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Biomedical subjects

Dieter Cremer

Publications and source records attributed to Dieter Cremer.

24 records · Page 2Linked to original sources
PubMed2003

The role of the HOOO(-) anion in the ozonation of alcohols: large differences in the gas-phase and in the solution-phase mechanism.

The mechanism of the ozonation of isopropyl alcohol was investigated for the gas and the solution phase using second-order many body perturbation theory and density functional theory (DFT) with the hybrid functional B3LYP and a 6-311++G(3df,3pd) basis set. A careful analysis of calculated energies (considering thermochemical corrections, solvation energies, BSSE corrections, the self-interaction error of DFT, etc.) reveals that the gas-phase mechanism of the reaction is dominated by radical or biradical intermediates while the solution-phase mechanism is characterized by hydride transfer and the formation of an intermediate ion pair that includes the HOOO(-) anion. The product distribution observed for the ozonation in acetone solution can be explained on the basis of the properties of the HOOO(-) anion. General conclusions for the ozonation of alcohols and the toxicity of ozone (inhaled or administered into the blood) can be drawn.

Journal Article↗
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.

Journal Article↗
PubMed2002

Evidence for the HOOO(-) anion in the ozonation of 1,3-dioxolanes: hemiortho esters as the primary products.

Low-temperature ozonation (-78 degrees C) of 2-methyl-1,3-dioxolane (1a) in acetone-d6, methyl acetate, and tert-butyl methyl ether produced both the corresponding acetal hydrotrioxide (3a, ROOOH) and the hemiortho ester (2a, ROH) in molar ratio 1:5. Both intermediates were fully characterized by 1H, 13C, and 17O NMR spectroscopy, and they both decomposed to the corresponding hydroxy ester at higher temperatures. The mechanism involving the HOOO- anion formed by the abstraction of the hydride ion by ozone to form an ion pair, R+ -OOOH, with its subsequent collapse to either the corresponding hemiortho ester (ROH) or the acetal hydrotrioxide (ROOOH) was proposed. This mechanism is supported by the PISA/B3LYP/6-311++G(3df,3pd) calculations.

Anions↗
PubMed2002

Peculiar structure of the HOOO(-) anion.

The HOOO(-) anion (1) can adopt a triplet state (T-1) or a singlet state (S-1), where the former is 9.8 kcal/mol (DeltaH(298) = 10.3 kcal/mol) more stable than the latter. S-1 possesses a strong O-OOH bond with some double bond character and a weakly covalent OO-OH bond (1.80 A) according to CCSD(T)/6-311++G(3df,3pd) calculations (the longest O-O bond ever found for a peroxide). In aqueous solution, S-1 adopts a geometry closely related to that of HOOOH (OO(O), 1.388 A; (O)OO(H), 1.509 A; tau(OOOH), 78.3 degrees ), justifying that S-1 is considered the anion of HOOOH. Dissociation into HO anion and O(2)((1)Delta(g)) requires 15.4 (DeltaH(298) = 14.3; DeltaG(298) = 8.9) kcal/mol. Structure T-1 corresponds to a van der Waals complex between HO anion and O(2)((3)Sigma(g)(-)) having a binding energy of 2.7 (DeltaH(298) = 2.1) kcal/mol. Modes of generating S-1 in aqueous solution are discussed, and it is shown that S-1 represents an important intermediate in ozonation reactions.

Anions↗
PubMed1996

Trishomocyclopropenylium Cations. Structure, Stability, Magnetic Properties, and Rearrangement Possibilities.

Potentially trishomoaromatic cations possessing the 6-X-bicyclo[3.1.0]hex-3-yl (X = CH(2), BH, NH, O) or bicyclo[3.2.0]hept-3-yl unit have been investigated at the Hartree-Fock, second-order, third-order, and fourth-order (single, double, quadruple excitations) Møller-Plesset perturbation level employing the 6-31G(d) basis set. IGLO/6-31G(d) chemical shift calculations have been carried out at optimized geometries. Through-space interactions between the symmetric Walsh orbital of the three-membered ring and ppi(C3) orbital have been analyzed as a function of orbital energies and orbital overlap. The best indicators for trishomoaromaticity are NMR chemical shifts and magnetic susceptibility. There is a simple relationship between the conformation of the trishomocyclopropenylium cation, its charge distribution, and delta(13)C3, which can be used to determine the conformation or the C1C3 interaction distance from NMR measurements. Trishomocyclopropylium cations investigated can rearrange to an envelope form of higher energy where the height of the inversion barrier and the chair-envelope energy difference are a measure for the homoaromatic stabilization energy. The bicyclo[3.1.0]hex-3-yl cation in its envelope form can rearrange with a barrier of just 1 kcal/mol to the bicyclo[3.1.0]hept-2-yl cation. In the case of the bicyclo[3.2.0]hept-3-yl cation, there exists just the envelope form, which can rearrange to a ethano-bridged center-protonated spirocyclopentyl cation. The later cation should be an interesting target of chemical synthesis since it contains a pentacoordinated carbon atom and possesses unusual properties.

Journal Article↗