Search

The anions C |$_7$| N |$^-$| and C |$_{10}$| H |$^-$| are the two longest of the linear (C,N)-bearing and (C,H)-bearing chains that have so far been detected in the interstellar medium (ISM). In order to glean information on their collision-induced rotational state-changing processes, we analyse the general features of new ab initio potentials describing the interaction of both linear anions with H |$_2$|⁠ , one of the most abundant partners in their ISM environment. We employ an artificial neural network fit of the reduced-dimensionality potential energy surface for C |$_7$| N |$^-$|⁠...H |$_2$| interaction and discuss in detail the spatial features in terms of multipolar radial coefficients. For the C |$_{10}$| H |$^-$|⁠...H |$_2$| interaction, we use the initial grid of two-dimensional raw points to generate by quadrature the Legendre expansion directly, further including the long-range terms as discussed in the main text. Quantum scattering calculations are employed to obtain rotationally inelastic cross-sections, for collision energies in the range of 10 |$^{-4}$| to 400 cm |$^{-1}$|⁠. From them we generate the corresponding inelastic rate coefficients as a function of temperature covering the range from 10 to 50 K. The results for the rate coefficients for the longest cyanopolyyne are compared with the earlier results obtained for the smaller terms of the same series, also in collision with H |$_2$|⁠. We obtain that the inelastic rate coefficients for the long linear anions are all fairly large compared with the earlier systems. The consequences of such findings on their non-equilibrium rotational populations in interstellar environments are illustrated in our conclusions. [ABSTRACT FROM AUTHOR]

The anions C |$_7$| N |$^-$| and C |$_{10}$| H |$^-$| are the two longest of the linear (C,N)-bearing and (C,H)-bearing chains that have so far been detected in the interstellar medium (ISM). In order to glean information on their collision-induced rotational state-changing processes, we analyse the general features of new ab initio potentials describing the interaction of both linear anions with H |$_2$|⁠ , one of the most abundant partners in their ISM environment. We employ an artificial neural network fit of the reduced-dimensionality potential energy surface for C |$_7$| N |$^-$|⁠...H |$_2$| interaction and discuss in detail the spatial features in terms of multipolar radial coefficients. For the C |$_{10}$| H |$^-$|⁠...H |$_2$| interaction, we use the initial grid of two-dimensional raw points to generate by quadrature the Legendre expansion directly, further including the long-range terms as discussed in the main text. Quantum scattering calculations are employed to obtain rotationally inelastic cross-sections, for collision energies in the range of 10 |$^{-4}$| to 400 cm |$^{-1}$|⁠. From them we generate the corresponding inelastic rate coefficients as a function of temperature covering the range from 10 to 50 K. The results for the rate coefficients for the longest cyanopolyyne are compared with the earlier results obtained for the smaller terms of the same series, also in collision with H |$_2$|⁠. We obtain that the inelastic rate coefficients for the long linear anions are all fairly large compared with the earlier systems. The consequences of such findings on their non-equilibrium rotational populations in interstellar environments are illustrated in our conclusions. [ABSTRACT FROM AUTHOR]

The O(3P) + D2 → OD(2Π) + D reaction presents the peculiarity of taking place on two different potential energy surfaces (PESs) of different symmetry, 3A′ and 3A′′, which become degenerate for collinear configurations where the saddle-point of the reaction is located. The degeneracy is broken for non-collinear approaches with the energy on the 3A′ PES rising more abruptly with the bending angle, making the frequency of this mode higher on the 3A′ state. Consequently, the 3A′ PES should be less reactive than the 3A′′ one. Nevertheless, quantum scattering calculations show that the cross section is higher on the 3A′ PES for energies close to the classical reaction threshold and rotationless reactant. It is found that the differences between the reactivity on the two PESs are greater for low values of total angular momentum, where the centrifugal barrier is lower and contribute to the higher population of the Π(A′) Λ-doublet states of OD at low collision energies. At high collision energies, the Π(A′) Λ-doublet state is also preferentially populated. Analysis of the differential cross sections reveals that the preponderance for the Π(A′) Λ-doublet at low energies comes from backward scattering, originating from the reaction on the 3A′ PES, while at high energies, it proceeds from a different mechanism that leads to sideways scattering on the 3A′′ PES and that populates the Π(A′) manifold. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In spite of being spin-forbidden, some enzymes are capable of catalyzing the incorporation of O2( Σ g − 3 ) to organic substrates without needing any cofactor. It has been established that the process followed by these enzymes starts with the deprotonation of the substrate forming an enolate. In a second stage, the peroxidation of the enolate formation occurs, a process in which the system changes its spin multiplicity from a triplet state to a singlet state. In this article, we study the addition of O2 to enolates using state-of-the-art multi-reference and single-reference methods. Our results confirm that intersystem crossing is promoted by stabilization of the singlet state along the reaction path. When multi-reference methods are used, large active spaces are required, and in this situation, semistochastic heat-bath configuration interaction emerges as a powerful method to study these multi-configurational systems and is in good agreement with PNO-LCCSD(T) when the system is well-represented by a single-configuration. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The anion C5N− is one of the largest linear (C,N)-bearing chains detected in the interstellar medium. Here we present and discuss the general features of new ab initio potential energy surfaces describing the interaction of this linear anion with He and H2. We employ a Legendre Polynomials expansion representation for the former and an artificial neural network fit for the latter. We then carry out quantum scattering calculations to yield rotationally inelastic cross-sections for collisions with He and H2, using relative translational energy values in the range of 0.1–300 cm−1. We then obtained the corresponding inelastic rate coefficients as a function of temperature covering the range from 1 to 100 K. The results for these two systems are compared with each other, as well as with the earlier results on the C3N− colliding with the same partners. We found that the final inelastic rate coefficients for this anion are all fairly large, those from collisions with H2 being the largest. The consequences of such findings on their non-equilibrium rotational populations in interstellar environments are discussed in our conclusions. [ABSTRACT FROM AUTHOR]

Using accurate ab initio calculations of the interaction forces, we employ a quantum mechanical description of the collisional state-changing processes that occur in a cold ion trap with He as a buffer gas. We generate the corresponding inelastic rates for rotational transitions involving three simple molecular anions OH−(1Σ), MgH−(1Σ), and C2H−(1Σ) colliding with the helium atoms of the trap. We show that the rotational constants of these molecular anions are such that within the low-temperature regimes of a cold ion trap (up to about 50 K), a different proportion of molecular states are significantly populated when loading helium as a buffer gas in the trap. By varying the trap operating conditions, population equilibrium at the relevant range of temperatures is reached within different time scales. In the modeling of the photodetachment experiments, we analyze the effects of varying the chosen values for photodetachment rates as well as the laser photon fluxes. Additionally, the changing of the collision dynamics under different buffer gas densities is examined and the best operating conditions, for the different anions, for yielding higher populations of specific rotational states within the ion traps are extracted. The present modeling thus illustrates possible preparation of the trap conditions for carrying out more efficiently state-selected experiments with the trapped anions. [ABSTRACT FROM AUTHOR]

The explicit consideration of the vector correlations is an essential step when it comes to determining the mechanism of chemical reactions. Usual vector correlations involve initial and final relative velocity vectors and rotational angular momenta. However, the correlation between the orbital, l, and rotational, j, angular momenta has seldom received any attention. In this article, we present a semiclassical methodology capable of describing the l-j correlation that may serve as a connection between the quantum and quasiclassical treatments. Using the scattering matrix in the orbital angular momentum representation, the l-j correlation is expressed as a probability density function of the angle formed by both vectors. This technique is exemplified through the H + D2 reaction and its accuracy is appraised by comparing with results derived from calculations based on quasiclassical trajectories. [ABSTRACT FROM AUTHOR]

A newly calculated ab initio potential energy surface is used to compute collision-driven state-changing cross sections and rate coefficients over a range from 5 to 100 K for CN−(1Σ), the smallest anion detected in the interstellar medium, interacting with He, an abundant species in this environment. We compare our presently computed rate coefficients with those previously published for the similar and important systems CN–He, CN-H2, and CN−–H2 to illustrate the broader network of inelastic, state-changing processes for these four systems. We also discuss the size-scaling effects that occur when changing partners from He to H2. We further analyze the differences in size between collision-driven rate coefficients when going from neutral CN to its anion. All the present results are discussed in detail, to provide accurate and realistic data for chemical networks that wish to include the CN− anion in their modeling of astrochemical environments. [ABSTRACT FROM AUTHOR]

While Cl + H2 reactive collisions have been a subject of numerous experimental and theoretical studies, inelastic collisions leading to rotational energy transfer and/or vibrational excitation have been largely ignored. In this work, extensive quantum mechanical calculations covering the 0.5-1.5 eV total energy range and various initial rovibrational states have been carried out and used to perform a joint study of inelastic and reactive Cl + H2 collisions. Quasiclassical trajectories calculations complement the quantum mechanical results. The analysis of the inelastic transition probabilities has revealed the existence of two distinct dynamical regimes that correlate with low and high impact parameters, b, and are neatly separated by glory scattering. It has been found that while high-b collisions are mainly responsible for |Δj| = 2 transitions which dominate the inelastic scattering, they are very inefficient in promoting higher |Δj| transitions. The effectiveness of this type of collision also drops with rotational excitation of H2. In contrast, reactive scattering, that competes with |Δj| > 2 inelastic transitions, is exclusively caused by low-b collisions, and it is greatly favored when the reactants get rotationally excited. Previous studies focusing on the reactivity of the Cl + H2 system established that the van der Waals well located in the entrance channel play a key role in determining the mechanism of the collisions. Our results prove this to be also a case for inelastic processes, where the origin of the double dynamical regime can be traced back to the influence exerted by this well that shapes the topology of the entrance channel of the Cl-H2 system. [ABSTRACT FROM AUTHOR]

Quantum coupled-channel scattering calculations have been carried out at ultralow energies (down to 10-5 K) for rotational quenching of ionic and spin-stretched states of the lithium dimer in collision with He atoms. Marked cross section differences, to be related to changes in their interactions with He, have been observed with respect to the singlet case while little changes in the collisional behavior are seen to occur upon ionization when the spin-stretched target is considered. Both effects originate from an interplay of structural and dynamical features of the colliding partners which are analyzed in detail. [ABSTRACT FROM AUTHOR]