.pi., .pi.-Biradicaloid hydrocarbons. o-Xylylene. Photochemical preparation from 1,4-dihydrophthalazine in rigid glass, electric spectroscopy, and calculations

Abstract

o-Xylylene has been prepared by irradiation of 1,4-dihydrophthalazine and five other precursors in glassy solutions at -196°. 1,4-Dihydrophthalazine is thermally unstable ( / * = 15.4 ± 1 kcal, 5 = = -7 • 2.5 eu). o-Xylylene appears to be a ground state singlet. It is stable indefinitely in rigid media but dimerizes fast as soon as the medium is softened. Irradiation converts it to benzocyclobutene. It shows characteristic absorption and fluorescence. The emitting state as well as the ground state are most likely planar. The triplet has not been detected. The spectral data are well accounted for by semiempirical 7r-electron calculations, which also predict the existence of a low-lying partly doubly excited state. Cl wave functions of the low-lying states of the three isomeric xylylenes are analyzed in terms of both delocalized and localized frontier orbitals as well as natural orbital occupation numbers to bring out the characteristic features of a biradicaloid species, and approximate relations to classical resonance formulas are established. Biradicalsare an important class of reaction intermediates. First, they are suspected to play a central role in numerous thermal reactions, particularly cycloadditions and isomerizations.3 Second, in photochemical reactions, the geometry possessed by the reacting species at the time of return from the excited to the ground state undoubtedly is of importance in determining the nature of final products. Such geometries are likely to correspond to minima in the excited state potential energy hypersurface and to crossings, or avoided crossings, between it and the ground state surface, and many of them probably are of "biradicaloid" nature.48 We use the adjective biradicaloid for those molecular geometries at which a simple MO picture of the species shows two approximately4*3 nonbonding molecular orbitals containing a total of two electrons in the ground state, irrespectively of the nature of their distribution in the ground state.5 Examples are squareplanar geometry of cyclobutadiene and open-chain geometry of tetramethylene, CH2CH2CH2CH2, both most likely corresponding to minima in the lowest triplet state but quite possibly not in the lowest singlet state. Most commonly, only tetramethylene would be considered a biradical, but our usage of the term biradicaloid geometry is meant to encompass both cases.Clearly, experimental data on the nature of the elec-(l) Presented in part at the 165th National Meeting of the American Chemical Society, Dallas, Texas, April 9, 1973. Support by the National Sicence Foundation (GP-26557) is gratefully acknowledged.(2) Alfred P.

Mechanistic studies on the photogeneration of o- and p-xylylenes from α,α′-dichloroxylenes

“…However, this could also reflect a different molar extinction coefficient of both xylylenes. Although e max of 3b has not been reported, electronic spectra calculated for xylylenes 9 indicate that a much higher intensity should be expected for the absorption of the para than for the ortho derivative. To establish the nature of the process leading to radical 2b and p-xylylene, an investigation of the effects of light intensity on the relative intensity of their signals was carried out by attenuating the laser beam with a set of calibrated neutral density filters.…”

Neutral and Charged Biradicals, Zwitterions, Funnels in S1, and Proton Translocation: Their Role in Photochemistry, Photophysics, and Vision

“…Here, given by Equation (21), is a parameter describing the mixing of 'IAZ-B2) and ']A2 + B2) as a result of the perturbation 6 / K (recall that 6 is always nonnegative). The singlet state energies are given by (22) and an example of their dependence on the perturbation parameter 6 is shown in Figure 3b.…”

“…Indeed, the gap increases rapidly upon motion away from a 90" twist in 1 or from a square toward a ration between the radical centers in a radical pair. 22) rectangular geometry in 2 and upon decrease of the sepa- In addition to the wave function 'IAB), which represents S,, or S,, we thus have the two functions 12) and Il), the latter of which represents either S, or So. For 6=0, we have p=O; i.e., the functions 'IA2-B2) and 'IA'fB') are not mixed, and the state wave functions remain those of a perfect biradical.…”

Luz: um raro produto de reação

“…79 Por uma abordagem alternativa, postulou-se que 2e não decompõe gerando o estado excitado num único passo, mas forma primeiramente o endoperóxido antiaromático 2f por uma reação tipo retro-Diels-Alder. 80 Analogamente, sugeriu-se a formação de 2f por uma abertura da hidrazida subsequente à formação de 2d, formando o perácido 2g. Por um ataque nucleofílico intramolecular, 2g formaria o endoperóxido cíclico 2h, que levaria finalmente à formação de 2f pela liberação de nitrogênio molecular.…”