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The hydrogen abstraction reaction between H and H2S, yielding HS and H2 as products, has been studied within the framework of interstellar surface chemistry. High-temperature rate constants below 2000 K are calculated in the gas phase and are in agreement with previously reported values. Subsequently, low-temperature rate constants down to 55 K are presented for the first time that are of interest to astrochemistry, i.e., covering both bimolecular and unimolecular reaction mechanisms. For this, a so-called implicit surface model is used. In strict terms, this is a structural gas-phase model in which the restriction of the rotation in the solid state is taken into account. The calculated kinetic isotope effects are explained in terms of the difference in activation and delocalization. All rate constants are calculated at the UCCSD(T)-F12/cc-VTZ-F12 level of theory. Finally, we show that the energetics of the reaction is affected to an only small extent by the presence of H2O or H2S molecular clusters that simulate an ice surface, calculated at the MPWB1K/def2-TZVP level of theory. [ABSTRACT FROM AUTHOR]

Recently, searches were made for H 2 CCS and HCCSH in a variety of interstellar environments─all of them resulted in nondetections of these two species. Recent findings have indicated the importance of destruction pathways, e.g., with atomic hydrogen, in explaining the consistent nondetection of other species, such as the H 2 C 3 O family of isomers. We have thus performed ab initio calculations looking at reactions of H 2 CCS, HCCSH, and related species with atomic hydrogen. Our results show that H 2 CCS and HCCSH are both destroyed barrierlessly by atomic hydrogen, thus providing a plausible explanation for the nondetections. We further find that subsequent reactions with atomic hydrogen can barrierlessly lead to CH 3 CH 2 SH, which has been detected. Astrochemical simulations including these reactions result not only in reproducing the observed abundance of H 2 CCS in TMC-1 but also show that CH 3 CH 2 SH, produced via our H-addition pathways and subsequently trapped on grains, can desorb in warmer sources up to abundances that match previous observations of CH 3 CH 2 SH in Orion KL. These results, taken together, point to the importance of grain-surface H-atom addition reactions and highlight the chemical links between cold prestellar cores and their subsequent, warmer evolutionary stages.

Atom probe tomography allows us to measure the three-dimensional composition of materials with up to atomic resolution by evaporating the material using high electric fields. Initially developed for metals, it is increasingly used for covalently bound structures. To aid the interpretation of the obtained fragmentation pattern, we modeled the fragmentation and desorption of self-assembled monolayers of thiolate molecules on a gold surface in strong electrostatic fields using density functional theory. We used a cluster model and a periodic model of amino-undecanethiolate, NH 2 (CH 2 ) 11 S, and fluoro-decanethiolate, CF 3 (CF 2 ) 7 (CH 2 ) 2 S. In the former molecule, the fragment CH 2 NH 2 + was found to evaporate at fields of 5.4-7.7 V/nm. It was followed by different hydrocarbon fragments. Fluoro-decanethiolate evaporates CF 3 + at fields of 5.7-6.7 V/nm in the cluster model and at 15.4-23.1 V/nm in the periodic model, followed by CF 2 + and C 2 F 4 2+ . Detailed analysis of the electronic structure during the evaporation process revealed a stepwise accumulation of the charge in the head groups exposed to the strongest fields, followed by dissociation of covalent bonds. These observations will facilitate the analysis of atom probe experiments of covalently bound structures.

We calculated reaction rate constants including atom tunneling for the hydrogen abstraction reaction CH 3 OH + H → CH 2 OH + H 2 with the instanton method. The potential energy was fitted by a neural network that was trained to UCCSD(T)-F12/VTZ-F12 data. Bimolecular gas-phase rate constants were calculated using microcanonic instanton theory. All H/D isotope patterns on the CH 3 group and the incoming H atom are studied. Unimolecular reaction rate constants, representing the reaction on a surface, down to 30 K, are presented for all isotope patterns. At 30 K, they range from 4100 for the replacement of the abstracted H by D to ∼8 for the replacement of the abstracting H to ∼2 to 6 for secondary KIEs. The 12 C/ 13 C kinetic isotope effect is 1.08 at 30 K, while the 16 O/ 18 O kinetic isotope effect is extremely small. A simple kinetic surface model using these data predicts high abundances of the deuterated forms of methanol.

The hydrogen abstraction reaction between H and H 2 S, yielding HS and H 2 as products, has been studied within the framework of interstellar surface chemistry. High-temperature rate constants below 2000 K are calculated in the gas phase and are in agreement with previously reported values. Subsequently, low-temperature rate constants down to 55 K are presented for the first time that are of interest to astrochemistry, i.e., covering both bimolecular and unimolecular reaction mechanisms. For this, a so-called implicit surface model is used. In strict terms, this is a structural gas-phase model in which the restriction of the rotation in the solid state is taken into account. The calculated kinetic isotope effects are explained in terms of the difference in activation and delocalization. All rate constants are calculated at the UCCSD(T)-F12/cc-VTZ-F12 level of theory. Finally, we show that the energetics of the reaction is affected to an only small extent by the presence of H 2 O or H 2 S molecular clusters that simulate an ice surface, calculated at the MPWB1K/def2-TZVP level of theory.

Quantum mechanical tunneling of atoms plays a significant role in many chemical reactions. The crossover temperature between classical and quantum movement is a convenient preliminary indication of the importance of tunneling for a particular reaction. Here we show, using instanton theory, that quantum tunneling is possible significantly above this crossover temperature for specific forms of the potential energy surface. We demonstrate the effect on an analytic potential as well as a chemical system. While protons move asynchronously along a Grotthuss chain in the classical high-temperature range, the onset of tunneling results in a synchronization of their movement.

We calculate, down to low temperature and for different isotopes, the reaction rate constants for the hydrogen abstraction reaction H + H(3)COH → H(2) + CH(2)OH/CH(3)O. These explain the known abundances of deuterated forms of methanol in interstellar clouds, where CH(2)DOH can be almost as abundant as CH(3)OH. For abstraction from both the C- and the O-end of methanol, the barrier-crossing motion involves the movement of light hydrogen atoms. Consequently, tunneling plays a dominant role already at relatively high temperature. Our implementation of harmonic quantum transition state theory with on the fly calculation of forces and energies accounts for these tunneling effects. The results are in good agreement with previous semiclassical and quantum dynamics calculations (down to 200 K) and experimental studies (down to 295 K). Here we extend the rate calculations down to lower temperature: 30 K for abstraction from the C-end of methanol and 80 K for abstraction from the OH-group. At all temperatures, abstraction from the C-end is preferred over abstraction from the O-end, more strongly so at lower temperature. Furthermore, the tunneling behavior strongly affects the kinetic isotope effects (KIEs). D + H(3)COH → HD + CH(2)OH has a lower vibrationally adiabatic barrier than H + H(3)COH → H(2) + CH(2)OH, giving rise to an inverse KIE (k(H)/k(D) < 1) at high temperature, in accordance with previous experiments and calculations. However, since tunneling is more facile for the light H atom, abstraction by H is favored over abstraction by D below ~135 K, with a KIE k(H)/k(D) of 11.2 at 30 K. The H + D(3)COD → HD + CD(2)OD reaction is calculated to be much slower than the D + H(3)COH → HD + CH(2)OH, in agreement with low-temperature solid-state experiments, which suggests the preference for H (as opposed to D) abstraction from the C-end of methanol to be the mechanism by which interstellar methanol is deuterium-enriched.

Geometry optimization, including searching for transition states, accounts for most of the CPU time spent in quantum chemistry, computational surface science, and solid-state physics, and also plays an important role in simulations employing classical force fields. We have implemented a geometry optimizer, called DL-FIND, to be included in atomistic simulation codes. It can optimize structures in Cartesian coordinates, redundant internal coordinates, hybrid-delocalized internal coordinates, and also functions of more variables independent of atomic structures. The implementation of the optimization algorithms is independent of the coordinate transformation used. Steepest descent, conjugate gradient, quasi-Newton, and L-BFGS algorithms as well as damped molecular dynamics are available as minimization methods. The partitioned rational function optimization algorithm, a modified version of the dimer method and the nudged elastic band approach provide capabilities for transition-state search. Penalty function, gradient projection, and Lagrange-Newton methods are implemented for conical intersection optimizations. Various stochastic search methods, including a genetic algorithm, are available for global or local minimization and can be run as parallel algorithms. The code is released under the open-source GNU LGPL license. Some selected applications of DL-FIND are surveyed.