Etude théorique du pK des états excités

Résumé On montre à partir de considérations théoriques qu'une molécule conjugée dont un substituant donneur d'électrons est le centre d'acidité doit devenir plus acide quand eile passe de l'état fondamental au premier état électronique excité. Le contraire doit se produire lorsque le substituant est accepteur d'électrons. Enfin une molécule conjugée contenant un hétéroatome centre de basicité doit devenir plus basique au cours de la mÊme transition. On suggère, par ailleurs, une interprétation du fait que dans cette série de molécules le premier état excité triplet possède une force acido-basique très differente de celle du premier état excité singlet.

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Theoretical study on the pK of excited states

From theoretical considerations it is shown, that a conjugated molecule, in which an electron-donator is the center of acidity, the first excited state is more acid than the ground state. If the substituent is an electron-acceptor, then the contrary will happen. If a conjugated molecule has a heteroatom as center of basicity, then the first excited state is more basic than the ground state. Further an interpretation is proposed for the fact, that in this series of molecules the first singlet has an acido-basic force, which is quite different from that of the first triplet.

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Zusammenfassung Mit Hilfe theoretischer überlegungen wird gezeigt, da\ ein konjugiertes Molekül, bei dem ein Elektronendonator als Substituent das AciditÄtszentrum ist, im ersten angeregten Zustand saurer ist, als im Grundzustand. Ist der Substituent ein Elektronenacceptor so tritt der entgegengesetzte Effekt ein. Besitzt ein konjugiertes Molekül ein Heteroatom als BasizitÄtszentrum so wird es bei demselben übergang basischer. Darüber hinaus wird eine Interpretation der Tatsache vorgeschlagen, da\ in dieser Reihe von Molekülen der erste Singulett-Zustand eine vom ersten Triplett-Zustand verschiedene SÄure-BasenstÄrke besitzt.

     

The symmetry energy γ parameter of relativistic mean-field models

The relativistic mean-field models tested in previous works against nuclear matter experimental values,critical parameters and macroscopic stellar properties are revisited and used in the evaluation of the symmetry energyγ parameter obtained in three different ways. We have checked that, independent of the choice made to calculate theγ values, a trend of linear correlation is observed between γ and the symmetry energy(S_0) and a more clear linear relationship is established between γ and the slope of the symmetry energy(L_0). These results directly contribute to the arising of other linear correlations between γ and the neutron star radii of R_(1.0) and R_(1.4), in agreement with recent findings. Finally, we have found that short-range correlations induce two specific parametrizations, namely,IU-FSU and DD-MEδ, simultaneously compatible with the neutron star mass constraint of 1.93≤M_(max)/M_☉≤2.05 and with the overlap band for the L_0 ×S_0 region, to present γ in the range of γ=0.25±0.05.     

Effects of chlorine gas exposure on the optical properties of rhodium phthalocyanine films

We investigated the effects of exposing rhodium phthalocyanine films deposited on glass substrates by the Lagmuir-Blodgett technique to chlorine gas. The visual aspect of the films is altered upon chlorination, changing in color from blue to transparent. We performed optical absorption and Raman Scattering measurements on our films prior to and after exposing it to chlorine gas. We observed a pronounced quenching of the characteristic triplet centered around the Q-absorption band at 662 nm as a result of chlorine incorporation. Another absorption band, in the near UV part of the spectrum, is not greatly affected by the process. No new optical structures appear as a consequence of chlorination. Equivalent effects were observed in the Raman spectra. Leaving the previously exposed films in air for several hours results in a slow partial recovery of the optical spectra. This recovery, as well as the amount of original quenching, depends on the amount of time during which the film was exposed to chlorine.     

Influence de la substitution et de la solvatation sur les forces acides. Approche Theorique.

The use of a theoretical method which takes account of solvent effects permits an original approach of importance of substitution and solvation on acid strengths.     

Interpretation theorique de la reaction de diels-alder dans la serie des dienes fonctionnels dissymetriquement substitues-I : Structure electronique et reactivite

The chemical reactivity of two series of butadienes in Diels-Alder reactions is given. It shows a substitution dissymmetry dependent on the substitution sites (1 or 2) of the functional group (ethoxy) on the butadiene skeleton. The differences in the behaviour of the two series are shown by means of semi-empirical correlations between Log K and theoretical data derived from CNDO/S calculations within the frame of perturbation theory. Furthermore, the existence of an excellent correlation between Log K and electronic affinity for a series of ethylene dienophiles reacting with the ethoxy-1 butadiene is demonstrated.     

An attempt at interpreting competitive Friedel-Crafts reactions : Role of solvation energy

Results of Olah et al. on benzylation of benzene and toluene are studied theoretically. The evolution of intermolecular k_T/k_B and intramolecular o/2p selectivities is interpreted with the aid of a model involving electron affinities of electrophilic entities. Our electrophilic affinities are calculated taking solvation into account, which reverses the order of affinities computed on the naked cations.     

FORMATION OF CYCLOBUTANE THYMINE DIMERS PHOTOSENSITIZED BY PYRIDOPSORALENS: A TRIPLET-TRIPLET ENERGY TRANSFER MECHANISM

The 365 nm irradiation of thymine thin films in the presence of pyridopsoralens is shown to induce the formation of cyclobutane thymine dimers, in contrast to other compounds such as 8- and 5-methoxypsoralen. In order to elucidate the mechanism of such a photosensitized reaction, we have determined the energy of the lowest triplet state (T1) of these compounds, using phosphorescence spectroscopy and CNDO/S quantum chemistry calculations. The T1 energy values were found to be significantly higher for pyridopsoralens--up to 0.3 eV--than for 8- and 5-methoxypsoralen (approximately 2.8 eV), which are not able to photoinduce cyclobutane thymine dimers. The determination of the relative efficiency of cyclobutane thymine dimer formation was performed using chromatographic analysis. A good correlation was found between the energy of the T1 state of the psoralen derivatives and the related cyclobutane thymine dimer formation. Moreover, the photosensitized cyclobutane thymine dimer formation appeared to be temperature-dependent. Our results are consistent with a mechanism involving a triplet energy transfer from the pyridopsoralen to thymine.     

Use of the CNDO/S method to elucidate the conformation of benzylidene aniline

The present study was aimed at clarifying the conformation of benzylidene aniline with the aid of the CNDO/S method. The non-planarity of benzylidene aniline is clearly established and the estimated deviation, 55°, is consistent with a recent experimental determination. An interpretation of the UV absorption of benzylidene aniline is presented. A linear relationship has been found between the orbital energies and the experimental ionization potentials, but the photoelectronic spectrum does not provide any definite information about the conformational problem.     

Etude théorique de dérivés substitués du benzène

Résumé La méthode semi-empirique de Pariser-Parr-Pople est appliquée à l'étude électronique des douze fluorobenzènes dans des conditions de calcul précédemment discutées. L'influence de la substitution sur les spectres est discutée en termes de transfert. La position et l'intensité des transitions électroniques sont en bon accord avec l'expérience. Enfin, les indices de charge calculés pour l'état fondamental permettent de vérifier la règle d'additivité.

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Theoretical study of substituted benzenesIII. The fluorobenzenes

The Pariser-Parr-Pople method is applied for the electronic study of twelve fluorobenzenes in previously detailed numerical conditions. The substitution effects on electronic spectra are discussed in transfer terms. The energies and intensities of electronic transitions are in good agreement with experiment. At last, the electronic densities computed for the ground states allow the verification of the additivity rule.

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Zusammenfassung 12 Fluorbenzole werden nach der PPP-Methode behandelt. Die Substituenteneinflüsse auf die Elektronenspektren werden unter dem Gesichtspunkt des Ladungsüberganges diskutiert. Die Energie und die Intensitäten der elektronischen Übergänge stimmen gut mit dem Experiment überein. Die für die Grundzustände berechneten Elektronendichten erlauben eine Verifikation der Additivitätsregel.