Discovery of the first Ca-bearing molecule in space: CaNC

Energetic and Spectroscopic Properties of Astrophysically Relevant MgC4H Radicals Using High-Level Ab Initio Calculations

Considering the importance of magnesium-bearing hydrocarbon molecules (MgCnH; n = 2, 4, and 6) in the carbon-rich circumstellar envelopes (e.g., IRC+10216), a total of 28 constitutional isomers of MgC4H have been theoretically investigated using density functional theory (DFT) and coupled-cluster methods. The zero-point vibrational energy corrected relative energies at the ROCCSD(T)/cc-pCVTZ level of theory reveal that the linear isomer, 1-magnesapent-2,4-diyn-1-yl (1, 2Σ+), is the global minimum geometry on the MgC4H potential energy surface. The latter has been detected both in the laboratory and in the evolved carbon star, IRC+10216. The calculated spectroscopic data for 1 match well with the experimental observations (error ∼ 0.78%) which validates our theoretical methodology. Plausible isomerization processes happening among different isomers are examined using DFT and coupled-cluster methods. CASPT2 calculations have been performed for a few isomers exhibiting multireference characteristics. The second most stable isomer, 1-ethynyl-1λ3-magnesacycloprop-2-ene-2,3-diyl (2, 2A1, μ = 2.54 D), is 146 kJ mol-1 higher in energy than 1 and possibly the next promising candidate to be detected in the laboratory or in the interstellar medium in future.

Nonadiabatic quantum dynamics explores non-monotonic photodissociation branching of N2 into the N(4S) + N(2D) and N(4S) + N(2P) product channels

Vacuum ultraviolet (VUV) photodissociation of N2 molecules is a source of reactive N atoms in the interstellar medium. In the energy range of VUV optical excitation of N2, the N-N triple bond cleavage leads to three types of atoms: ground-state N(4S) and excited-state N(2P) and N(2D). The latter is the highest reactive and it is believed to be the primary participant in reactions with hydrocarbons in Titan's atmosphere. Experimental studies have observed a non-monotonic energy dependence and non-statistical character of the photodissociation of N2. This implies different dissociation pathways and final atomic products for different wavelength regions in the sunlight spectrum. We here apply ab initio quantum chemical and nonadiabatic quantum dynamical techniques to follow the path of an electronic state from the excitation of a particular singlet 1Σ+u and 1Πu vibronic level of N2 to its dissociation into different atomic products. We simulate dynamics for two isotopomers of the nitrogen molecule, 14N2 and 14N15N for which experimental data on the branching are available. Our computations capture the non-monotonic energy dependence of the photodissociation branching ratios in the energy range 108 000-116 000 cm-1. Tracing the quantum dynamics in a bunch of electronic states enables us to identify the key components that determine the efficacy of singlet to triplet population transfer and therefore predissociation lifetimes and branching ratios for different energy regions.

New potential energy surface and rotational deexcitation cross-sections of CNNC by para-H2 (jp = 0)

The objective of this study is to enhance our understanding of the existence of molecules in interstellar space by determining the collisional rate coefficients with the most prevalent species. The study examines the impact of para-H2 collisions, specifically when it is in its ground vibrational state with a nuclear spin of para-H2, i.e., jp = 0, on causing the rotational deexcitation of the diisocyanogen (CNNC) molecule. These scattering data are obtained as a result of spherically averaging a four-dimensional potential energy surface (4DPES) over the H2 orientations. Using the CCSD(T)-F12a approach and aug-cc-pVTZ basis sets, the ab initio 4DPES for the CNNC-H2 van der Waals system is calculated. The CNNC-para-H2 4DPES attains a global minimum of 221.38 cm-1 at the CNNC and H2 center of mass distance (R) of 3.1 Å. The method of close coupling calculations is employed for the purpose of calculating the cross-sectional data of CNNC with para-H2 (jp = 0), for total energies up to 1000 cm-1. Rate coefficients are computed over the temperature range of 1 K to 100 K. Propensity suggests that even Δj transitions are strongly preferred. The rate coefficients for CNNC-H2 are determined to be 0.90-2.95 times those of CNNC-He, which implies it is not reliable to estimate the H2 rate coefficients by multiplying the rate coefficients for CNNC-He collision with a scaling factor of 1.38.

New theoretical investigation of rotational inelastic (de)-excitation of calcium isocyanide CaNC(2Σ+) in collision with He(1S)

The theoretical study of collisions between atoms and molecules provides a detailed description of the involved mechanisms and greatly contributes to improving atmospheric and astrophysics models. In the present paper, we focus on the new calculation of rate coefficients for the first 25 rotational levels of the CaNC molecule in collision with He. A new 2D potential energy surface (2D-PES), for the CaNC-He system, was determined using the single, double and perturbative triple excitation restricted coupled-cluster method [rccsd(t)] and the standard aug-cc-pVQZ basis sets. This PES presents a global minimum with a well depth of -21.93 cm^-1 located at R = 8a₀ and γ = 116°. State-to-state collisional excitation cross-sections of the fine-structure levels of CaNC(²Σ⁺)-He are calculated for energies up to 305 cm^-1, which yield, after thermal averaging, rate coefficients up to 70 K. A ΔJ = ΔN propensity rule was observed. A comparison of the CaNC rates with those of valence isoelectronic MgNC has been investigated. These new data are necessary for the CaNC abundance determination and interpretation of its observed lines.

Electronic structure calculations and quantum dynamics of rotational deexcitation of CNNC by He

The quantum dynamics of rotational transitions of the diisocyanogen (CNNC) molecule undergoing collision with the helium (He) atom occurring in the interstellar medium (ISM) has been studied. The rotational deexcitation cross sections are extracted by first computing an ab initio potential energy surface of CNNC-He using the coupled-cluster with single and double and perturbative triple excitations with the F12a method (CCSD(T)-F12a) employing the aug-cc-pVTZ basis set. Utilizing the multipole expansions, collisional cross sections are determined for total energies of up to 1000 cm^-1 by the close coupling equations. The discussion on propensity rules suggests that the transitions have even Δj values, while odd Δj valued transitions are forbidden due to C and N nuclei spin statistics. Quasi-bound states present in the CNNC-He van der Waals complex resulted in the resonances coming from the rapid oscillations in the values of the cross sections in the region of low energy. Rotational deexcitation rate coefficients are further worked out by averaging the calculated cross sections at temperatures below 200 K. The new findings of the study will be beneficial in modeling the abundance of diisocyanogen in the ISM.

Discovery of two new magnesium-bearing species in IRC+10216: MgC3N and MgC4H

We report on the detection of two series of harmonically related doublets in IRC +10216. From the observed frequencies, the rotational constant of the first series is B = 1380.888 MHz and that of the second series is B = 1381.512 MHz. The two series correspond to two species with a 2Σ electronic ground state. After considering all possible candidates, and based on quantum chemical calculations, the first series is assigned to MgC3N and the second to MgC4H. For the latter species, optical spectroscopy measurements support its identification. Unlike diatomic metal-containing molecules, the line profiles of the two new molecules indicate that they are formed in the outer layers of the envelope, as occurs for MgNC and other polyatomic metal-cyanides. We also confirm the detection of MgCCH that was previously reported from the observation of two doublets. The relative abundance of MgC3N with respect to MgNC is close to one while that of MgC4H relative to MgCCH is about ten. The synthesis of these magnesium cyanides and acetylides in IRC +10216 can be explained in terms of a two-step process initiated by the radiative association of Mg+ with large cyanopolyynes and polyynes followed by the dissociative recombination of the ionic complexes.