Josef Kalcher

Accurate ab initio calculations of radiative transition probabilities between the A 3Σ+u, B 3Πg, W 3Δu, B′ 3Σ−u, and C 3Πu states of N2

Potential energy and electronic transition moment functions for the lowest five triplet states of N2 have been calculated from highly correlated multiconfiguration‐reference CI wave functions. From the calculated transition moments and RKR potential energy functions, radiative transition probabilities and lifetimes have been evaluated which are believed to be accurate within 15%. The theoretical lifetime of 36.7 ns of the C 3Πu state is in close agreement with the most recent experimental values. The calculated lifetimes of the B 3Πg state decrease from 13.4 μs for v′=0 to 4.82 μs for v′=12. The values for v′=5 to v′=12 are in good agreement with recent LIF measurements. The lifetimes for v′=0 to v′=4, however, are considerably larger than most previously measured values. The fact that the calculated values are very stable with respect to improvements of the electronic wave functions leads us to conclude that the experimental values are too low. Empirical transition moment functions for the B–A system are...

Amperometric determination of bonded glucose with an MnO2 and glucose oxidase bulk-modified screen-printed electrode using flow-injection analysis

A screen-printed amperometric biosensor based on carbon ink double bulk-modified with MnO(2) as a mediator and glucose oxidase as a biocomponent was investigated for its ability to serve as a detector for bonded glucose in different compounds, such as cellobiose, saccharose, (-)-4-nitrophenyl-beta-d-glucopyranoside, as well as in beer samples by flow-injection analysis (FIA). The biosensor could be operated under physiological conditions (0.1M phosphate buffer, pH 7.5) and exhibited good reproducibility and stability. Bonded glucose was released with glucosidase in solution, and the free glucose was detected with the modified screen-printed electrode (SPE). The release of glucose by the aid of glucosidase from cellobiose, saccharose and (-)-4-nitrophenyl-beta-d-glucopyranoside in solution showed that stoichiometric quantities of free glucose could be monitored in all three cases. The linear range of the amperometric response of the biosensor in the FIA-mode flow rate 0.2mLmin(-1), injection volume 0.25mL, operation potential 0.48V versus Ag/AgCl) extends from 11 to 13,900mumolL(-1) glucose in free form. The limit of detection (3sigma) is 1mumolL(-1) glucose. A concentration of 100mumolL(-1) yields a relative standard deviation of approximately 7% with five injections. These values correspond to the same concentrations of bonded glucose supposed that it is liberated quantitatively (incubation for 2h with glucosidase). Bonded glucose could be determined in beer samples using the same assay. The results corresponded very well with the reference procedure.

Small carbanions and their parent neutral systems in the multiconfigurational SCF + CI approach

Abstract The carbanions C−, CH−, CH−2, and CH−3 and their parent neutral systems are investigated in order to obtain molecular structures and electron affinities. At the most sophisticated level of theory, MC SCF + CI, the calculated electron affinities deviate at most by 0.15 eV from the experimental values. Althouh the stability of CH−3 is not achieved the results query its hitherto predicted non-planarity.

Ab initio and pseudopotential investigations on the SiHn (n = 1–3) radicals and their anions

Abstract Ab initio and pseudopotential (PP) calculations on the SiH n radicals and their anions show close agreements at all levels of theory. The calculated dipole moment for SiH of about −0.1 D is in excellent agreement with recent studies at comparable levels of theory and strongly queries the large experimental estimate of more than 1 D. The PP CI(SD) calculations on the 1 A 1 , 3 B 1 , and 2 B 1 states of SiH 2 and SiH − 2 allow a consistent interpretation of the photodetachment spectrum of the anionic ground state. At the same level of theory the classical inversion barriers of SiH 3 and SiH − 3 are found to be 5.39 and 25.07 kcal/mol, which is in perfect agreement with the experimental estimates. All calculations document a satisfactory performance of the effective potential at all levels of theory. At the highest level of theory the calculated electron affinities are at variance with the experimentally determined values by only 0.15 eV or even less.

Stabilities and stationary points for singlet and triplet states of the Si2H− anion

Abstract Various stationary points on the 2A′ and 2A″ potential energy surfaces (PES) of the Si2H radical and the 1A′, 3A′, 3A″, and 1A″ PES of the Si2H− anion are discussed. The ground state electron affinity of Si2H is calculated larger than 200 kJ/mol. The vast majority of the stationary points are strongly non-linear or even symmetrically bridged (C2v) structures.

Singlet-triplet splittings and electron affinities of some substituted silylenes

Abstract The lowest electronic states of methyl-, silyl-, and lithium-substituted silylene have been investigated. CH3 acts as an electronegative substituent and increases the singlet-triplet splitting, SiH3 behaves as an electropositive substituent and decreases this energy gap, and the lithium-substituted silylenes even have triplet electronic ground states. It is found that the effects of the electropositive substituents prevail against those of the electronegative ones. Silyl substitution increases the electron affinity significantly, whereas methyl substituents strongly decrease it, or even make it vanish. Despite the highly electropositive lithium substituent, HSiLi and SiLi2 both exhibit unexpectedly large electron affinities, because Li facilitates the delocalization of the extra electron.

Ab initio quantum chemical study of the SN2 reaction, CH3F+H− → CH4+F−, in the gas phase

Abstract The reaction profile of the title S N 2 reaction has been studied by the ab initio method at various levels. The geometries of the van der Waals complexes of reactants and products and of the transition state have been optimized at the MP2/6-311G** level. It has been found that the reaction path loses C 3v symmetry and the reaction path is bifurcated in the product region. All the stationary point energies have been evaluated at the MP2 and MCSCF CI levels with various basis sets. The vibrational frequencies of the stationary points have been calculated using the SCF and, for some modes, MP2 methods.

On the stabilities of the singly and doubly negative atomic ions of Si, P, S, and Cl

Abstract The first and second electron affinities of four second-period elements, Si, P, S, and Cl were investigated with the CI(SD), CEPA, MC SCF and MC CI(SD) methods. The most accurate MC CI(SDQ) calculations yield errors of 0.08, 0.23, and 0.18 eV in the first electron affinity values of Si, S, and Cl, respectively. The error for the P atom is 0.4 eV even at the MC CI(SDQ) level of approximation. All calculated second electron affinities were obtained negative, irrespective of the inclusion of electron correlation, i.e., the doubly negative atomic ions are not stable in the gas phase. The investigation of a few high spin states of Si 2− and P 2− gives no hint for metastable states.

Intramolecular H-transfer reactions in Si2Hn (for n=3–5)

Abstract Intramolecular rearrangement reactions for doublet Si 2 H 5 and Si 2 H 3 , quartet Si 2 H 3 , and singlet Si 2 H 4 have been studied. aim of the study was to characterize a series of intramolecular H-transfer reactions in silicon hydrides with vrying degrees of saturation. The transition states belonging to the reactions presented in this work possess a monobridged Si 2 H moiety. Structural features of the transition states and relative barrier heights have been examined; the geometry optimizations were performed with the use of CAS-SCF wavefunctions and the barrier height estimates were obtained with single-point CI calculations.

The electron affinities of Si2H2, Si2H3, and Si2H4

Abstract The three title compounds are found to form stable negatively charged states. Si 2 H 2 exhibits two anion states ( 2 A g and 2 B g ) corresponding to adiabatic electron affinities (EAs) of 1.76 and 1.23 eV. Si 2 H 3 has two stable anion states ( 1 A and 3 A) associated with EAs of 2.20 and 1.17 eV. Disilene shows a remarkably high adiabatic EA of 1.03 eV, but this electron attachment energy is very sensitive to the out-of-plane bending of the molecule.