New reactive intermediates in organic chemistry

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Fragmentation of a dioxolanyl radical via nonstatistical reaction dynamics: characterization of the vinyloxy radical by ns time-resolved laser flash photolysis

The photochemistry of two Barton esters, one derived from a dioxolane carboxylic acid and the other from pivalic acid, was investigated by product analysis and nanosecond laser flash photolysis (LFP). As expected, photolysis of the pivalate ester resulted in formation of the pyridine-2-thiyl and the t-butyl radical. Photolysis of the Barton ester of 2,2-dimethyl-1,3-dioxolane-4-carboxylic acid, on the other hand, revealed a complex multi-step fragmentation. In addition to the pyridine-2-thiyl and dioxolanyl radical, we gained evidence for the formation of the vinyloxy radical, CH2[double bond, length as m-dash]CHO˙. The latter was identified in the LFP by its π-complexes with benzene and diphenylether, its rapid quenching by electron-rich arenes and tri-n-butyl tin hydride, and its oxidative power in presence of trifluoroacetic acid as demonstrated by the oxidation of ferrocene to ferrocenium. Formation of CH2[double bond, length as m-dash]CHO˙ can be rationalized via fragmentation of the dioxolanyl radical. As the calculated barriers are too high for the reaction sequence to occur on the LFP time scale, we investigated the fragmentation of the photoexcited Barton ester via Born–Oppenheimer molecular dynamics simulations. In one trajectory, we could observe all reaction steps including ring opening of the dioxolanyl radical, suggesting that the excess energy gained in the ester cleavage and decarboxylation may lead to fragmentation of the hot dioxolanyl radical

Dimethylberyllium + CO2 → Fire! A DFT and ab initio study into the photon emission observed in a gas phase carbon dioxide activation reaction

Dimethylberyllium, Me2Be, is known to ignite when the neat compound is reacted with CO2. In this contribution, we present evidence from DFT and ab initio calculations demonstrating that, while the two-stage gas phase carboxylation of Me2Be to yield beryllium acetate is strongly exothermic, it is not sufficiently so to result in the formation of excited states, as required by a combustion process. The reaction, however, will liberate sufficient heat to drive endothermic unimolecular decomposition reactions. In the case of the reaction of diethylberyllium Et2Be, this results in the formation of beryllium hydride via a β-hydride elimination reaction, and potentially of Be atoms. Pyrolysis of Me2Be, which lacks β-hydrogen atoms, is predicted to give the extremely reactive methyleneberyllium CH2═Be, a ground state triplet species. All reactive intermediates generated by pyrolysis of either Me2Be or Et2Be are calculated to react with CO2 in exothermic reactions. With one possible exception, however, none of the carboxylation reactions are predicted to be sufficiently exothermic to yield a product in an excited state. The photon emission observed experimentally is rationalized via the oligomerization of monomeric BeO, which was studied up to two different tetramers. Formation of (BeO)2, (BeO)3, and (BeO)4 ring structures was found to be so intensely exothermic that even relatively high-lying (5.6 eV) excited states will be populated with ease. Finally, the reaction of Be atoms with CO2, previously studied by matrix isolation spectroscopy (Andrews, L.; Tague, T. J., Jr. J. Am. Chem. Soc. 1994, 116, 6856−6859), was found to proceed via initial formation of a four-membered ring carbene-type structure, which had not been taken into account in the earlier experimental work

Thermochemistry and photochemistry of spiroketals derived from indan-2-one: stepwise processes versus coarctate fragmentations

Coarctate reactions are defined as reactions that include atoms at which two bonds are made and two bonds are broken simultaneously. In the pursuit of the discovery of new coarctate reactions we investigate the fragmentation reactions of cyclic ketals. Three ketals with different ring sizes derived from indan-2-one were decomposed by photolysis and pyrolysis. Particularly clean is the photolysis of the indan-2-one ketal 1, which gives o-quinodimethane, carbon dioxide and ethylene. The mechanism formally corresponds to a photochemically allowed coarctate fragmentation. Pyrolysis of the five-ring ketal yields a number of products. This is in agreement with the fact that coarctate fragmentation observed upon irradiation would be thermochemically forbidden, although this exclusion principle does not hold for chelotropic reactions. In contrast, fragmentation of the seven-ring ketal 3 is thermochemically allowed and photochemically forbidden. Upon pyrolysis of 3 several products were isolated that could be explained by a coarctate fragmentation. However, the reaction is less clean and stepwise mechanisms may compete

Detection and identification of reaction intermediates in the photorearrangement of pyridazine N-oxide: discrepancies between experiment and theory

Photolysis of pyridazine N-oxide (PNO) results in the detection of a complex series of transient phenomena. On the ultrafast (fs) timescale, we could detect the decay of the first singlet excited state of PNO, and the formation of a short-lived transient species, that, based on its time-resolved resonance Raman (TR3) spectrum, we assign to the oxaziridine 1,2-diaza-7-oxa-bicyclo[4.1.0]hepta-2,4-diene. On a longer (hundreds of ns) timescale, this species rearranges to a ring-opened diazo compound, which we have also detected by time-resolved infrared and TR3 spectroscopy. In addition, we identify 1-oxa-3,4-diazepine as a long-lived species formed in the photochemistry of PNO. This species is formed via its oxirane isomer, which in turn is likely formed directly from the S1 state of PNO via a conical intersection. The barrier determined experimentally for the decay of 1,2-diaza-7-oxa-bicyclo[4.1.0]hepta-2,4-diene (Ea = (7.1 + 0.5) kcal mol-1) is far larger than any barrier calculated by any method that includes dynamic electron correlation, but very close to the barriers calculated at the RHF or CASSCF levels of theory. Many methods (B3LYP, MP2, MP4) fail to give a minimum structure for 1,2-diaza-7-oxa-bicyclo[4.1.0]hepta-2,4-diene, while M06, M06-2X, MP3, CCSD, or CCSD(T) yield activation energies for its electrocyclic ring opening that are far too small. In addition, we note that several important geometric parameters, both of 1,2-diaza-7-oxa-bicyclo[4.1.0]hepta-2,4-diene and of the transition state of its ring opening reaction clearly have reached no convergence, even at the fully optimized CCSD(T)/cc-pVTZ level of theory. We therefore suggest that the transient species described in this contribution might be excellent test molecules for further development of highly correlated and DFT methods

Inhomogenous Primordial Baryon Distributions on Sub-Galactic Scales: High-z Galaxy Formation with WDM

For the Warm Dark Matter (WDM) cosmological model the implications of strongly inhomogenous, primordial baryon distribution on sub-galactic scales for Big Bang Nucleosynthesis, Cosmic Microwave Background anisotropies and Galaxy Formation (including fully non-linear evolution to z=0) are discussed, and the inflationary theory leading to such distributions is briefly reviewed. It is found that Big Bang Nucleosynthesis is essentially unaffected relative to SBBN and that the change in recombination history at z~1500-700 relative to ``standard'' theory leads to differences in the anisotropy and polarization power spectra, which should be detectable by the Planck satellite provided systematic effects can be accounted for. Moreover, it is shown by fully cosmological, hydro/gravity simulations that the formation of galactic discs is only weakly affected by going from smooth to highly non-homogenous, initial baryon distributions. In particular, the final disc angular momenta at z=0 are as large as for the standard case and the ``disc angular momentum problem'' is solved to within a factor of two or better without invoking (hypothetical) energetic feedback events. A very desirable difference relative to the the standard WDM model, however, is that the on-set of star (and AGN) formation happens earlier. For the ``optimal'' free-streaming mass scale of M_f~1.5x10^11 h^-1 Msun the redshift of formation of the first stars increases from z_*=4-5 to >6.5, in much better agreement with observational data on high-redshift galaxies and QSOs. It will, however, not be possible to push z_* above ~10, because at higher redshifts the gas velocity field is nowhere compressive. Probing the ``dark ages'' will hence enable a direct test of this theory.Comment: 13 pages, 10 figures, submitted to MNRA

Oval Domes: History, Geometry and Mechanics

An oval dome may be defined as a dome whose plan or profile (or both) has an oval form. The word Aoval@ comes from the latin Aovum@, egg. Then, an oval dome has an egg-shaped geometry. The first buildings with oval plans were built without a predetermined form, just trying to close an space in the most economical form. Eventually, the geometry was defined by using arcs of circle with common tangents in the points of change of curvature. Later the oval acquired a more regular form with two axis of symmetry. Therefore, an “oval” may be defined as an egg-shaped form, doubly symmetric, constructed with arcs of circle; an oval needs a minimum of four centres, but it is possible also to build polycentric ovals. The above definition corresponds with the origin and the use of oval forms in building and may be applied without problem until, say, the XVIIIth century. Since then, the teaching of conics in the elementary courses of geometry made the cultivated people to define the oval as an approximation to the ellipse, an “imperfect ellipse”: an oval was, then, a curve formed with arcs of circles which tries to approximate to the ellipse of the same axes. As we shall see, the ellipse has very rarely been used in building. Finally, in modern geometrical textbooks an oval is defined as a smooth closed convex curve, a more general definition which embraces the two previous, but which is of no particular use in the study of the employment of oval forms in building. The present paper contains the following parts: 1) an outline the origin and application of the oval in historical architecture; 2) a discussion of the spatial geometry of oval domes, i. e., the different methods employed to trace them; 3) a brief exposition of the mechanics of oval arches and domes; and 4) a final discussion of the role of Geometry in oval arch and dome design