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The rotational dynamics of isocyanogen (CNCN) is studied for its collision with para (p-) and ortho (o-) hydrogen (H2) in the temperature range of 1–100 K. These temperatures correspond to the cold dense molecular clouds in the interstellar medium where molecular hydrogen is the primary collider. An ab initio 4D potential energy surface (PES) is constructed keeping the two molecules under rigid rotor approximation. The PES is generated using the CCSD(T)-F12b/AVTZ level of theory. The 4D PES is further fitted into a neural network (NN) model, which can augment the surface and account for missing data points within spectroscopic accuracy. This NN-fitted PES is then expanded over a bispherical harmonics function to get radial terms, which are expressed into analytic functions. Thereafter, the cross sections (σ) are computed for rotational transitions of CNCN (j → j′) using the close-coupling and centrifugal sudden methods for both p-H2 (jc = 0) and o-H2 (jc = 1) collision till 194 cm−1. In addition, p-H2 (jc = 0, 2) cross sections are also computed using the centrifugal sudden approximation method. The collisional rates are achieved by taking the Boltzmann distribution of σ over the translational energy of H2 till 100 K. Finally, the CNCN–H2 rates are compared to CNCN–He and NCCN–H2 collisional rates. Comparing even and odd transitions for the CNCN–H2 rates show a propensity toward higher rates for even transitions especially for o-H2 collisions considering low-order transitions. [ABSTRACT FROM AUTHOR]

The rotational excitation rates of NCCN species are studied for its collision with hydrogen (H2) in temperatures ranging from 1 to 100 K. Such collisions can occur in the interstellar medium with H2 in either para (p-) or ortho (o-) state, of which the p-H2 state can be approximated via its collision with He (using a scaling factor) or with a reduced rigid rotor-H2 surface (by averaging over various orientations of H2). In the current work, a four-dimensional (4D) ab initio potential energy surface (PES) is considered to study the collision dynamics of H2 in both p- and o-states and the results are compared with previous approximations. The 4D surface is constructed using the explicitly correlated coupled-cluster method CCSD(T)-F12b with the augmented triple zeta basis AVTZ and then fitted into an artificial neural networks (NN) model to augment the surface and account for missing data points. The radial coefficients are obtained from this NN fitted 4D PES via a least square fit over two spherical harmonics functions. The cross sections (σ) are computed using the close-coupling (CC) method (until 230 cm−1) for both p- and o-H2 collisions, and the rates are obtained by Boltzmann distribution over the translational energy of H2 until 100 K. The o-H2 rates are found to be higher by 25%–30% and 10%–20% compared to the p-H2 rates for Δj = 2 and higher order transitions, respectively. The coupled-state/centrifugal sudden approximated rates are also computed and found to have deviations as large as 40% when compared to CC rates, thus making quantitative descriptions unreliable. [ABSTRACT FROM AUTHOR]

The effect of para -hydrogen (j p = 0) collisions on the rotational de-excitation transitions of molecule NCCP is investigated in this study. The scattering information is obtained by spherically averaging a four-dimensional potential energy surface (4DPES) over various H2 molecule orientations. The calculations used the CCSD(T)-F12a method and aug-cc-pVTZ basis set to generate a 4DPES for the NCCP–H2 van der Waals system. Within the NCCP– para -H2 4DPES, a minimum energy point of 191.82 cm−1 is attained at a distance of 3.6 Å from the centre of mass of H2 and NCCP. To compute cross-sectional data for NCCP interacting with para -H2 (j p = 0), close coupling calculations are employed, encompassing total energies up to 600 cm−1. The resulting rate coefficients [ |$k_{j{\rightarrow }j^{\prime }}(T$|⁠)] are calculated across a temperature range spanning from 5 K to 200 K. In accordance with propensity, even Δ j = −2 transitions are highly preferred. Comparatively, the derived |$k_{j{\rightarrow }j^{\prime }}(T$|⁠) for NCCP–H2 are determined to be 1.5–4.5 times of NCCP–He. This observation implies that relying on a scaling factor of 1.38 to extrapolate rate coefficients for H2 collisions from those of NCCP–He is not a reliable approach. [ABSTRACT FROM AUTHOR]

Quantum dynamics of the molecule cyanogen (NCCN) and its collision with helium taking place in the interstellar medium has been studied. An ab initio potential energy surface of NCCN—He, a van der Waals complex, is generated using the high-level single reference coupled-cluster with single and double and perturbative triple excitation method and aug-cc-pVQZ basis sets. Using the multipole expansion, Legendre coefficients have been calculated and utilized in determining collisional cross sections. Close-coupling calculations have been performed to study rotational excitations for He collision with NCCN. Due to nuclear spin statistics, collision induced transitions have even Δj, while odd Δj transitions are forbidden. The presence of resonances arising from rapid oscillation of cross sections in the low energy region is the result of quasi-bound states in the NCCN—He van der Waals complex. Among all the transitions, Δj = 2 are found to be predominant for excitation. Thereafter, for each transition, the rate coefficients have been calculated which decrease with increasing values of j and Δj. The result of this work will be helpful to accurately model the abundance of cyanogen in stellar atmospheres and interstellar gas. [ABSTRACT FROM AUTHOR]

NCCP, a phosphorous species, is believed to have been discovered in the carbon-rich star IRC+10216. Understanding collisional properties, such as cross-sections (⁠|$\sigma _{j\rightarrow j^{\prime }}$|⁠) and rate coefficients (⁠|$k_{j\rightarrow j^{\prime }}$|⁠), is important for the reliable determination of molecular abundance. The non-reactive collisions between NCCP and He species at low temperatures are the subject of this study. Calculations are based on new ab initio potential energy surface (PES) of NCCP–He. The PES calculations are carried out at the CCSD(T)–F12a in conjunction with the aug-cc-pVTZ basis set. The PES is found to have a global minimum towards the N end with a well depth of −46.40 cm−1. The ab initio points are analytically fitted on to the Legendre polynomial relevant for quantum scattering. From this fitted PES, the integral inelastic rotational cross-sections of NCCP with He collisions are computed for total energies up to 550 cm−1, using the accurate close coupling approach of quantum mechanics. The resonances are observed at low total energies due to quasi-bound states of the NCCP−He complex. Rate coefficients are determined among the 19 lowest rotational levels of NCCP by thermally averaging the cross-sections at low temperatures. The de-excitation rate coefficients increase with decreasing Δ j. [ABSTRACT FROM AUTHOR]