HeH+ Collisions with H2: Rotationally Inelastic Cross Sections and Rate Coefficients from Quantum Dynamics at Interstellar Temperatures.

We report for the first time an accurate ab initio potential energy surface for the HeH+-H2 system in four dimensions (4D) treating both diatomic species as rigid rotors. The computed ab initio potential energy point values are fitted using an artificial neural network method and used in quantum close coupling calculations for different initial states of both rotors, in their ground electronic states, over a range of collision energies. The state-to-state cross section results are used to compute the rate coefficients over a range of temperatures relevant to interstellar conditions. By comparing the four dimensional quantum results with those obtained by a reduced-dimensions approach that treats the H2 molecule as an averaged, nonrotating target, it is shown that the reduced dimensionality results are in good accord with the four dimensional results as long as the HeH+ molecule is not initially rotationally excited. By further comparing the present rate coefficients with those for HeH+-H and for HeH+-He, we demonstrate that H2 molecules are the most effective collision partners in inducing rotational excitation in HeH+ cation at interstellar temperatures. The rotationally inelastic rates involving o-H2 and p-H2 excitations are also obtained and they turn out to be, as in previous systems, orders of magnitude smaller than those involving the cation. The results for the H2 molecular partner clearly indicate its large energy-transfer efficiency to the HeH+ system, thereby confirming its expected importance within the kinetics networks involving HeH+ in interstellar environments.

Efficiency of rovibrational cooling of HeH+ by collisions with He: Cross sections and rate coefficients from quantum dynamics.

By extending an earlier study [Gianturco et al., J. Chem. Phys. 154, 054311 (2021)] on the purely rotational excitation of HeH+ by He atoms, we report in this paper integral cross sections and rate coefficients for rovibrational excitation and de-excitation processes in HeH+ due to collisions with He. The data were obtained using a new ab initio potential energy surface that includes the vibrational degree of freedom. The results are compared with those computed using the earlier potential energy surface by Panda and Sathyamurthy [J. Phys. Chem. A 107, 7125 (2003)] that additionally accounts for the proton-exchange reaction between HeH+ and He. It is shown that the exchange channel contributes nearly as much as the inelastic channel to the vibrational excitation and de-excitation processes and that the total rate constants pertaining to the purely inelastic processes are largely of the same magnitude as those obtained when both inelastic and reactive channels are included in the dynamics. The inelastic rovibrational rate coefficients involving this astrophysical cation are also found to be much larger than those obtained for anions present in similar interstellar environments.

Dynamics of HeHHe+ Rotational State Changes Induced by Collision with He: A Possible New Path in Early Universe Chemistry.

Ab initio calculations are employed to generate the rigid rotor (RR) potential energy surface (PES) describing the interaction of the linear molecular cation HeHHe+, at its equilibrium geometry, with the neutral He atom. The resulting interaction is employed to investigate the efficiency of rotational state-changing collisions at the temperatures relevant to the early universe conditions, where the latter molecule has been postulated to exist, albeit not yet observed. The inelastic rate coefficients are found to be fairly large and are compared with those found for another important cation just recently observed in the interstellar medium: the HeH+ polar molecule. The possibility for this cation to provide new options to energy dissipation routes under early universe conditions after the recombination era is briefly discussed.

Energy-transfer quantum dynamics of HeH+ with He atoms: Rotationally inelastic cross sections and rate coefficients.

Two different ab initio potential energy surfaces are employed to investigate the efficiency of the rotational excitation channels for the polar molecular ion HeH+ interacting with He atoms. We further use them to investigate the quantum dynamics of both the proton-exchange reaction and the purely rotational inelastic collisions over a broad range of temperatures. In current modeling studies, this cation is considered to be one of the possible cooling sources under early universe conditions after the recombination era and has recently been found to exist in the interstellar medium. The results from the present calculations are able to show the large efficiency of the state-changing channels involving rotational states of this cation. In fact, we find them to be similar in size and behavior to the inelastic and reaction rate coefficients obtained in previous studies, where H atoms were employed as projectiles. The same rotational excitation processes, occurring when free electrons are the collision partners of this cation, are also compared with the present findings. The relative importance of the reactive, proton-exchange channel and the purely inelastic channels is also analyzed and discussed. The rotational de-excitation processes are also investigated for the cooling kinetics of the present cation under cold trap conditions with He as the buffer gas. The implications of the present results for setting up more comprehensive numerical models to describe the chemical evolution networks in different environments are briefly discussed.

Solvation of coronene oligomers by para-H2 molecules: the effects of size and shape.

The stepwise solvation of various cationic coronene oligomers by para-hydrogen (p-H2) molecules was computationally investigated using a united-atom model for the p-H2 molecules and the Silvera-Goldman potential, together with a polarizable description for the interaction with the hydrocarbon molecules. A survey of the energy landscape for oligomers containing between 1 and 4 coronene molecules and possible different conformers was carried out using standard global optimization, the hydrocarbon complex being kept as rigid. The most stable structures provided the starting configuration of systematic path-integral molecular dynamics simulations at 2 K. The variations of the geometric and energetic properties of the solvation shell were determined with increasing number of para-hydrogen molecules. The relative stability of the solvation shell is generally found to be more robustly determined by the energy increment (or dissociation energy) than by geometrical indicators, especially when the oligomers have less ordered structures. In agreement with recent mass spectrometry experiments, the size at which the first solvation shell is complete is found to vary approximately linearly with the oligomer size when the coronene molecules stack together, with a slope that is related to the offset between two successive molecules.

On the Formation of Interstellar CH- Anions: Exploring Mechanism and Rates for CH2 Reacting with H.

We present accurate ab initio calculations on the structural properties of a gas-phase reaction of possible interest for Saturn's outer atmosphere chemistry, in which the CH2 molecule has been detected. In the present study, that molecule is made to react with the H- anion to form the CH- species, one considered as a possible intermediate in ionic processes networks. The results indicate that this reaction is markedly exothermic and proceeds with the formation of an intermediate, which occurs via only a shallow barrier below the reagents and progresses directly to the product region. The corresponding rate coefficients of reactions are also computed by making use of the variational transition state theory modeling and found to efficiently lead to the formation of the final anion even at the lower temperatures of interstellar medium conditions.

Modeling Ionic Reactions at Interstellar Temperatures: The Case of NH2- + H2 ⇔ NH3 + H.

Structural features and enthalpy details are presented for the title reactions, both for the exothermic (forward) path to NH3 formation and for the endothermic (reverse) reaction to NH2- formation. Both pathways have relevance for the nitrogen chemistry in the interstellar medium (ISM). They are also helpful to document the possible role of H- in molecular clouds at temperatures well below room temperature. The structural calculations are carried out using different ab initio methods and are further employed to obtain the reaction rates down to the interstellar temperatures detected in earlier experiments. The reaction rates are obtained from the computed minimum energy path (MEP) using the variational transition-state theory (VTST) approach. The results indicate very good accord with experiment results at room temperature, while measured low temperature data down to 8 K are well described using an appropriately modified VTST approach. This is done by employing a temperature-dependent scaling, from room temperature conditions down to the lower ISM temperatures, which acknowledges the noncanonical behavior of the fast, barrierless exothermic reaction. The reasons for this behavior and the need for improving on the VTST method when used away from room temperatures are discussed.

N2+(2Σg) and Rb(2S) in a hybrid trap: modeling ion losses from radiative association paths.

By employing ab initio computed intermolecular potential energy surfaces we calculate the radiative association probabilities and rates for two different associative mechanisms involving trapped molecular ions N2+(2Σg) interacting either directly with ultracold Rb atoms or undergoing charge-exchange (CE) processes leading to the formation of complexes of the strongly exothermic products N2(X1Σg) plus Rb+(1S0). The two processes are expected to provide possible paths to ion losses in the trap within the timescale of experiments. The present calculations suggest that the associative rates for the 'vibrational' direct process are too small to be of any significant importance at the millikelvin temperatures considered in the experiments, while the 'vibronic' path into radiatively associating the CE products has a probability of occurring which is several orders of magnitude larger. However the reaction rate constants attributed to non-adiabatic CE [F. H. J. Hall and S. Willist, Phys. Rev. Lett., 2012, 109, 233202] are in turn several orders of magnitude larger than the radiative ones calculated here, thereby making the primary experimental process substantially unaffected by the radiative losses channel.

Quantum Chemical View on the Growth Mechanisms of Odd-Sized Nitrogen Cluster Anions.

The stable structures of odd-numbered anionic nitrogen clusters, N2 n+3-, have been theoretically investigated in the size range n = 1-9 using a variety of quantum chemistry methods that include perturbation theory, coupled cluster, and density-functional theory with different exchange-correlation functionals. We generally find that the clusters are composed of an azide chromophore N3- surrounded by essentially neutral nitrogen molecules. The growth initially proceeds by placing the neutral molecules parallel to the azide anion, completing a first shell at N13-, above which the extra molecules arrange on the side but with a significantly lower binding energy. Comparison with the cyclic N5- anionic core shows that the latter is unfavorable, the spectral signatures of both N5- and N2N3- being provided in both the infrared and ultraviolet ranges. The trend of these clusters to be highly stable as (N2) nN3- agrees with recent mass spectrometry experiments under the cryogenic environment of helium droplets. The issues associated with the successful development of a nonreactive force field for such clusters are also highlighted.

The quantum structure of anionic hydrogen clusters.

A flexible and polarizable interatomic potential has been developed to model hydrogen clusters interacting with one hydrogen anion, (H2)nH-, in a broad range of sizes n = 1-54 and parametrized against coupled cluster quantum chemical calculations. Using path-integral molecular dynamics simulations at 1 K initiated from the putative classical global minima, the equilibrium structures are found to generally rely on icosahedral shells with the hydrogen molecules pointing toward the anion, producing geometric magic numbers at sizes n = 12, 32, and 44 that are in agreement with recent mass spectrometry measurements. The energetic stability of the clusters is also connected with the extent of vibrational delocalization, measured here by the fluctuations among inherent structures hidden in the vibrational wave function. As the clusters grow, the outer molecules become increasingly free to rotate, and strong finite size effects are also found between magic numbers, associated with more prominent vibrational delocalization. The effective icosahedral structure of the 44-molecule cluster is found to originate from quantum nuclear effects as well, the classical structure showing no particular symmetry.

Physisorption of H2 on Fullerenes and the Solvation of C60 by Hydrogen Clusters at Finite Temperature: A Theoretical Assessment.

The interaction between hydrogen and carbonaceous nanostructures is of fundamental interest in various areas of physical chemistry. In this contribution we have revisited the physisorption of hydrogen molecules and H2 clusters on fullerenes, following a first-principles approach in which the interaction is quantitatively evaluated for the C20 system using high-level electronic structure methods. Relative to coupled cluster data at the level of single, double, and perturbative triple excitations taken as a benchmark, the results for rotationally averaged physisorbed H2 show a good performance of MP2 variants and symmetry-adapted perturbation theory, but significant deviations and basis set convergence issues are found for dispersion-corrected density functional theory. These electronic structure data are fitted to produce effective coarse-grained potentials for use in larger systems such as C60-H2. Using path-integral molecular dynamics, the potentials are also applied to parahydrogen clusters solvated around fullerenes, across the regime where the first solvation shell becomes complete and as a function of increasing temperature. For C60 our findings indicate a sensible dependence of the critical solvation size on the underlying potential. As the temperature is increased, a competition is found between the surface and radial expansions of the solvation shell, with one molecule popping away at intermediate temperatures but getting reinserted at even higher temperatures.

Solvation of carbonaceous molecules by para-H2 and ortho-D2 clusters. II. Fullerenes.

The coating of various fullerenes by para-hydrogen and ortho-deuterium molecules has been computationally studied as a function of the solvent amount. Rotationally averaged interaction potentials for structureless hydrogen molecules are employed to model their interaction with neutral or charged carbonaceous dopants containing between 20 and 240 atoms, occasionally comparing different fullerenes having the same size but different shapes. The solvation energy and the size of the first solvation shell obtained from path-integral molecular dynamics simulations at 2 K show only minor influence on the dopant charge and on the possible deuteration of the solvent, although the shell size is largest for ortho-D2 coating cationic fullerenes. Nontrivial finite size effects have been found with the shell size varying non-monotonically close to its completion limit. For fullerenes embedded in large hydrogen clusters, the shell size and solvation energy both follow linear scaling with the fullerene size. The shell sizes obtained for C60 (+) and C70 (+) are close to 49 and 51, respectively, and agree with mass spectrometry experiments.

Solvation of carbonaceous molecules by para-H2 and ortho-D2 clusters. I. Polycyclic aromatic hydrocarbons.

This work theoretically examines the progressive coating of planar polycyclic aromatic hydrocarbon (PAH) molecules ranging from benzene to circumcoronene (C54H18) by para-hydrogen and ortho-deuterium. The coarse-grained Silvera-Goldman potential has been extended to model the interactions between hydrogen molecules and individual atoms of the PAH and parametrized against quantum chemical calculations for benzene-H2. Path-integral molecular dynamics simulations at 2 K were performed for increasingly large amounts of hydrogen coating the PAH up to the first solvation shell and beyond. From the simulations, various properties were determined such as the size of the first shell and its thickness as well as the solvation energy. The degree of delocalization was notably quantified from an energy landscape perspective, by monitoring the fluctuations among inherent structures sampled by the trajectories. Our results generally demonstrate a high degree of localization owing to relatively strong interactions between hydrogen and the PAH, and qualitatively minor isotopic effects. In the limit of large hydrogen amounts, the shell size and solvation energy both follow approximate linear relations with the numbers of carbon and hydrogen in the PAH.

Possible Formation of Metastable PAH Dimers upon Pickup by Helium Droplets.

Using path-integral molecular dynamics simulations and two quantum-mechanical-based force fields, we have investigated the conformational stability of dimers of a polycyclic aromatic hydrocarbon, perylene (C20H12), produced under typical experimental conditions of successive pick-up under helium nanodroplet environment. The most stable configurations are found to be of the stacked form with different relative orientations of the main molecular axes, perpendicular or T-shaped dimers being energetically much disfavored; however, in the presence of helium our simulations suggest that the time for rearrangement and π-stacking may be rather long and exceed hundreds of picoseconds. In addition, highly metastable dimers that are stacked but with a helium monolayer sandwiched between the two molecules are also found as likely products upon successive pickup. This stabilization occurs owing to the stronger localization of the helium atoms facing the aromatic rings, which is further enhanced in the dimer. The implications of the present results are discussed in the perspective of possible identification by spectroscopic methods.

Forming NCO(-) in Dense Molecular Clouds: Possible Gas-Phase Chemical Paths From Quantum Calculations.

The existence of NCO(-) anions in the interstellar medium (ISM) has been suggested and searched for over the years but without any formal definitive sighting of that molecule. We discuss in this work the possible formation of either NCO(-) directly or of NCO neutral as a precursor to NCO(-) formation by electron attachment. We follow simple, gas-phase chemical reactions for which the general features are obtained from accurate quantum calculations. The results are shedding some additional light on the likely presence of this anion in the ISM environment, drawing further information from the specific features of the considered reactions on the additional chemical options that exist for its formation.

Evidence for broken ergodicity due to chemical alloying from the dissociation kinetics of binary clusters.

The interplay between thermal relaxation and statistical dissociation in binary Morse clusters (AB)N has been investigated using numerical simulations and simple statistical approaches, for a variety of interaction parameters covering miscible and non-miscible regimes. While all clusters exhibit a core/shell phase separation pattern in their most stable, T = 0 structure, different melting mechanisms are identified depending on the ranges and their mismatch, including two-step melting of the surface and the core or premelting as alloying. The preference for emitting A or B particles upon evaporation has been evaluated assuming that the cluster is either thermally equilibrated or vibrationally excited in its ground state structure, and compared to the predictions of the Weisskopf theory. The variations of the dissociation rate constants with increasing energy and the branching ratio between the two channels show significant differences in both cases, especially when the clusters are miscible and bound by short-range forces, which indicates that the time scale for evaporation is much shorter than the equilibration time. Our results suggest that dissociation properties could be used to test the ergodic hypothesis in such compounds.

Weakly bound finite systems: (4He)N-Rb2(³Σu), clustering structures from a quantum Monte Carlo approach.

We report here ((4)He)(N)-Rb(2)((3)Σ(u)) complexes, 2 ≤ N ≤ 20, analysed through a quantum diffusion Monte Carlo stochastic approach. The calculations show that the spin stretched dimer molecule is bound outside the pure He sub-complex, due to the stronger He-He potential as compared with the He-Rb(2) interaction, while the rare gas atom moiety presents, in turn, a shell-like structure with ten He adatoms completing the first shell. Our results agree with previous findings on this and similarly weakly interacting systems.

Spin-polarized Rb2 interacting with bosonic He atoms: potential energy surface and quantum structures of small clusters.

A new full-dimension potential energy surface of the three-body He-Rb2(³Σ(u)(+)) complex and a quantum study of small (4He)(N)-Rb2(³Σ(u)(+)) clusters, 1 ≤ N ≤ 4, are presented. We have accurately fitted the ab initio points of the interaction to an analytical form and addressed the dopant's vibration, which is found to be negligible. A Variational approach and a Diffusion Monte Carlo technique have been applied to yield energy and geometric properties of the selected species. Our quantum structure calculations show a transition in the arrangements of the helium atoms from N = 2, where they tend to be separated across the diatomic bond, to N = 4, in which a closer packing of the rare gas particles is reached, guided by the dominance of the He-He potential over the weaker interaction of the latter adatoms with the doping dimer. The deepest well of the He-Rb2 interaction is placed at the T-shape configuration, a feature which causes the dopant to be located as parallel to the helium "minidroplet". Our results are shown to agree with previous findings on this and on similar systems.

Energy landscapes of ion clusters in isotropic quadrupolar and octupolar traps.

The energy landscapes of ion clouds confined in isotropic quadrupolar and octupolar traps are characterized for several representative cluster sizes. All clusters exhibit stable multishell structures that belong to separate funnels. Quadrupolar confinement leads to more homogeneous clusters and denser distributions of isomers than octupolar confinement. Statistical analysis of the transition states indicates that the barriers associated with intrashell motion are lower but more asymmetric and more cooperative compared to intershell motion. The relaxation between low-energy funnels with different arrangements of shells mostly exhibits Arrhenius kinetics, with a weak variation of the activation energy at higher temperatures.