Watching photoinduced chemistry and molecular energy flow in solution in real time

Energized molecules are the essential actors in chemical transformations in solution. As the rearrangement of bonds requires a movement of nuclei, vibrational energy is often the driving force for a reaction. Vibrational energy can be redistributed within the "hot" molecule, or relaxation can occur when molecules interact. Both processes govern the rates, pathways, and quantum yields of chemical transformations in solution. Unfortunately, energy transfer and the breaking, formation, and rearrangement of bonds take place on ultrafast timescales. This Review highlights experimental approaches for the direct, ultrafast measurement of photoinduced femtochemistry and energy flow in solution. In the first part of this Review, we summarize recent experiments on intra- and intermolecular energy transfer. The second part discusses photoinduced decomposition of large organic peroxides, which are used as initiators in free radical polymerization. The mechanisms and timescales of their decarboxylation determine the initial steps of polymerization and the microstructure of the polymer product.     

Comparative analysis of local spin definitions

This work provides a survey of the definition of electron spin as a local property and its dependence on several parameters in actual calculations. We analyze one-determinant wave functions constructed from Hartree-Fock and, in particular, from Kohn-Sham orbitals within the collinear approach to electron spin. The scalar total spin operators S2 and Sz are partitioned by projection operators, as introduced by Clark and Davidson, in order to obtain local spin operators SASB and SzA, respectively. To complement the work of Davidson and co-workers, we analyze some features of local spins which have not yet been discussed in sufficient depth. The dependence of local spin on the choice of basis set, density functional, and projector is studied. We also discuss the results of Sz partitioning and show that SzA values depend less on these parameters than SASB values. Furthermore, we demonstrate that for small organic test molecules, a partitioning of Sz with preorthogonalized Lowdin projectors yields nearly the same results as one obtains using atoms-in-molecules projectors. In addition, the physical significance of nonzero SASB values for closed-shell molecules is investigated. It is shown that due to this problem, SASB values are useful for calculations of relative spin values, but not for absolute local spins, where SzA values appear to be better suited.     

Asymmetric Synthesis of 3-Hydroxyprolines by Photocyclization of N-(2-Benzoylethyl)glycinamides

The chiral N-(2-benzoylethyl)-N-tosylglycinamides 1a-c were prepared from the C₂-symmetric pyrrolidines 5a-c . Irradiation of these ketones 1a-c gave cis-3-hydroxyprolinamides 10-12 in moderate to good yields (Scheme 3). The de of the photocyclizations depended on the size of the substituents in positions C(2) and C(5) of the chiral pyrrolidine auxiliaries. In addition, the de varied with the reaction temperature, allowing the determination of activation-parameter differences. The structure of products 10-12 were established by NMR and X-ray analyses.     

Diastereoselectivity of the mercuration of acyclic allylic alcohols

The diastereoselectivity of the mercuration of acyclic alkenes $\text{4}$ can be reversed by changing the substituent in the allylic position; with alcohols the erythro isomers $\text{5}$ and with esters or hemiacetals the threo isomers $\text{6}$ and $\text{8}$ are formed predominantly (Table I).     

ASSESSMENT OF GENOTOXICITY AND MUTAGENICITY OF 1,2-DIOXETANES IN HUMAN CELLS USING A PLASMID SHUTTLE VECTOR

Abstract— 1,2-Dioxetanes are efficient sources of triplet excited carbonyl compounds on thermal decomposition. They cause photochemical and photobiological transformations in the dark. In order to study the genotoxicity and mutagenicity of 1,2-dioxetanes, the replicating shuttle vector pZ189 was damaged with 3,3,4-trimethyl-l,2-dioxetane(TrMD) or 3-hydroxymethyl-3,4,4-trimethyl-l,2-dioxetane (HTMD) in vitro and subsequently transfected into normal human lymphoblasts. We found a dose-dependent increase of genotoxicity (decrease of plasmid survival) and increase of mutation frequency with both dioxetanes. However, TrMD was less mutagenic than HTMD at similar genotoxicity. Sequence analysis of the supF gene revealed more point mutations than deletions. Single base substitutions occurred exclusively at G:C sites: 94.6% of point mutations with TrMD and 100% with HTMD were G:C to T:A and G:C to C:G transversions. These are the typical mutations following 7,8-dihydro-8-oxoguanine (8-oxo-G) formation, the main DNA lesion induced by TrMD and HTMD. Only with TrMD we found 5.4% G:C to A:T transitions, probably reflecting the more pronounced ability of TrMD to form some pyrimidine dimers. Our results indicate that 8-oxo-G is also the most relevant modification in in vivo mutagenesis.     

The preparation of a cabbage reference material for environmental monitoring and food analysis

 Cabbage is frequently used in environmental monitoring and food control, and, hence, cabbage reference materials (RMs) are required for ensuring quality assurance. A cabbage RM was prepared in view of certification of specific elements from the "black list" of high toxicological interest and nutritive importance. All tasks of the RM production (production of the plant material, cutting and freeze-drying, determination of the residual water content, preparation of the RM, homogeneity testing, stability testing, certification measurements) are described in detail. Received: 12 November 1999 / Accepted: 29 January 2000     

Dampfdruck- und Leitfähigkeitsuntersuchungen der Ionensolvatation in Amiden

The solvating properties of formamide, N-methylformamide, dimethylformamide, N-ethylformamide and diethylformamide have been investigated by means of vapour pressure and conductivity measurements. For the salts and ions, resp. average solvation numbers have been calculated from vapour pressure data and limiting ionic conductances. The solvation numbers are used for a discussion of the influence of N-substituents on the solvating properties of the amides.

---

---

Mit 2 Abbildungen     

Electron binding energies of aqueous alkali and halide ions: EUV photoelectron spectroscopy of liquid solutions and combined ab initio and molecular dynamics calculations

Photoelectron spectroscopy combined with the liquid microjet technique enables the direct probing of the electronic structure of aqueous solutions. We report measured and calculated lowest vertical electron binding energies of aqueous alkali cations and halide anions. In some cases, ejection from deeper electronic levels of the solute could be observed. Electron binding energies of a given aqueous ion are found to be independent of the counterion and the salt concentration. The experimental results are complemented by ab initio calculations, at the MP2 and CCSD(T) level, of the ionization energies of these prototype ions in the aqueous phase. The solvent effect was accounted for in the electronic structure calculations in two ways. An explicit inclusion of discrete water molecules using a set of snapshots from an equilibrium classical molecular dynamics simulations and a fractional charge representation of solvent molecules give good results for halide ions. The electron binding energies of alkali cations computed with this approach tend to be overestimated. On the other hand, the polarizable continuum model, which strictly provides adiabatic binding energies, performs well for the alkali cations but fails for the halides. Photon energies in the experiment were in the EUV region (typically 100 eV) for which the technique is probing the top layers of the liquid sample. Hence, the reported energies of aqueous ions are closely connected with both structures and chemical reactivity at the liquid interface, for example, in atmospheric aerosol particles, as well as fundamental bulk solvation properties.     

Sequentially photocleavable protecting groups in solid-phase synthesis

[reaction: see text] A sequential solid-phase peptide synthesis was developed using both photolabile linker and protecting groups. The chromatic sequential lability between a tert-butyl ketone-derived linker (sensitive to irradiation at 305 nm) and a nitroveratryloxycarbonyl (NVOC) group (sensitive at 360 nm) was exploited to prepare Leu-Enkephalin in a 55% overall yield. This new strategy allows the preparation of peptides in essentially neutral medium, by avoiding the use of common deprotection reagents such as trifluoroacetic acid or piperidine.