Karina Sendt

Failure of density-functional theory and time-dependent density-functional theory for large extended π systems

Density-functional theory (DFT) is widely used for studying large systems such as metals, semiconductors, and large molecules, with time-dependent density-functional theory becoming a very powerful tool for investigating molecular excited states. As part of a systematic study of both the intrinsic weaknesses of DFT and the weaknesses of present implementations, we consider its application to the one and two-dimensional conjugated π systems: polyacetylene fragments and oligoporphyrins, respectively. Very poor results are obtained for the calculated spectra, and polyacetylene is predicted by all functionals considered, including gradient-corrected functionals, to have a triplet ground state. The cause of this is linked to known problems of existing density functionals concerning nonlocality and asymptotic behavior which result in the highest-occupied molecular-orbital being too high in energy so that semiconductors and low-band-gap insulators are predicted to have metal-like properties. The failure of modern density functionals to predict qualitatively realistic molecular hyperpolarizabilities for extended systems is closely related.

Spectroscopic constants of the X̃(1A1), ã(3B1), and Ã(1B1) states of CF2, CCl2, and CBr2 and heats of formation of selected halocarbenes: An ab initio quantum chemical study

The geometries, rotational constants, harmonic force constants and frequencies, dissociation and term energies of CF2, CCl2, and CBr2 in their respective X(1A1), a(3B1) and A(1B1) states, computed by complete active space self-consistent field (CASSCF), complete active space second-order purturbation (CASPT2), and coupled-cluster with single, double and perturbative triple excitations [CCSD(T)] methods and cc-pVTZ basis sets, are reported. For CCl2 and CBr2 the barriers to linearity are also characterized. The computed spectroscopic constants are in good agreement with the available experimental data. The atomization energies and hence heats of formation at 0 and 298 K of these molecules as well as of CHF, CHCl, and CFCl, all in their lowest singlet ground states were also computed by the CCSD(T) method utilizing basis sets ranging from cc-pVDZ to aug-cc-pVQZ, cc-pCVQZ and G3large, enabling the extrapolation of the energies to a complete basis set (CBS) limit and the inclusion of core–valence correlation...

Chemical kinetic modeling of the H/S system: H2S thermolysis and H2 sulfidation

A detailed chemical mechanism to describe reactions in the H 2 -S 2 -H 2 S system has been constructed.The mechanism comprises 21 reactions among the species H 2 S, S 2 , H 2 , HSSH, HSS, SH, S, and H. The structure of the mechanism resembles closely that of the H 2 -O 2 system. For a few reactions, experimental values for rate constants were taken from the literature, but the kinetics of most of the reactions have been studied theoretically, using a combination of transition state theory for bimolecular reactions, master equation calculations for unimolecular decompositions, and QRRK methods for chemically activated reactions. The mechanism has been validated against a diverse collection of published data for H 2 S thermolysis in a static cell or in flow reactors, for temperatures ranging from 873 to 1423 K, pressures from 0.04 to 3 bar, and H 2 S mole fraction from 0.02 to 1. The predictions of the mechanism are sensitive only to the rates of the processes responsible for S-S bond formation, HSS + H ⇌ 2SH ( R 8 ) HSSH ( + M ) ⇌ 2SH ( + M ) ( R 9 ) HSSH + H ⇌ H 2 S + SH ( R 16 ) Slight adjustment of these rates allows the data to be modeled accurately. Data for the reverse, hydrogen sulfidation reaction (H 2 +S 2 ) are also modeled very accurately. This comprehensive chemical kinetic mechanism for the H/S system not only describes a wide range of experimental data but also provides the basis for the construction of accurate models for H 2 S oxidation in combustion and related systems.

Computational study of the reaction SH + O2.

The reaction of SH + O2 has been characterized using multireference methods, with geometries and vibrational frequencies determined at the CASSCF/cc-pVTZ level and single-point energies calculated at the MRCI/aug-cc-pV(Q+d)Z level. The dominant product channels are found to be SO + OH and HSO + O. Whereas the formation of SO + OH has a barrier of approximately 81 kJ mol-1, it is energetically more favorable than the formation of HSO + O, which is barrierless beyond the endothermicity of approximately 89 kJ mol-1 at 0 K. Thus, the reaction SH + O2 --> SO + OH is 2 orders of magnitude faster than the reaction SH + O2 --> HSO + O at room temperature, revealing that the atmospheric oxidation of SH leads directly to the formation of SO + OH with the rate coefficient of approximately 1.0 x 10(-2) cm3 mol-1 s-1. At temperatures above 1000 K, however, the rates of the two channels become comparable. This may be attributed to the entropy effects leading to the higher pre-exponential factor for the channel (forming HSO + O) via a more loose transition state than that (forming SO + OH) entailing a four-centered transition state. Whereas the hydrogen abstraction reaction producing S + HO2 is found to proceed on the quartet surface, the substantial barrier of approximately 165 kJ mol-1 means that it occurs as a minor product channel. Finally, the formation of possible products SO2 + H is prohibited due to the lack of a transition state for the direct sulfur insertion.

Theoretical study of reactions in the multiple well H2/S2 system.

The potential energy surface of the H(2)/S(2) system has been characterized at the full valence MRCI+Davidson/aug-cc-pV(Q+d)Z level of theory using geometries optimized at the MRCI/aug-cc-pVTZ level. The analysis includes channels occurring entirely on either the singlet or the triplet surface as well as those involving an intersystem crossing. RRKM-based multiple well calculations allow the prediction of rate constants in the temperature range of 300-2000 K between 0.1 and 10 bar. Of the SH recombined on the singlet surface, the stabilization of the rovibrationally excited adduct HSSH is at the low-pressure limit at 1 bar, but it has a rate comparable to that forming another major set of products H(2)S + S (via an intersystem crossing) at temperatures below 1000 K; at higher temperatures, HSS + H becomes the dominant product. For the reaction H(2)S + S, the presence of an intersystem crossing allows the formation of the singlet excited adduct H(2)SS, most of which rearranges and stabilizes as HSSH under atmospheric conditions. At high temperatures, the majority of excited HSSH dissociates to SH + SH and HSS + H. Compared to reported shock tube measurements of the reaction H(2)S + S, most of the S atom consumption can be described by the triplet abstraction route H(2)S + S --> SH + SH, especially at high temperatures, but inclusion of the singlet insertion channel provides a better description of the experimental data. The reaction HSS + H was found to proceed predominantly on the singlet surface without a chemical barrier. The formation of the major product channel SH + SH is very fast at room temperature (approximately 4 x 10(15) cm(3) mol(-1) s(-1)). While the formation of H(2)S + S or S(2) + H(2) via an isomerization or an intersystem crossing, respectively, are minor product channels, their rates are significantly higher than those of the corresponding direct triplet channels, except at elevated temperatures. Finally, due to the relatively shallow nature of its well, the stabilization of H(2)SS is negligible under conditions of likely interest.

Theoretical Study of Hydrogen Abstraction and Sulfur Insertion in the Reaction H2S + S

The reaction of H2S + S has been characterized at the multireference configuration interaction level with the geometries optimized using the aug-cc-pVTZ basis set and the single-point energy calculated using the aug-cc-pV(Q+d)Z basis set. As in the analogous reaction of H2 + S, the presence of an intersystem crossing enables products (SH + SH) to be formed on the singlet surface through S insertion, which bypasses the triplet barrier (19.1 kJ mol-1 relative to SH + SH) of the H abstraction route. This provides theoretical evidence for SH + SH formation without barrier beyond endothermicity at sufficiently low temperatures. The H abstraction route, however, is expected to be competitive at higher temperatures due to a much higher Arrhenius pre-exponential factor (6.9 x 10(14) cm3 mol-1 s-1 derived from TST calculation) than that of S insertion channel (3.7 x 10(13) cm3 mol-1 s-1, derived by a least-squares fit to the measurements). With a slightly higher transition-state barrier than that of the H abstraction channel, the production of S2 + H2 is less favored due to proceeding via intersystem crossing and insertion. While the formation of HSS + H is energetically unfavorable relative to SH + SH, recombination channels producing H2SS or the more stable HSSH are expected to occur under collisional stabilization conditions at high pressures.