Collision Complex Research Articles

Rotational Excitation of the CP(Χ2Σ+) Open Shell Molecule Due to Collision with He(1S).

Phosphorus bearing molecules have been discovered in the circumstellar and interstellar media. Modeling their abundance accurately requires computations of rate coefficients induced by collision with He and H2 (i.e., the most abundant gaseous components). These calculations may be carried out by first determining highly accurate potential energy surface (PES) and cross sections. In this paper, we present the first PES of the CP(X2Σ+)-He(1S) van der Waals collisional complex. The ab initio interaction potential was performed using the explicitly correlated restricted coupled cluster approach with simple, double, and perturbative triple excitation (RCCSD(T)-F12) in connection with the augmented-correlation consistent-polarized valence triple-ζ Gaussian basis set (aug-cc-pVTZ). The potential presents two minima of -18.62 cm-1 and -18.72 cm-1. From the PES obtained, we have computed state-to-state excitation cross sections of CP due to collision with He for energies up to 500 cm-1. Rotational transitions involving the fine-structure levels of the CP molecule were treated with a recoupling technique based on the scattering matrix calculated with the exact quantum mechanical close coupling method. Discussions on the propensity rules between the fine-structure levels were made and we found that the Δj = ΔN transitions are favored with respect to the Δj ≠ ΔN ones. The data presented in this paper may have a great impact on the accurate determination of the CP abundance in space.

Using a modified Marcus model to analyze the Brønsted correlation of gas-phase reactions of the deprotonation of para-substituted toluenes by CH2Cl−, H(O)C−, CH3O−, H(F)N−, NH2C≡C−, and СH3C≡C− anions

Ab initio calculations of the energy parameters for a series of gas-phase reactions of the deprotonation of para-substituted toluenes (OH, CH3, H, Cl, NC, CN, and NO2 substituents) by CH2Cl–, H(O)C–, CH3O–, H(F)N–, NH2C≡C–, and CH3C≡C– anions (the conjugated acids are chloromethane (CH3Cl), formaldehyde (Н2CO), methanol (CH3OH), fluoroamine (NH2F), aminoacetylene (NH2C≡CН), and methylacetylene (CH3C≡CН), respectively) is performed using a technique of intramolecular reorganization proposed earlier. The equilibrium (E a eq) and nonequilibrium (E a neq ) contributions to energy of activation E а are found for distance Q(C…B) = 3.0 A (where С and В are the central atoms of the acid and base). At this value of Q, the tunneling mechanism of proton transfer predominates. The results from calculations, together with similar data obtained earlier for reactions of the deprotonation of toluenes by СН2CN– anions, are considered from the viewpoint of the harmonic Marcus model. It is found that in the graph ΔE 0 − E a neq (where ΔE 0 is the internal energy of the reaction (for a collision complex)), the points for five reaction series with C-bases fall on the same quadratic (within the −11 to 12 kcal/mol range of ΔE 0) curve; the second curve, which includes points for the series with N- and O-bases (OCH3 – and H(F)N–) is shifted below the first curve by 1.5 kcal/mol. For most reaction series (except the one with the strongest bases, CH2Cl– and H(O)C–), the curvature and slope of graph ΔE 0 − E a neq are in agreement with the theory. The correlations of energies E a neq , E a eq, E а, and ΔE 0 with the energy of reaction ΔE 00 are satisfactorily described by quadratic equations as well. Differences between the structures of bases are reflected most by energy E a eq and the height of the proton tunneling barrier in the activated complex (AC) E b. For the series of reactions of toluene with a set of bases, graphs ΔE 00 − E a eq and ΔE 0 − E b show good linear correlation between the values (opposite in sign) of departures from straight lines drawn through the points for two C-bases of the same type (CH2Cl− and СН2CN−).

Monitoring shipping emissions in the German Bight using MAX-DOAS measurements

Abstract. A 3-year time series of ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements of NO2 and SO2 on the island Neuwerk has been analyzed for contributions from shipping emissions. The island is located in the German Bight, close to the main shipping lane (at a distance of 6–7 km) into the river Elbe towards the harbor of Hamburg. Measurements of individual ship plumes as well as of background pollution are possible from this location. A simple approach using the column amounts of the oxygen molecule dimer or collision complex, O4, for the determination of the horizontal light path length has been applied to retrieve path-averaged volume mixing ratios. An excellent agreement between mixing ratios determined from NO2 retrievals in the UV and visible parts of the spectrum has been found, showing the validity of the approach. Obtained mixing ratios of NO2 and SO2 are compared to co-located in situ measurements showing good correlation on average but also a systematic underestimation by the MAX-DOAS O4 scaling approach. Comparing data before and after the introduction of stricter fuel sulfur content limits (from 1 to 0.1 %) on 1 January 2015 in the North Sea Emission Control Area (ECA), a significant reduction in SO2 levels is observed. For situations with wind from the open North Sea, where ships are the only local source of air pollution, the average mixing ratio of SO2 decreased by a factor of 8, while for NO2 in the whole time series from 2013 to 2016, no significant change in emissions was observed. More than 2000 individual ship emission plumes have been identified in the data and analyzed for the emission ratio of SO2 to NO2, yielding an average ratio of 0.3 for the years 2013/2014 and decreasing significantly, presumably due to lower fuel sulfur content, in 2015/2016. By sorting measurements according to the prevailing wind direction and selecting two angular reference sectors representative for wind from the open North Sea and coast excluding data with mixed air mass origin, relative contributions of ships and land-based sources to air pollution levels in the German Bight have been estimated to be around 40 % : 60 % for NO2 as well as SO2 in 2013/2014, dropping to 14 % : 86 % for SO2 in 2015/2016.

Mechanistic investigation of the atmospheric reaction of NH2 with NO2 and study of the catalytic effects of water molecule on kinetic path

The gas-phase reaction mechanism of NO2 with NH2 on the singlet potential energy surface is investigated at the B3LYP, MP2, CCSD(T) and G3B3 levels of calculation in connection with the 6-311[Formula: see text]G(3df, 3pd) basis sets. Seven different kinds of products have been obtained. The results reveal that P7 [N2[Formula: see text]2O] is the main product in thermodynamic viewpoint with the Gibbs free energy of [Formula: see text][Formula: see text]kcal/mol. The product P2 due to less number of transition states and low level barrier height of transition states is introduced as the kinetic product. The calculation of rate constants in different temperatures confirms our predication for P7 adduct in comparison with other products. In the first step, the reactions without catalytic effects are considered. All of these reactions begin with pre-reactive collision complex CR [NO2–NH2]. This complex has been produced from barrier-less process via collision of reactants. From variation of bond lengths and angles or dissociation of this complex, other species are produced. In the second step, the catalytic effect of water and water dimer molecules on the reaction rate of kinetic path, P2, have been attended. Calculation of the rate constant was done in the temperature range from 500[Formula: see text]K to 3000[Formula: see text]K. Our results showed that the rate constant of path P2 increases with temperature and the number of water molecules. So, we can conclude that the number of water molecules corresponding to relative humidity of atmosphere has catalytic effect on the mentioned reaction rate.

Ephemeral collision complexes mediate chemically termolecular transformations that affect system chemistry.

Termolecular association reactions involve ephemeral collision complexes-formed from the collision of two molecules-that collide with a third and chemically inert 'bath gas' molecule that simply transfers energy to/from the complex. These collision complexes are generally not thought to react chemically on collision with a third molecule in the gas-phase systems of combustion and planetary atmospheres. Such 'chemically termolecular' reactions, in which all three molecules are involved in bond making and/or breaking, were hypothesized long ago in studies establishing radical chain branching mechanisms, but were later concluded to be unimportant. Here, with data from ab initio master equation and kinetic-transport simulations, we reveal that reactions of H + O2 collision complexes with other radicals constitute major kinetic pathways under common combustion situations. These reactions are also found to influence flame propagation speeds, a common measure of global reactivity. Analogous chemically termolecular reactions mediated by ephemeral collision complexes are probably of significance in various combustion and planetary environments.

Singlet Oxygen Photophysics in Liquid Solvents: Converging on a Unified Picture.

Singlet oxygen, O2(a1Δg), the lowest excited electronic state of molecular oxygen, is an omnipresent part of life on earth. It is readily formed through a variety of chemical and photochemical processes, and its unique reactions are important not just as a tool in chemical syntheses but also in processes that range from polymer degradation to signaling in biological cells. For these reasons, O2(a1Δg) has been the subject of intense activity in a broad distribution of scientific fields for the past ∼50 years. The characteristic reactions of O2(a1Δg) kinetically compete with processes that deactivate this excited state to the ground state of oxygen, O2(X3Σg-). Moreover, O2(a1Δg) is ideally monitored using one of these deactivation channels: O2(a1Δg) → O2(X3Σg-) phosphorescence at 1270 nm. Thus, there is ample justification to study and control these competing processes, including those mediated by solvents, and the chemistry community has likewise actively tackled this issue. In themselves, the solvent-mediated radiative and nonradiative transitions between the three lowest-lying electronic states of oxygen [O2(X3Σg-), O2(a1Δg), and O2(b1Σg+)] are relevant to issues at the core of modern chemistry. In the isolated oxygen molecule, these transitions are forbidden by quantum-mechanical selection rules. However, solvent molecules perturb oxygen in such a way as to make these transitions more probable. Most interestingly, the effect of a series of solvents on the O2(X3Σg-)-O2(b1Σg+) transition, for example, can be totally different from the effect of the same series of solvents on the O2(X3Σg-)-O2(a1Δg) transition. Moreover, a given solvent that appreciably increases the probability of a radiative transition generally does not provide a correspondingly viable pathway for nonradiative energy loss, and vice versa. The ∼50 years of experimental work leading to these conclusions were not easy; spectroscopically monitoring such weak and low-energy transitions in time-resolved experiments is challenging. Consequently, results obtained from different laboratories often were not consistent. In turn, attempts to interpret molecular events were often simplistic and/or misguided. However, over the recent past, increasingly accurate experiments have converged on a base of credible data, finally forming a consistent picture of this system that is resonant with theoretical models. The concepts involved encompass a large fraction of chemistry's fundamental lexicon, e.g., spin-orbit coupling, state mixing, quantum tunneling, electronic-to-vibrational energy transfer, activation barriers, collision complexes, and charge-transfer interactions. In this Account, we provide an explanatory overview of the ways in which a given solvent will perturb the radiative and nonradiative transitions between the O2(X3Σg-), O2(a1Δg), and O2(b1Σg+) states.

Universality and chaoticity in ultracold K+KRb chemical reactions

A fundamental question in the study of chemical reactions is how reactions proceed at a collision energy close to absolute zero. This question is no longer hypothetical: quantum degenerate gases of atoms and molecules can now be created at temperatures lower than a few tens of nanokelvin. Here we consider the benchmark ultracold reaction between, the most-celebrated ultracold molecule, KRb and K. We map out an accurate ab initio ground-state potential energy surface of the K2Rb complex in full dimensionality and report numerically-exact quantum-mechanical reaction dynamics. The distribution of rotationally resolved rates is shown to be Poissonian. An analysis of the hyperspherical adiabatic potential curves explains this statistical character revealing a chaotic distribution for the short-range collision complex that plays a key role in governing the reaction outcome.

Influence of Molecular Oxygen on Ortho-Para Conversion of Water Molecules

The mechanism of influence of molecular oxygen on the probability of ortho-para conversion of water molecules and its relation to water magnetization are considered within the framework of the concept of paramagnetic spin catalysis. Matrix elements of the hyperfine ortho-para interaction via the Fermi contact mechanism are calculated, as well as the Maliken spin densities on water protons in H2O and O2 collisional complexes. The mechanism of penetration of the electron spin density into the water molecule due to partial spin transfer from paramagnetic oxygen is considered. The probability of ortho-para conversion of the water molecules is estimated by the quantum chemistry methods. The results obtained show that effective ortho-para conversion of the water molecules is possible during the existence of water-oxygen dimers. An external magnetic field affects the ortho-para conversion rate given that the wave functions of nuclear spin sublevels of the water protons are mixed in the complex with oxygen.

Spectral lineshapes of collision-induced absorption (CIA) using isotropic intermolecular potential for mixtures of molecular hydrogen with helium

Quantum mechanical lineshapes of collision-induced absorption (CIA) at different temperatures for the collisional complex of a hydrogen molecule and a helium atom are computed using the numerical results of the induced dipole moment and isotropic intermolecular potential as input. Comparison with measured spectra and first three spectral moments shows good agreement over the rototranslational band in the far infrared. The quality of the present potentials have been checked by comparing between calculated and experimental thermo-physical and transport properties, which are found to be in good agreement.

Theoretical investigation of rotationally inelastic collisions of CH(X2Π) with hydrogen atoms.

We report calculations of state-to-state cross sections for collision-induced rotational transitions of CH(X2Π) with atomic hydrogen. These calculations employed the four adiabatic potential energy surfaces correlating CH(X2Π) + H(2S), computed in this work through the multi-reference configuration interaction method [MRCISD + Q(Davidson)]. Because of the presence of deep wells on three of the potential energy surfaces, the scattering calculations were carried out using the quantum statistical method of Manolopoulos and co-workers [Chem. Phys. Lett. 343, 356 (2001)]. The computed cross sections included contributions from only direct scattering since the CH2 collision complex is expected to decay predominantly to C + H2. Rotationally energy transfer rate constants were computed for this system since these are required for astrophysical modeling.

Induction of radiative forbidden transitions in an oxygen molecule in O2–H2O collision complexes

By the DFT/B3LYP method the equilibrium structures of oxygen complexes with water are calculated in various geometric conformations with symmetries C 2v and C s . By the MRCI/CASSCF method potential energy surface cross-sections of the 1.3[O2–H2O] complexation reaction are constructed. With taking into account the spin-orbit coupling, the forbidden transition moments a 1Δ g –X 3Σ g −, b 1Σ g +–a 1Δ g , c 1Σ u −–a 1Δ g , A 3Σ u +–X 3Σ g − of the complexes are calculated and changes in their intensities at different geometric configurations of the complex are revealed.

Standard deviations of chemical reaction rates and validity ranges of the statistical theory

ABSTRACTValidity ranges of the statistical theory of chemical reactions are discussed. On the basis of the quantum mechanical scattering theory, the rate of the chemical reaction of the complex-formation mode is formulated. The interaction matrix elements between resonance states and scattering continua are regarded as stochastic variables. The expectation value of the reaction rate is shown to coincide with the prediction of the conventional statistical theory if the condition of overlapping resonance is fulfilled. The standard deviation is found to be inversely proportional to the number of resonance states involved in the width of the collision energy. High density of vibrational energy levels of the collision complex serves to suppress the standard deviation and to make the statistical theory accurate. The condition for the conventional statistical theory to hit the correct value with probabilities more than 99% is obtained as a relation between the number of vibrational modes and the depth of the potential energy well of the collision complex.

Steric Effects of Solvent Molecules on SN2 Substitution Dynamics.

Influences of solvent molecules on SN2 reaction dynamics of microsolvated F-(H2O)n with CH3I, for n = 0-3, are uncovered by direct chemical dynamics simulations. The direct substitution mechanism, which is important without microsolvation, is quenched dramatically upon increasing hydration. The water molecules tend to force reactive encounters to proceed through the prereaction collision complex leading to indirect reaction. In contrast to F-(H2O), reaction with higher hydrated ions shows a strong propensity for ion desolvation in the entrance channel, diminishing steric hindrance for nucleophilic attack. Thus, nucleophilic substitution avoids the potential energy barrier with all of the solvent molecules intact and instead occurs through the less solvated barrier, which is energetically unexpected because the former barrier has a lower energy. The work presented here reveals a trade-off between reaction energetics and steric effects, with the latter found to be crucial in understanding how hydration influences microsolvated SN2 dynamics.

Singlet Oxygen Production in the Caudodorsal Medulla of Rat Brain Slices Increases During Hyperoxia at Normobaric Pressure and Following Decompression from Iso‐oxic Hyperbaric Nitrogen

Previous studies in brain slices harvested from the medulla oblongata of Sprague‐Dawley (SD) rats indicate that neurons in the caudal solitary complex (cSC) are stimulated by normobaric hyperoxia (NBO2) and hyperbaric O2 (HBO2). We've also reported that during NBO2, cSC cells produce a variety of reactive O2 and N2 species, including superoxide, nitric oxide, peroxynitrite, hydrogen peroxide, hydroxyl radicals, carbonate radicals, & nitrogen dioxide. These findings support the premise that cSC neurons employ redox signaling mechanisms that are activated by increasing pO2. Presumably, this explains, in part, the paradoxical stimulation of breathing by prolonged exposures to NBO2 and HBO2. We have tested the hypothesis that singlet oxygen (1O2) production increases in cSC cells exposed to a) NBO2 in the presence of nominal N2 and b) iso‐oxic hyperbaric nitrogen (HBN2). In the case of HBN2, we wanted to determine if inert gas molecules react with O2 molecules at pressure to form collision complexes that yield 1O2. The fluorogenic dye, Singlet Oxygen Sensor Green (SOSG, 2.5μM), was used to measure SOSG intensity of cSC neurons (P16–40 M/F SD rats, 400μm). Fluorescent images were captured at 1–2.5 min intervals and correlated with 1O2 production using one of two Nikon TE2000 inverted microscopes; one microscope was automated for use inside a hyperbaric chamber. The HCO3−‐buffered nutrient medium was equilibrated at room pressure with either (in atmospheres absolute, ATA) 0.4 O2 + 0.05 CO2 + 0.55 N2 (control) or 0.95 O2 + 0.05 CO2 + “0.0” N2 (NBO2).* For iso‐oxic HBN2, the air‐filled hyperbaric chamber (0.8 ATA N2) was compressed (0.25 ATA/min) with 100% N2 to 80–85 psig (5.4–5.8 ATA N2), thereby increasing atmospheric pN2 to 6.2–6.6 ATA (0.95 O2 + CO2 in the nutrient medium; pN2 increases in the medium and slice from HBN2 diffusing in from the overlying atmosphere). Nutrient media were delivered to the brain slice at 1.5 ml/min using an HPLC pump (30–31°C). Results show that increasing pO2 from 0.4 to 0.95 ATA at room pressure increased the rate of 1O2 fluorescence. 1–4 mM ascorbate, but not <1mM, decreased the rate of 1O2 fluorescence in 0.4 ATA O2 and NBO2 in a dose‐dependent manner. Exposure to iso‐oxic (0.95 ATA O2) HBN2 (10–30 min) did not increase 1O2; however, decompression (0.5ATA/min) back to sea level induced a post‐dive increase in 1O2 fluorescence that persisted for 10–30 min. The post‐dive increase in 1O2 fluorescence was inhibited by 4mM ascorbate. These findings suggest that 1O2 production is activated in cSC cells by hyperoxia, compression to at least 6.2 ATA N2, decompression, or the combination of compression/decompression in hyperoxic HBN2. We postulate that 1O2 generated in cSC neurons may contribute, in part, to CNS O2 toxicity, inert gas narcosis, and decompression sickness.Support or Funding InformationONR N000141512572 & N000141612537

Effect of internal excitations of reagent diatom on initial state-selected dynamics of C + OH reaction on its second excited (14A″) electronic state

ABSTRACTInitial state-selected total reaction probabilities and integral reaction cross sections (ICSs) of, C(3P) + OH (X2Π) → CO(a3Π) + H (2S), reaction on its second excited electronic state (14A″) are calculated with the aid of a time-dependent wave packet propagation method within the coupled states approximation. Partial wave contributions for the total angular momentum quantum number, J=0–48, were necessary to obtain converged ICSs for OH (v=0, j=0) upto a collision energy of ∼0.25 eV. In case of rotationally and vibrationally excited OH, many more partial wave contributions had to be included. Dense oscillatory structures are found in total reaction probabilities due to the formation of quasibound collision complexes inside the wells (of depth ∼2.25 eV and ∼1.85 eV) present on the potential energy surface. While reagent vibrational excitation promotes the reaction, no such general trend is found with rotational excitation. The results of the present study are compared with the literature data. Mechanistic details of the C + OH reaction occurring on its ground, first and second excited electronic states are compared and discussed.

Nonadiabatic Renner-Teller quantum dynamics of OH(X2Π) + H+ reactive collisions.

Following previous studies on the O(3P) + H2+(X2Σg+) collisions, we present the nonadiabatic quantum dynamics of the reactions OH(X2Π) + H'+ → OH'(X2Π) + H+, exchange (e), → OH+(X3Σ-) + H'(2S), quenching (q), and → OH'+ (X3Σ-) + H(2S), exchange-quenching (eq). The reactants and products correlate via the ground X[combining tilde]2A'' and first excited Ã2A' electronic states of OH2+, which are the degenerate components of linear 2Π species. Therefore, they are strongly perturbed by nonadiabatic Renner-Teller (RT) effects, opening the (q) and (eq) channels that are closed in the Born-Oppenheimer approximation. Using accurate potential energy surfaces (PESs) and RT matrix elements, initial-state-resolved reaction probabilities, real-time dynamics, cross sections, and rate constants of the product channels are obtained through the time-dependent real wavepacket (WP) method and full coupled-channel calculations. Owing to the nonadiabatic couplings, the WP jumps from the excited Ã2A' surface to the X[combining tilde]2A'' ground PES, avoiding any barrier, opening the quenching channels, and giving many collision complexes into the deep minima of both PESs, as it is clearly shown by the oscillations of the reaction probabilities and by the time-dependent WP dynamics. All the results show that the nonadiabatic-RT channels (q) and (eq) are highly reactive, much more than the adiabatic one (e), pointing out large RT effects. The reactivity of the quenching channels is similar, accounting for 97% of the overall reactivity. In fact, the maximum values of the (q) and (eq) cross sections σq and σeq are equal to 31.6 Å2, whereas the maximum σe value equals 1.34 Å2, and the maximum values of the rate constants kq, keq, and ke are 2.07 × 10-10, 2.45 × 10-10, and 0.23 × 10-10 cm3 s-1. Some calculations show that the centrifugal-sudden and the truncated coupled-channel approximations cannot be employed for the (q) channel. After a sharp increase at the threshold, σq and σeq decrease at larger collision energies while σe and the rate constants depend slightly on the collision energy and temperature, respectively. These findings are consistent with the barrierless nature of both PESs and the exoergicity of the quenching products, with the small role played by the centrifugal and RT barriers in the reactant channel, and with the large RT couplings in the OH2+ intermediates. Finally, we contrast the present results with those for the opposite reactions O + H2+ and for the nonadiabatic-induced quenchings NH + H' and OH + H'.

Photochemistry and Spectroscopy of Singlet Oxygen in Solvents. Recent Advances which Support the Old Theory

Molecular oxygen is a paramagnetic gas with the triplet O2( ) ground state which exhibits just sluggish chemical reactivity in the absence of radical sources. In contrast, the excited metastable singlet oxygen O2( ) is highly reactive; it can oxygenate organic molecules in a wide range of specific reactions which differ from those of the usual triplet oxygen of the air. This makes the singlet oxygen an attractive reagent for new synthesis and even for medical treatments in photodynamic therapy. As an important intermediate O2( ) has attracted great attention of chemists during half-century studies of its reactivity and spectroscopy, but unusual properties of singlet oxygen makes it difficult to unravel all mysterious features. The semiempirical theory of spin-orbit coupling in dioxygen and in collision complexes of O2 with diamagnetic molecules proposed in 1982 year has explained and predicted many photochemical and spectral properties of dioxygen produced by the dye sensitization in solvents. Recent experiments with direct laser excitation of O2 in solvents provide a complete support of the old theory. The present review scrutinizes the whole story of development and experimental verification of this theory.

An Empirical Dynamical Barrier for Statistical Theory of Low-Energy Reactive S(1D) + HD(j = 0), H2(j = 0) Collisions.

A simple model potential is proposed to describe the dynamical barrier in the mean interaction potential at small distances between the reactants in S(1D) + HD(1Σ, v = 0, j = 0) reaction. The statistical theory of collision complex formation and complex decay is applied to calculate the total reaction cross sections and the cross sections for SH and SD productions in the range of low collision energies Ec = (0.4-60) meV. The results are compared with measured cross sections and results of hyperspherical close coupling calculations. As a check of consistency the same comparisons are presented for the case of S(1D) + H2(1Σ, v = 0, j = 0) reaction.

Gas-Phase Synthesis of the Elusive Trisilicontetrahydride Species (Si3H4).

The bimolecular gas-phase reaction of ground-state atomic silicon (Si; 3P) with disilane (Si2H6; 1A1g) was explored under single-collision conditions in a crossed molecular beam machine at a collision energy of 21 kJ mol-1. Combined with electronic structure calculations, the results suggest the formation of Si3H4 isomer(s) along with molecular hydrogen via indirect scattering dynamics through Si3H6 collision complex(es) and intersystem crossing from the triplet to the singlet surface. The nonadiabatic reaction dynamics can synthesize the energetically accessible singlet Si3H4 isomers in overall exoergic reaction(s) (-93 ± 21 kJ mol-1). All reasonable reaction products are either cyclic or hydrogen-bridged suggesting extensive isomerization processes from the reactants via the initially formed collision complex(es) to the fragmenting singlet intermediate(s). The underlying chemical dynamics of the silicon-disilane reaction are quite distinct from the isovalent carbon-ethane system that does not depict any reactivity at all, and open the door for an unconventional gas phase synthesis of hitherto elusive organosilicon molecules under single-collision conditions.

Theoretical investigation of rotationally inelastic collisions of CH(X2Π) with molecular hydrogen.

We report calculations of state-to-state cross sections for collision-induced rotational transitions of CH(X2Π) with molecular hydrogen. These calculations employed the diabatic matrix elements of the interaction potential determined by Dagdigian [J. Chem. Phys. 145, 114301 (2016)], which employed the multi-reference configuration-interaction method [MRCISD+Q(Davidson)]. Because of the presence of a deep well on the lower potential energy surface, the scattering calculations were carried out using the quantum statistical method of Manolopoulos and co-workers [Chem. Phys. Lett. 343, 356 (2001)]. The computed cross sections included contributions from direct scattering, as well as from the formation and decay of a collision complex. The magnitude of latter contribution was found to decrease significantly with increasing collision energy. Rotationally energy transfer rate constants were computed for this system since these are required for astrochemical modeling.