The role of small molecular cations in the chemical flow of the interstellar environments.

Molecular ions have been ubiquitous in a variety of environments in the interstellar medium, from Circumstellar Envelopes to Dark Molecular Clouds and to Diffuse Clouds. Their role in the multitude of molecular processes which have been found to occur in those environments has been the subject of many studies over the years, so that we have acquired by now a complex body of data on their chemical structures, their possible function within chemical reactions and their most likely paths to formation. In the present work we review a broad range of such molecular ions, focusing exclusively on positive ions involving the smallest and simplest cations which have been either detected or conjectured as present in the interstellar medium (ISM). We therefore consider mainly molecular cations formed with components like H, H+, He and He+, atomic species which are by far the most abundant baryons in the ISM in general. Their likely structures and their roles in a variety of chemical energy flow paths are discussed and presented within the context of their interstellar environments.

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.

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.

Study of Topological Effects Concerning the Lowest A″ and the Three A' States for the CO2(+) Ion.

A study of the topological effects, viz., the Jahn-Teller (JT) and Renner-Teller (RT) effects, in CO2(+) has been carried out by calculating nonadiabatic coupling terms (NACTs) at the state-averaged CASSCF level using the cc-pVTZ basis set for the lowest three A' states and one A″ state along a circular contour. Using the NACTs, the privileged adiabatic-to-diabatic transformation (ADT) angles (γ12) for 1A' and 2A' states of CO2(+) have been calculated along various circular contours. Employing one of the oxygen atoms as the test particle exposed two conical intersections (ci) located on each side of the CO diatom. The main purpose of this study is to explore the possibility of forming reliable diabatic potential energy surfaces for this system. Success in achieving this goal is guaranteed by the ability to calculate quantized privileged ADT angles along closed contours covering large regions in configuration space (see, e.g., J. Phys. Chem. A 2014 , 118 , 6361 ). The calculations were carried out for two and three JT states. In most cases very nice quantization has been achieved although the calculations were frequently done, as required, for large regions in configuration space (sometimes ≥18 Å(2)). In one case, for which the quantization was not gratifying, the inclusion of the RT effect modified it considerably.

Interpretation of the accidental predissociation of the E(1) Π state of CO.

A special case of predissociation, known as indirect or accidental predissociation observed in the Rydberg E(1)Π bound state of CO is discussed. We resort to ab initio potentials in order to determine the plausible mechanism for this predissociation. Values of the predissociation width for the valence k(3)Π state of CO, as obtained from Fermi's golden rule, are also reported. The predissociation width obtained for the mixed E(1)Π (v = 1, J = 7) state is 0.033 cm(-1) compared to the experimental value of 0.034 cm(-1). The mixed E - E(') state with J = 28, v = 0 is found to be in near resonance condition with the k(3)Π (v = 4, J = 28) state, thus providing the means to indirect predissociation.

Ab initio potential energy curves for the ground and low-lying excited states of OH and OH(-) and a study of rotational fine structure in photodetachment.

Complete basis set extrapolated ab initio potential energy curves obtained from multireference configuration interaction (MRCI) level calculations for the ground state (X(1)Σ(+)) of OH(-), and the ground state (X(2)Π) and the first excited state (A(2)Σ(+)) of OH are reported. The potential energy curves for the excited states A(1)Π, a(3)Π, and b(3)Π of OH(-) have been computed using the V6Z basis set at the MRCI level. Λ-doubling parameters p and q were calculated for the ground and the first excited vibrational states of the ground electronic state of OH using second-order perturbation theory. Using the computed potential energy curves and the rovibrational spectra for photodetachment including the fine splitting, the threshold for electron detachment has been computed. The result is in agreement with the experimental results of Goldfarb et al. [J. Chem. Phys. 1985, 83, 4364].

Ab initio potential energy curves for the ground and low lying excited states of NH(-) and the effect of (2)Σ(±) states on Λ-doubling of the ground state X(2)Π.

Complete basis set extrapolated ab initio potential energy curves obtained from multireference configuration interaction (MRCI) level calculations for the ground state (X(2)Π) and the a(4)Σ(-) state of NH(-) and the ground state (X(3)Σ(-)) of NH are reported. The potential energy curves for the A'(2)Σ(-) and A(2)Σ(+) states of NH(-) have been computed using the V6Z basis set at the MRCI level. Λ-doubling parameters p and q are calculated for the ground and the first excited vibrational states of the ground electronic state of NH(-) using second-orderperturbation theory. The effect of the (2)Σ(+) and (2)Σ(-) states on the Λ-doubling values is discussed. Earlier experiments had not considered the influence of the (2)Σ(-) state on p and q while fitting the spectral data. Using the computed potential energy curves and the ro-vibrational spectra including the fine splitting, we have computed the threshold for electron detachment. The result is in agreement with the experimental values of Neumark et al. [J. Chem. Phys. 1985, 83, 4364] and Farley et al. [Phys. Rev. A 1987, 35, 1099].

Radiative lifetimes of spin forbidden a1Δ â†’ X3Σ- and spin allowed A3Π â†’ X3Σ- transitions and complete basis set extrapolated ab initio potential energy curves for the ground and excited states of CH-.

The spin forbidden transition a(1)Δ â†’ X(3)Σ(-) in CH(-) has been studied using the Breit-Pauli Hamiltonian for a large number of geometries. This transition acquires intensity through spin-orbit coupling with singlet and triplet Π states. The transition moment matrix including more than one singlet and triplet Π states was calculated at the multi-reference configuration interaction/aug-cc-pV6Z level of theory. The computed radiative lifetime of 5.63 s is in good agreement with the experimental (5.9 s) and other theoretical (6.14 s) results. Transition moment values of the spin allowed A(3)Π â†’ X(3)Σ(-) transition have also been calculated at the same level of theory. Calculations show that the corresponding radiative lifetime is considerably low, 2.4 × 10(-7) s. Complete basis set extrapolated potential energy curves for the ground state of CH and the ground state and six low lying excited states (a(1)Δ, b(1)Σ(+), two (3)Π, and two (1)Π) of CH(-) are reported. These curves are then used to calculate the vibrational bound states for CH and CH(-). The computed electron affinity of CH supports the electron affinity bounds reported by Okumura et al. [J. Chem. Phys. 85, 1971 (1986)].

Collision-induced dissociation in (He, H2(+)(v = 0-2; j = 0-3)) system: a time-dependent quantum mechanical investigation.

The collision-induced process He + H(2)(+)(v = 0-2; j = 0-3) → He + H + H(+) has been investigated using a time-dependent quantum mechanical wave packet approach, within the centrifugal sudden approximation. The exchange reaction He + H(2)(+) → HeH(+) + H, which has a lower threshold, dominates over the dissociation process over the entire energy range considered in this study. The reaction cross section for both the exchange and dissociation channels and the branching ratio between the two channels have been computed on the McLaughlin-Thompson-Joseph-Sathyamurthy potential-energy surface and compared with the available experimental and quasiclassical trajectory results.

Theoretical studies of host-guest interaction in gas hydrates.

Ab initio calculations and atoms-in-molecules (AIM) analysis have been used to investigate the host-guest interaction in dodecahedral water cages using a variety of guest species that include monatomic (He, Ne, Ar, Kr, and Xe), diatomic (CO, H(2), N(2), O(2), and NO), triatomic (CO(2), NO(2), and O(3)), and polyatomic (CH(4) and NH(3)) molecules. Geometry optimization for the guest species, host cage, and their complexes was carried out using the second order Møller-Plesset perturbation method with the 6-31G** basis set. Single point energy calculations using the same method but different basis sets (6-31++G**, 6-311++G**, aug-cc-pVDZ, and aug-cc-pVTZ) were carried out for the MP2/6-31G** optimized geometries. The interaction energy between the guest species and the host cage has been obtained in the complete basis set limit by basis set extrapolation.

Stacking and spreading interaction in N-heteroaromatic systems.

π-π interactions in heteroaromatic systems are ubiquitous in biological systems. In the present study, stabilization energies of stacked and hydrogen-bonded dimers of N-heteroaromatic systems (pyridine, pyrazine, sym-triazine, and sym-tetrazine) have been computed using a benchmark quality coupled cluster through the perturbative triples (CCSD(T)) method at the estimated complete basis set (CBS) limit. In the case of stacking, monomer units are found to be stacked in parallel planes with displaced geometries. The stabilization energies for the most stable stacked geometry of pyridine, pyrazine, sym-triazine, and sym-tetrazine dimers are found to be -3.39, -4.14, -4.02, and -3.90 kcal/mol, respectively at the est. CCSD(T)/CBS level of theory, which is clearly larger than the stabilization energy for the most stable geometry of the benzene dimer. In the case of spreading, hydrogen bonded dimers and trimers are stabilized by weak C-H···N interactions. The stabilization energies for the stacked and the spread out complexes are found to be comparable. The stabilization energy for the trimers is computed using the MP2, MP3, and B3LYP-D methods. The present study is aimed at unraveling the basis of preferred conformations of N-heteroaromatic dimers. These model systems explain partly the stability of double helical DNA and RNA structures that are formed by stacking and hydrogen bonding between nucleic acid bases.

Importance of Coriolis Coupling in Isotopic Branching in (He, HD+) collisions.

A three-dimensional time-dependent quantum mechanical wave packet approach is used to calculate the reaction probability (P(R)) and integral reaction cross section values for both channels of the reaction He + HD(+)(v = 1; j = 0) --> HeH (D)(+) + D (H) over a range of translational energy (E(trans)) on the McLaughlin-Thompson-Joseph-Sathyamurthy potential energy surface including the Coriolis coupling (CC) term in the Hamiltonian. The reaction probability plots as a function of translational energy for different J values exhibit several oscillations, which are characteristic of the system. The sigma(R) values obtained by including CC and not including it are nearly the same over the range of E(trans) investigated for the HeD(+) channel. For the HeH(+) channel, on the other hand, sigma(R) values obtained from CC calculations are significantly smaller than those obtained from coupled state calculations. These results are compared with the available experimental results. The computed branching ratios (Gamma(sigma) = sigma(R) (HeH(+))/sigma(R) (HeD(+))) are also compared with the available experimental results.

Structure and stability of water chains (H2O)n, n = 5-20.

The structure and stability of linear (helical) water chains (H2O)n, n = 5-20 as obtained from ab initio/DFT calculations are reported along with an atoms-in-molecules (AIM) analysis of hydrogen bond critical points and their characteristics. The resulting helical chain arrangement is one of the predominant motifs in different host environments; although they may not be the most stable, it is shown that these linear water chain clusters could exist in their own right.

The self-assembly of metaboric acid molecules into bowls, balls and sheets.

The structural motifs responsible for the formation of bowls, balls and sheets of orthoboric acid were pointed out in an earlier publication (Elango et al. J. Phys. Chem. A 2005, 109, 8587). It is shown in the present study that metaboric acid forms similar bowls, balls and sheets, despite the fact that the basic unit for cluster formation is different.

Structure, energetics, and reactivity of boric acid nanotubes: a molecular tailoring approach.

Cardinality guided molecular tailoring approach (CG-MTA) [Ganesh et al. J. Chem. Phys. 2006, 125, 104019] has been effectively employed to perform ab initio calculations for large molecular clusters of boric acid. It is evident from the results that boric acid forms nanotubes, structurally similar to carbon nanotubes, with the help of an extensive hydrogen-bonding (H-bonding) network. Planar rosette-shaped hexamer of boric acid is the smallest repeating unit in such nanotubes. The stability of these tubes increases due to enhancement in the number of H-bonding interactions as the diameter increases. An analysis of molecular electrostatic potential (MESP) of these systems provides interesting features regarding the reactivity of these tubes. It is predicted that due to alternate negative and positive potentials on O and B atoms, respectively, boric acid nanotubes will interact favorably with polar systems such as water and can also form multiwalled tubes.

Hydrogen bonding in protonated water clusters: an atoms-in-molecules perspective.

This article highlights the results of a detailed study of hydrogen bonding in the first and the second solvation shells of Eigen (H3O+) and Zundel (H5O2+) cations solvated by water in a stepwise manner. It is evident from the results that an electron density analysis clearly distinguishes the first and the second solvation shell and helps in quantifying the strength of hydrogen bonding in these clusters.

Time-dependent density functional theoretical study of the absorption properties of BN-substituted C60 fullerenes.

Time-dependent density functional theoretical calculations using the B3LYP functional and 6-31G* basis set for a series of BN-substituted C60 fullerenes reveal that, unlike C60, these molecules would absorb in the visible region and that the optical and electronic properties of fullerenes can be fine-tuned with proper BN substitution.

Dissociative double ionization of CO2: dynamics, energy levels, and lifetime.

In a kinematically complete experiment on the dissociative double ionization of CO2 by electron impact, spontaneous and metastable decay have been observed via the channel CO2(2+) --> CO+ + O+. The metastable decay shows a lifetime of 5.8 +/- 1.5 micros. The measured kinetic energy release spectrum of the dissociation shows one broad peak. To understand the observed features, ab initio potential energy surface (PES) for the ground electronic state of CO2(2+) was computed using a multireference configuration interaction method and a correlation-consistent polarized-valence quadruple-zeta basis set, for a range of internuclear distances and O-C-O bond angles, and an analytic fit of the PES was obtained. The computed PES clearly indicates the metastability of the dication and yields a barrier height and an asymptotic limit in fair agreement with the reported data. A time-dependent quantum mechanical approach was used to compute the ground vibrational state wave function of CO2 in its ground electronic state. Assuming a Franck-Condon transition, the same function was taken to be the initial wave function at time t = 0 for the time evolution on the fitted PES for the ground electronic state of CO2(2+). The autocorrelation function was computed and Fourier transformed to obtain the excitation spectrum. Upon convolution with the instrument resolution function, the kinetic energy release spectrum was obtained, in good agreement with the experimental results, particularly at lower energies. The discrepancies at higher energies are attributed to the noninclusion of the excited states of CO2(2+) in the dynamical study.

Van der Waals complexes of small molecules with benzenoid rings: influence of multipole moments on their mutual orientation.

Intermolecular interaction between some small molecules (HF, H2O, NH3, and CH4) and certain benzenoid ring systems (benzene, hexafluorobenzene, and 1,3,5-trifluorobenzene) has been investigated in detail at MP2 level of theory using 6-311++G** basis set, and the results are corrected for basis set superposition error (BSSE). Vibrational frequencies were calculated for all the geometries at the same level of theory and basis sets to ensure that the geometries obtained correspond to true minima. In the complexes with benzene, which has a large negative quadrupole moment, the preferred geometry has the electropositive end of the small molecule (HF, H2O, and NH3) pointing toward the ring and the corresponding interaction energies are -3.24, -2.43, and -1.57 kcal/mol, respectively. For the complexes with hexafluorobenzene which has a large positive quadrupole moment, the most stable geometries are those in which the electropositive end of HF, H2O, and NH3 points away from the ring and the corresponding interaction energies are -1.59, -2.73, and -3.14 kcal/mol, respectively. Methane, which has neither a dipole nor a quadrupole moment, is weakly bound and is oriented differently in different systems. 1,3,5-Trifluorobenzene has a negligible quadrupole moment, and the complexes with small molecules are stabilized by cyclic hydrogen bonding. Although the point dipole-quadrupole and point quadrupole-quadrupole interactions present in these complexes account qualitatively for the preferred orientations, distributed multipole moments of the constituent atoms are found to give a quantitative description of the interaction in such complexes.