Faraday Discussions最新文献

Despite extensive experimental and theoretical studies on the kinetics of the O(³P) + C₇H₈ (toluene) reaction and a pioneering crossed molecular beam (CMB) investigation, the branching fractions (BFs) of the CH₃C₆H₄O(methylphenoxy) + H, C₆H₅O(phenoxy) + CH₃, and spin-forbidden C₅H₅CH₃ (methylcyclopentadiene) + CO product channels remain an open question, which has hampered the proper inclusion of this important reaction in the chemical modelling of various chemical environments. We report a CMB study with universal soft electron-ionization mass-spectrometric detection of the reactions O(³P,¹D) + toluene at the collision energy of 34.7 kJ mol⁻¹. From CMB data we have inferred the reaction dynamics and quantified the BFs of the primary products and the role of intersystem crossing (ISC). The CH₃-elimination channel dominates (BF = 0.69 ± 0.22) in the O(³P) reaction, while the H-displacement and CO-formation channels are minor (BF = 0.22 ± 0.07 and 0.09 ± 0.05, respectively), with ISC accounting for more than 50% of the reactive flux. Synergistic transition-state theory (TST)-based master equation simulations including nonadiabatic TST on ab initio coupled triplet/singlet potential energy surfaces were employed to compute the product BFs and assist in the interpretation of the CMB results. In the light of the good agreement between the theoretical predictions for the O(³P) + toluene reaction and the CMB results as well as the absolute rate constant as a function of temperature (T) (from literature), the so-validated computational methodology was used to predict channel-specific rate constants as a function of T at 1 atm.

In molecular dynamics, a fundamental question is how the outcome of a collision depends on the relative orientation of the collision partners before their interaction begins (the stereodynamics of the process). The preference for a particular orientation of the reactant complex is intimately related to the idea of a collision mechanism and the possibility of control, as revealed in recent experiments. Indeed, this preference holds not only for chemical reactions involving complex polyatomic molecules, but also for the simplest inelastic atom–diatom collisions at cold collision energies. In this work, we report how the outcome of rotationally inelastic collisions between two D₂ molecules can be controlled by changing the alignment of their internuclear axes under the same or different polarization vectors. Our results demonstrate that a higher degree of control can be achieved when two internuclear axes are aligned, especially when both molecules are relaxed in the collision. The possibility of control extends to very low energies, even to the ultracold regime, when no control could be achieved just by the alignment of the internuclear axis of one of the colliding partners.

We propose to coherently control the ultracold 2KRb → K₂ + Rb₂ reaction product state distribution via quantum interference. By leveraging that the nuclear spin degrees of freedom in the reaction maintain coherence, which was demonstrated in Liu, Zhu et al., arXiv, 2023, arXiv:2310.07620, https://doi.org/10.48550/arXiv.2310.07620, we explore the concept of a “reaction interferometer”. Such an interferometer involves splitting one KRb molecular cloud into two, imprinting a well-defined relative phase between them, recombining the clouds for reactions, and measuring the product state distribution. We show that the interference patterns provide a mechanism to coherently control the product states, and specific product channels also serve as an entanglement witness of the atoms in the reactant KRb molecule.

Studying larger nucleophiles in bimolecular nucleophilic substitution (S_N2) reactions bridges the gap from simple model systems to those relevant to organic chemistry. Therefore, we investigated the reaction dynamics between the methoxy anion (CH₃O⁻) and iodomethane (CH₃I) in our crossed-beam setup combined with velocity map imaging at the four collision energies 0.4, 0.7, 1.2, and 1.6 eV. We find the two ionic products I⁻ and CH₂I⁻, which can be attributed to the S_N2 and proton transfer channels, respectively. The proton transfer channel progresses in a previously observed fashion from indirect to direct scattering with increasing collision energy. Interestingly, the S_N2 channel exhibits direct dynamics already at low collision energies. Both the direct stripping, leading to forward scattering, and the direct rebound mechanism, leading to backward scattering into high angles, are observed.

A magnetically manipulated molecular beam technique is used to change the rotational orientation of H₂ molecules which collide with a stepped Cu(511) surface and explore how the polarisation dependence of molecules scattering into the specular channel changes as a function of surface temperature. At all temperatures, H₂ molecules that are rotating like cartwheels are more likely to be scattered into the specular channel than those that are rotating like helicopters. Furthermore, the scattered molecules are more likely to be rotating like cartwheels, regardless of their state before the collision. Increasing the temperature of the Cu(511) surface causes the polarisation effects to become stronger, with the scattering becoming more selective for H₂ with cartwheel like rotation. Therefore, scattering a molecular beam of H₂ from a Cu(511) surface and taking the molecules scattered into the specular channel provides a method to create a rotationally polarised beam of H₂, where the polarisation can be tuned by changing the surface temperature. In contrast, the rotational orientation dependence observed for specular scattering from a flat Cu(111) surface is independent of surface temperature within the same temperature range.

Sulfur atoms serve as key players in diverse chemical processes, from astrochemistry at very low temperature to combustion at high temperature. Building upon our prior findings, showing cyclization to thiophenes following the reaction of ground-state sulfur atoms with dienes, we here extend this investigation to include many additional reaction products, guided by detailed theoretical predictions. The outcomes highlight the complex formation of products during intersystem crossing (ISC) to the singlet surfaces. Here, we employed crossed-beam velocity map imaging and high-level ab initio methods to explore the reaction of S(³P) with 1,3-butadiene and isoprene under single-collision conditions and in low-temperature flows. For the butadiene reaction, our experimental results show the formation of thiophene via H₂ loss, a 2H-thiophenyl radical through H loss, and thioketene through ethene loss at a slightly higher collision energy compared to previous observations. Complementary Chirped-Pulse Fourier-Transform mmWave spectroscopy (CP-FTmmW) measurements in a uniform flow confirmed the formation of thioketene in the reaction at 20 K. For the isoprene reaction, we observed analogous products along with the 2H-thiophenyl radical arising from methyl loss and C₃H₄S (loss of ethene or H₂ + acetylene). CP-FTmmW detected the formation of thioformaldehyde via loss of 1,3-butadiene, again in the 20 K flow. Coupled-cluster calculations on the pathways found by the automated kinetic workflow code KinBot support these findings and indicate ISC to the singlet surface, leading to the generation of various long-lived intermediates, including 5-membered heterocycles.

A crossed beam velocity-map ion-imaging apparatus has been used to determine differential cross sections (DCSs) for the rotationally inelastic scattering of NO(A²Σ⁺, v = 0, j = 0.5) with CO₂, as a function of both NO(A, v = 0, N′) final state and the coincident final rotational energy of the CO₂. The DCSs are dominated by forward-peaked scattering for all N′, with significant rotational excitation of CO₂, and a small backward scattered peak is also observed for all final N′. However, no rotational rainbow scattering is observed and there is no evidence for significant product rotational angular momentum polarization. New ab initio potential energy surface calculations at the PNO-CCSD(T)-F12b level of theory report strong attractive forces at long ranges with significant anisotropy relative to both NO and CO₂. The absence of rotational rainbow scattering is consistent with removal of low-impact-parameter collisions via electronic quenching, in agreement with the literature quenching rates of NO(A) by CO₂ and recent electronic structure calculations. We propose that high-impact-parameter collisions, that do not lead to quenching, experience strong anisotropic attractive forces that lead to significant rotational excitation in both NO and CO₂, depolarizing product angular momentum while leading to forward and backward glory scattering.

Dissolved helium atoms evaporate from liquids in super-Maxwellian speed distributions because their interactions are too weak to enforce full thermal equilibration at the surface as they are “squeezed” out of solution. The excess speeds of these He atoms reflect their final interactions with solvent and solute molecules at the surfaces of water and other liquids. We extend this observation by monitoring He atom evaporation from salty water solutions coated with surfactants. These surface-active molecules span neutral, anionic, and cationic amphiphiles: butanol, 3-methyl-1-butanol, pentanol, pentanoic acid, pentanoate, tetrabutylammonium, benzyltrimethylammonium, hexyltrimethylammonium, and dodecyltrimethylammonium, each characterized by surface tension measurements. The helium energy distributions, recorded in vacuum using a salty water microjet, reveal a sharp distinction between neutral and ionic surfactant films. Helium atoms evaporate through neutral surfactant monolayers in speed distributions that are similar to a pure hydrocarbon, reflecting the common alkyl chains of both. In contrast, He atoms appear to evaporate through ionic surfactant layers in distributions that are closer to pure salty water. We speculate that the ionic surfactants distribute themselves more loosely and deeply through the top layers of the aqueous solution than do neutral surfactants, with gaps between the surfactants that may be filled with salty water. This difference is supported by prior molecular dynamics simulations and ion scattering measurements of surfactant solutions.