relative rates of 1:125 for 6 and 7, respectively. Since structural constraints prohibit the formation of an orthoquinonoid intermediate in these systems, formation of the usual nitrosoalcohol 13 represents the first substantiation of the biradical route to product (3′ → 4, Scheme 1). Absorption of the intermediate triplet biradical resulting from hydrogen abstraction by the triplet excited state was not observed, and is therefore believed to be either very weak in the 400–700 nm region, or located outside the detection region. The kinetic results together with structural parameters determined by molecular mechanics show good correlation between structure and reactivity. Hydrogen atom transfer can proceed at up to 53° deviation from linearity of the C—H—O nuclei, with transfer rates decreasing sharply as the [Formula: see text] distance increases from 1.3 to 1.6 Å. It is suggested that the conditions of the reaction from the excited triplet state meet those required for a hydrogen tunneling mechanism. Key words: o-nitrotoluene photorearrangement, mechanism of; o-nitrobenzyl systems, mechanism of photorearrangement; [Formula: see text] distance, influence of, in hydrogen abstraction mechanism; picosecond flash absorption spectroscopy study of o-nitrobenzyl photorearrangement; radical pathway to product in o-nitrotoluene photorearrangement.

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