Computed Rotational Collision Rate Coefficients for Recently Detected Anionic Cyanopolyynes

We report new results from quantum calculations of energy-transfer processes taking place in interstellar environments and involving two newly observed molecular species: C$_5$N$^-$ and C$_7$N$^-$ in collision with He atoms and the p-H$_2$ molecules. These species are part of the anionic molecular chains labeled as cyanopolyynes which have been observed over the years in molecule-rich Circumstellar Envelopes and in molecular clouds. In the present work, we first carry out new $ab$ $initio$ calculations for the C$_7$N$^-$ interaction potential with He atom and then obtain state-to-state rotationally inelastic cross sections and rate coefficients involving the same transitions which have been observed experimentally by emission in the interstellar medium (ISM) from both of these linear species. For the C$_5$N$^-$/He system we extend the calculations already published in our earlier work (see reference below) to compare more directly the two molecular anions. We extend further the quantum calculations by also computing in this work collision rate coefficients for the hydrogen molecule interacting with C5N$^-$, using our previously computed interaction potential. Additionally, we obtain the same rate coefficients for the C$_7$N$^-$/H$_2$ system by using a scaling procedure that makes use of the new C$_7$N$^-$/He rate coefficients, as discussed in detail in the present paper. Their significance in affecting internal state populations in ISM environments where the title anions have been found is analyzed by using the concept of critical density indicators. Finally, similarities and differences between such species and the comparative efficiency of their collision rate coefficients are discussed. These new calculations suggest that, at least for the case of these longer chains, the rotational populations could reach local thermal equilibrium conditions within their observational environments

Ab initio potential energy surface for HeF<SUB>2</SUB> in its ground electronic state

The ground state potential energy surface for He-F<SUB>2</SUB> has been generated using the coupled-cluster singles and doubles excitation approach with perturbative treatment of triple excitations [CCSD(T)] and multi-reference configuration interaction (MRCI) methodologies, with augmented correlation consistent quadruple zeta basis set and diffused functions. Both the CCSD(T) and MRCI surfaces are compared and the results analyzed. The CCSD(T) surface exhibits van der Waals minima at different distances for different orientations of He approaching F<SUB>2</SUB> and is adequate to describe accurately only in the region around the equilibrium bond distance of F<SUB>2</SUB>. The MRCI surface, on the other hand, yields reliable results for a wider range of F-F bond distances leading to the correct asymptote. Davidson correction to the MRCI surface makes it purely repulsive over the regions investigated

Ab initio quantum chemical investigation of the ground and excited states of salicylic acid dimer

The structure and stability of different forms of salicylic acid dimer have been examined by Hartree-Fock and density functional theoretic calculations using 6-31G(d,p) and 6-311++g(d,p) basis sets. Vertical excitation energies for the monomer as well as the dimer have been computed using the time-dependent density functional theory using 6-311++G(d,p) basis set. The predicted absorption maxima for the first excited singlet state of salicylic acid monomer and the dimer of the primary form are in reasonable agreement with the experimental result. There is a slight red shift (~6 nm) in the absorption maximum in going from the monomer to the dimer, in accord with the experimental observation. Configuration-interaction calculations including single excitation have been carried out to map the potential-energy profile for the intra- as well as the intermolecular proton transfer in different forms of the dimer. The barrier for proton transfer in the ground state as well as the excited states makes it clear that most of the processes take place in the primary form and largely by intramolecular proton transfer

Stereomutation in Tetracoordinate Centers via Stabilization of Planar Tetracoordinated Systems

The quest for stabilizing planar forms of tetracoordinate carbon started five decades ago and intends to achieve interconversion between [R]- and [S]-stereoisomers without breaking covalent bonds. Several strategies are successful in making the planar tetracoordinate form a minimum on its potential energy surface. However, the first examples of systems where stereomutation is possible were reported only recently. In this study, the possibility of neutral and dications of simple hydrocarbons (cyclopentane, cyclopentene, spiropentane, and spiropentadiene) and their counterparts with the central carbon atom replaced by elements from groups 13, 14, and 15 are explored using ab initio MP2 calculations. The energy difference between the tetrahedral and planar forms decreases from row II to row III or IV substituents. Additionally, aromaticity involving the delocalization of the lone pair on the central atom appears to help in further stabilizing the planar form compared to the tetrahedral form, especially for the row II substituents. We identified 11 systems where the tetrahedral state is a minimum on the potential energy surface, and the planar form is a transition state corresponding to stereomutation. Interestingly, the planar structures of three systems were found to be minimum, and the corresponding tetrahedral states were transition states. The energy profiles corresponding to such transitions involving both planar and tetrahedral states without the breaking of covalent bonds were examined. The systems showcased in this study and research in this direction are expected to realize molecules that experimentally exhibit stereomutation

Ground and excited states of the monomer and dimer of certain carboxylic acids

The ground-state properties of the monomer and the dimer of formic acid, acetic acid, and benzoic acid have been investigated using Hartree-Fock (HF) and density functional theory (DFT) methods using the 6-311++G(d,p) basis set. Some of the low-lying excited states have been studied using the time-dependent density functional theory (TDDFT) with LDA and B3LYP functionals and also employing complete-active-space-self-consistent-field (CASSCF) and multireference configuration interaction (MRCI) methodologies. DFT calculations predict the ground-state geometries in quantitative agreement with the available experimental results. The computed binding energies for the three carboxylic acid dimers are also in accord with the known thermodynamic data. The TDDFT predicted wavelengths corresponding to the lowest energy n-π∗ transition in formic acid (214 nm) and acetic acid (214 nm) and the π-π∗ transition in benzoic acid (255 nm) are comparable to the experimentally observed absorption maxima. In addition, TDDFT calculations predict qualitatively correctly the blue shift (4-5 nm) in the excitation energy for the π-π∗ transition in going from the monomer to the dimer of formic acid and acetic acid and the red shift (~19 nm) in π-π∗ transition in going from benzoic acid monomer to dimer. This also indicates that the electronic interaction arising from the hydrogen bonds between the monomers is marginal in all three carboxylic acids investigated

Time-dependent density functional theoretical study of low lying excited states of F<SUB>2</SUB>

The utility of time-dependent density functional theory (TDDFT) in predicting excitation energies is tested for the low lying excited states of F<SUB>2</SUB>, a system that has posed severe challenges to ab initio quantum theory. It is shown that TDDFT using B3LYP functional predicts the excitation energies in good agreement with experiment. In some cases, the agreement is better than that for the post-Hartree-Fock methods like CASSCF and MRCI

Determination of stability and degradation in polysilanes by an electronic mechanism

Polysilanes are potential candidates for active materials in light emitting diodes because of possible emission in the near-ultraviolet to blue region. Unfortunately, they degrade rapidly upon exposure to light because of scission of sigma bonds. Relative stability of four polysilanes, for example, poly(di-n-butylsilane) (PDBS), poly(di-n-hexylsilane) (PDHS), poly(methylphenylsilane) (PMPS), and poly[bis(p-butylphenyl)silane] (PBPS), which have been reported as active materials in light emitting diodes, have been investigated theoretically through semiempirical (AM1) and ab initio (HF/6-31g∗) methods and density functional theory using B3LYP parametrization. The AM1 level of calculation predicts the absorption maxima reasonably, but it fails to explain the relative stabilities of the four polysilanes in the excited state. However, calculations based on configuration interaction with single excitation and time-dependent density functional theory suggest additional stabilization in the excited states through intersystem crossing to triplets for PMPS and PBPS, consistent with the experimental observation. In contrast, no such stabilization is predicted for PDBS and PDHS. Furthermore, the existence of a stable triplet state in PMPS may also explain the visible emission observed experimentally in PMPS

Stability in polysilanes for light emitting diodes

Theoretical investigations to determine the stability among different polysilanes have been attempted. Semiempirical (AM1) and ab initio calculations at Hartree Fock (HF)/3-21g<SUP>∗</SUP> level have been performed for four polymers namely poly(di-n-butylsilane) (PDBS), poly(di-n-hexylsilane) (PDHS), poly(methylphenylsilane) (PMPS) and poly[bis(p-butylphenyl)silane] (PBPS) that have been reported as candidates for light emitting diodes. Configuration interaction (single excitation) has been carried out to predict the stability of the excited states of polysilanes. Based on the ab initio calculations, we are proposing a possibility of stabilization of PBPS and PMPS by intersystem crossing from S<SUB>1</SUB> to T<SUB>1</SUB> excited state, which in turn leads to higher stability of these two polymers

An improved artificial neural network fit of the ab initio potential energy surface points for HeH+ + H2 and its ensuing rigid rotors quantum dynamics

Artificial neural networks (ANN) have been shown for the last several years to be a versatile tool for fitting ab initio potential energy surfaces. We have demonstrated recently how a 60-neuron ANN could successfully fit a four-dimensional ab initio potential energy surface for the rigid rotor HeH+ - rigid rotor H2 system with a root-mean-squared deviation (RMSD) of 35 cm−1. We show in the present study how a (40, 40) neural network with two hidden layers could achieve a better fit with an RMSD of 5 cm−1. Through a follow-up quantum dynamical study of HeH+(j1)-H2(j2) collisions, it is shown that the two fits lead to slightly different rotational excitation and de-excitation cross sections but are comparable to each other in terms of magnitude and dependence on the relative translational energy of the collision partners. When averaged over relative translational energy, the two sets of results lead to rate coefficients that are nearly indistinguishable at higher temperatures thus demonstrating the reliability of the ANN method for fitting ab initio potential energy surfaces. On the other hand, we also find that the de-excitation rate coefficients obtained using the two different ANN fits differ significantly from each other at low temperatures. The consequences of these findings are discussed in our conclusions