Multirate hierarchical time integration for electronic circuits

Electrical circuits are hierarchically defined, in which electrical components are combined with submodels. These last ones may contain submodels themselves. This construction gives options for dedicated time integration procedures. We describe an extension that allows for a dynamically partitioned multirate time integration that retains a number of hierarchical efficiencies. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Model order reduction for nonlinear problems in circuit simulation

Electrical circuits usually contain nonlinear components. Hence we are interested in MOR methods that can be applied to a system of nonlinear Differential-Algebraic Equations (DAEs). In particular we consider the TPWL (Trajectory PieceWise Linear) and POD (Proper Orthogonal Decomposition) methods. While the first one fully exploits linearity, the last method needs modifications to become efficient in evaluation. We describe a particular technique based on Missing Point Estimation. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Oscillator models for multiphoton optical bistability and phase conjugacy

Nonlinear oscillator models are constructed to treat the bistability in situations involving elementary excitations in solids. Such models are shown to be useful not only in describing single photon but also multiphoton bistability. The resulting bistability both with and without cavity is considered. The two-photon excitonic bistability in CuCl is in detail. The effect of local field corrections can also be incorporated, in a simple manner, in such models.     

The relative sign of through-bond and through-space interactions; “sigma assistance” of cyclization and intramolecular hydrogen transfer

A perturbational frontier MO model indicates that the relative sign of the through-bond and through-space interaction in nonconjugated bifunctional molecules alternates with the number (N) of intervening sigma bonds. The qualitative predictions from the FMO model are in full accord with quantitative computational and experimental literature data, the latter being mainly available for N<4. It is suggested that alternation of the relative sign of through-bond and through-space interactions with N effect—for which the name “sigma assistance” is proposed—may account for the preference of radical cyclizations to yield odd membered instead of even membered rings and for the preference of hydrogen transfer to occur via an even membered transition state.     

Synthesis of oligosaccharides by using levulinic ester as an hydroxyl protecting group

The fast, selective and mild removal of levulinyl groups with hydrazine from galactose, which also carries hydroxyl functions protected with acetyl groups, enables, under Koenigs-Knorr conditions, the synthesis of a trimer containing β-linked galactoses.     

Electron transfer at the single-molecule level in a triphenylamine-perylene imide molecule.

Photoinduced electron transfer (ET) processes in a donor-acceptor system based on triphenylamine and perylene imide have been studied at the single-molecule (SM) and ensemble levels. The system exists as two isomers, one of which undergoes forward and reverse ET in toluene with decay constants of 3.0 and 2.2x10(9) s(-1), respectively, resulting in the dual emission of quenched and delayed fluorescence while the other isomer remains ET-inactive. The fluorescence of both isomers is heavily quenched in the more polar solvent, diethyl ether, by ET. A broad range of ET dynamics is seen at the SM level in polystryene with the two isomers nonresolvable indicating that the local nanoenvironment of the SMs varies considerably throughout the polymer matrix. Both the electronic coupling and the driving force for ET are shown to influence the ET dynamics. Many fluorescence trajectories of SMs show long periods (tens of milliseconds to seconds) where the count rate is attenuated either partly (a "dim" state) or to the background level (an "off-time"). During these periods, the reduction or interruption of emission is attributed to cycles of rapid charge separation followed by charge recombination to the ground state reducing the fluorescence quantum yield of the SM.     

Isoerization of allylic acetates catalyzed by palladium. New method for stereocontrol.

Reaction of allylic acetates with tetrakis (triphenylphosphine) palladium leads to positional and stereochemical isomerism. A mechanism is proposed. Elimination of allyl acetates to dienes is also observed and constitutes a useful synthetic procedure. The use of bis(benzenesulfonyl)methane as a ^ΘCH₂^Θ equivalent is reported.     

Chromophoric systems based upon through-bond interaction IV : Intramolecular charge-transfer interaction via three sigma bonds

Ketones containing a β-substituent with readily ionizable lone pair or π-electrons are studied by UV spectroscopy. When the geometry favours through-bond interaction between the carbonyl group and the γ-substituent a sigma-coupled transition is observed in the near UV region. Replacement of the CO group by more electronegative systems such as the gem-dicyanovinylidene group is shown to enhance the intensity of the sigma-coupled transition and to shift it bathochromically. These phenomena are interpreted to result from a charge-transfer character of the sigma transition. Furthermore it is shown that intensity borrowing from locally excited states plays an important role in determining the intensity of this sigmal-coupled transition.     

The capacity of an α-nitroalkyl radical for hydrogen abstraction

Photochemical decomposition of 2-iodo-2-nitroadamantane in several hydrogen donating solvents, gives rise to formation of α-nitroalkyl radicals. Such ambident radicals can abstract hydrogen from the solvent via oxygen, resulting in a nitronic acid which decomposes exlusively into adamantanone. Alternately the abstraction can take place via carbon to give 2-nitroadamantane. The product distribution is strongly influenced by electron-withdrawing substituents in the hydrogen donor. The oxidation products derived from the solvent have been detected. All the experiments point towards a minor stabilisation of a carbon radical by a nitro group. INDO-calculations on the nitromethyl radical are in good agreement with this lack of stabilization.