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Wolfram Sander

Publications and source records attributed to Wolfram Sander.

25 records · Page 2Linked to original sources
PubMed2002

Vibrational spectrum of m-benzyne: a matrix isolation and computational study.

m-Benzyne (2) was generated in low-temperature matrices and IR spectroscopically characterized from four different precursors. To assign the IR absorptions, the perdeuterated derivative 2-d(4) was also investigated. By comparison with CCSD(T) calculations all vibrations between 200 and 2500 cm(-)(1) with a predicted relative intensity >2% could be assigned. All experimental and theoretical results are in accordance with a biradicaloid structure for 2, while there is no evidence for a bicyclic closed-shell structure. While benzyne 2 is stable under the conditions of matrix isolation at low temperature, flash vacuum pyrolysis at high temperatures or UV irradiation results in the rearrangement to cis-enediyne. A mechanism involving ring opening accompanied by hydrogen migration is proposed.

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PubMed1998

Photochemistry of Diaryl Vicinal Tetraketones and Chemistry of Intermediate (Aroyloxy)arylketenes(1).

Photolysis of diaryl vic-tetraketones results in formation of tricyclic-gamma-lactones 3 and 4 with low quantum but high chemical yield. (Aroyloxy)arylketenes and carbon monoxide are the initial photochemical products of these photolyses. Subsequent reaction of ketene with ground-state tetraketone results in formation of the observed photoproducts via intermediate beta-lactones. The latter are formed with a high degree of stereoselectivity. The failure of a cyclic tetraketone to react is attributed to its inability to undergo the cyclic mechanism proposed for formation of ketenes from tetraketones. Dimesityl and di-tert-butyl tetraketones react by a competing intramolecular hydrogen atom transfer mechanism. Dimerization of (benzoyloxy)phenylketene and its reactions with a number of tetraketones are described.

Journal Article↗
PubMed1996

Photochemistry of Fluorinated Phenyl Nitrenes: Matrix Isolation of Fluorinated Azirines.

2,6-Difluorophenylnitrene and pentafluorophenylnitrene were generated in solid argon at 10 K by irradiation of the corresponding phenyl azides and characterized by IR and UV spectroscopy. Selective irradiation with lambda = 444 nm results in the formation of the corresponding azirines, while ketene imines are not produced. On lambda = 366 nm irradiation the azirines rearrange back to the nitrenes. The assignment of the azirines is confirmed by ab-initio calculations.

Journal Article↗
PubMedDate not supplied

A TRIR, TREPR and computational study on the reactivity and structure of the 2,2,2-trifluoroethoxycarbonyl radical.

The 2,2,2-trifluoroethoxycarbonyl radical, 3b, has been generated by pulsed irradiation of 9-fluorenone oxime 2,2,2-trifluoroethyl oxalate 1b in carbon tetrachloride and acetonitrile solution. It was characterized by time-resolved electron paramagnetic resonance spectroscopy (EPR) and infrared spectroscopy. The radical has a lifetime in the range of microseconds and can be detected within the rise time of our time-resolved equipment before undergoing recombination or reactions with the solvent. No decarbonylation or decarboxylation was observed. In the presence of oxygen, the radical is quenched to yield the 2,2,2-trifluoroethoxycarbonylperoxy radical 4b, which has again a lifetime in the range of several microseconds. Time-resolved electron paramagnetic resonance spectroscopy (TREPR) allowed for the detection of a 1 : 1 : 1 triplet of the fluorene-9-iminyl radical 7 at g = 2.0032 and a 1 : 3 : 3 : 1 quartet with additional hyperfine splitting (HFS) due to proton coupling at g = 2.001 for the trifluoroethoxycarbonyl radical 3b. Calculations indicate that alkoxycarbonyl radicals can exist in conformations that are s-trans or s-cis with respect to the R-O-C(O) x dihedral. A comparison of experimental TREPR spectra with simulations indicates that the s-trans conformer is observed in the case of the ethoxycarbonyl radical, 3a. In the case of the trifluorethoxycarbonyl radical, 3b, however, the additional proton HFS observed shows that it is the s-cis conformer that is formed. As calculations give evidence for a fairly high activation enthalpy for s-cis-s-trans interconversion of alkoxycarbonyl radicals, this discrepancy is likely due to differing conformational preferences of the precursor molecules.

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