Phenyl Radical Activates Molecular Hydrogen Through Protium and Deuterium Tunneling

Activating dihydrogen, H2, is a challenging endeavor typically achieved using transition metal centers. Pure main-group compounds capable of this are rare and have emerged in recent decades. These systems rely on synergistic donor-acceptor interactions with H2's antibonding σ* and bonding σ orbital. An alternative (hydrocarbon) radical-mediated activation is problematic because the H-H bond is stronger (104.2 kcal mol-1) than most C-H bonds. Here, we explore using the phenyl radical to tackle this, forming benzene with a C-H bond energy (112.9 kcal mol-1) that provides a thermodynamic driving force. We mainly observe a benzene-HI complex upon photolysis of iodobenzene in an H2-doped neon matrix at 4.4 K despite a barrier of 7.6 kcal mol-1, while phenyl radical forms in case of the heavier D2 isotopologue. When D2 molecules are allowed to diffuse, mono-deuterated benzene accumulates within hours. Computations using path integral-based instanton theory highlight that primarily the transferred hydrogen atom is moving during the reaction which greatly increases the tunneling probability. In excellent agreement with the experimental results, we predict significant tunneling rate constants for both isotopologues, H2 and D2, featuring a strong kinetic isotope effect of up to four orders of magnitude at the lowest temperatures.

Grain-Surface Hydrogen-Addition Reactions as a Chemical Link Between Cold Cores and Hot Corinos: The Case of H2CCS and CH3CH2SH

Recently, searches were made for H₂CCS 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 H₂C₃O family of isomers. We have thus performed ab initio calculations looking at reactions of H₂CCS, HCCSH, and related species with atomic hydrogen. Our results show that H₂CCS 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 CH₃CH₂SH, which has been detected. Astrochemical simulations including these reactions result not only in reproducing the observed abundance of H₂CCS in TMC-1 but also show that CH₃CH₂SH, produced via our H-addition pathways and subsequently trapped on grains, can desorb in warmer sources up to abundances that match previous observations of CH₃CH₂SH 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.

A cryogenic ice setup to simulate carbon atom reactions in interstellar ices

The design, implementation, and performance of a customized carbon atom beam source for the purpose of investigating solid-state reaction routes in interstellar ices in molecular clouds are discussed. The source is integrated into an existing ultrahigh vacuum setup, SURFace REaction SImulation DEvice (SURFRESIDE²), which extends this double atom (H/D, O, and N) beamline apparatus with a third atom (C) beamline to a unique system that is fully suited to explore complex organic molecule solid-state formation under representative interstellar cloud conditions. The parameter space for this system is discussed, which includes the flux of the carbon atoms hitting the ice sample, their temperature, and the potential impact of temperature on ice reactions. Much effort has been put into constraining the beam size to within the limits of the sample size with the aim of reducing carbon pollution inside the setup. How the C-atom beam performs is quantitatively studied through the example experiment, C + ¹⁸O₂, and supported by computationally derived activation barriers. The potential for this source to study the solid-state formation of interstellar complex organic molecules through C-atom reactions is discussed.

Parallel Implementation of Semiclassical Transition State Theory

This paper presents the parsctst code, an efficient parallel implementation of the semiclassical transition state theory (SCTST) for reaction rate constant calculations. Parsctst is developed starting from a previously presented approach for the computation of the vibrational density of states of fully coupled anharmonic molecules ( Nguyen et al. Chem. Phys. Lett. 2010 , 499 , 915 ). The parallel implementation makes it practical to tackle reactions involving more than 100 fully coupled anharmonic vibrational degrees of freedom and also includes multidimensional tunneling effects. After describing the pseudocode and demonstrating its computational efficiency, we apply the new code for estimating the rate constant of the proton transfer isomerization reaction of the 2,4,6-tri- tert-butylphenyl to 3,5-di- tert-butylneophyl. Comparison with both theoretical and experimental results is presented. Parsctst code is user-friendly and provides a significant computational time saving compared to serial calculations. We believe that parsctst can boost the application of SCTST as an alternative to the basic transition state theory for accurate kinetics modeling not only in combustion or atmospheric chemistry, but also in organic synthesis, where bigger reactive systems are encountered.

Direct Synthesis of Heavy Grignard Reagents: Challenges, Limitations, and Derivatization

The direct synthesis of organocalcium compounds (heavy Grignard reagents) by the reduction of organyl halides with activated calcium powder succeeded in a straightforward manner for organic bromides and iodides that are bound at sp² -hybridized carbon atoms. Extension of this strategy to alkyl halides was very limited, and only the reduction of trialkylsilylmethyl bromides and iodides with activated calcium allowed the isolation of the corresponding heavy Grignard reagents. Substitution of only one hydrogen atom of the methylene moiety by a phenyl or methyl group directed this reduction toward the Wurtz-type coupling and the formation of calcium halide and the corresponding C-C coupling product. The stability of the methylcalcium and benzylcalcium derivatives in ethereal solvents suggests an unexpected reaction behavior of the intermediate organyl halide radical anions. Quantum chemical calculations verify a dependency between the ease of preparative access to organocalcium complexes and the C-I bond lengths of the organyl iodides. The bulkiness of the trialkylsilyl group is of minor importance. Chloromethyltrimethylsilane did not react with activated calcium; however, halogen-exchange reactions allowed the isolation of [Ca(CH₂ SiMe₃ )(thf)₃ (μ-Cl)]₂ . Furthermore, the metathetical approach of reacting [Ca(CH₂ SiMe₃ )I(thf)₄ ] with KN(SiMe₃ )₂ and the addition of N,N,N',N'',N''-pentamethyldiethylenetriamine (pmdeta) allowed the isolation of heteroleptic [CaCH₂ SiMe₃ {N(SiMe₃ )₂ }(pmdeta)]. In the reaction of this derivative with phenylsilane, the trimethylsilylmethyl group proved to be more reactive than the bis(trimethylsilyl)amido substituent.