Energetics and Spectroscopic Properties of Low‐lying CaC₆H₂ Isomers: An Astrochemical Perspective

Abstract

The spectroscopic constants and molecular properties of lowlying isomers of CaC 6 H 2 have been studied using dispersioncorrected density functional theory, MP2 and high-level coupled-cluster (CC) methods. 60 isomers have been identified, among which three low-lying isomers having energy differences within 1 eV have been re-optimized at MP2/cc-pVTZ level. Isomer 1 (1-calciacyclohepta-4-en-2,6-diyne) is found to be the most stable one, followed by isomer 3 (3-calciahepta-1,4,6-triyne) and isomer 2 (1-calciacyclohepta-2,3,4-trien-6-yne) that lie 20.9 and 19.7 kcal/mol above 1, respectively at MP2/cc-pVTZ level. All three isomers are polar with a permanent electric dipole moment. Molecular properties like electron affinity, ionization potential as well as GCRD parameters, i. e., chemical potential, global hardness, etc. have been calculated at ae-CCSD(T)/cc-pVTZ level. NBO and AIM analyses have been performed to predict the nature of bonds in the isomers. The spectroscopic constants such as rotational and centrifugal distortion constants, inertial axis dipole moment components, etc. have been calculated using the frozen core (fc)-CCSD(T) and all electron (ae)-CCSD(T) methods with cc-pVTZ basis. The ae-CCSD(T) method with cc-pwCVTZ-X2C basis set has also been used to calculate spectroscopic constants. The predicted spectroscopic constants and molecular properties will facilitate in detecting these isomers in the laboratory as well as in interstellar medium (ISM).