Why Are MgC 3 H Isomers Missing in the Interstellar Medium?

Considering the recent findings of linear doublet ( 2 Σ + ) MgC n H isomers ( n = 2, 4, and 6) in the evolved carbon star IRC+10216, various structural isomers of MgC 3 H and MgC 3 H + are theoretically investigated here. For MgC 3 H, 11 doublet and 8 quartet stationary points ranging from 0.0 to 71.8 and 0.0 to 110.1 kcal mol -1 , respectively, have been identified initially at the UωB97XD/6-311++G(2d,2p) level. To get accurate relative energies, further energy evaluations are carried out for all isomers with coupled cluster methods and thermochemical modules such as G3//B3LYP, G4MP2, and CBS-QB3 methods. Unlike the even series, where the global minima are linear molecules with a Mg atom at one end, in the case of MgC 3 H, the global minimum geometry turns out to be a cyclic isomer, 2-magnesabicyclo[1.1.0]but-1,3,4-triyl ( 1 , C 2 v , 2 A 1 ). In addition, five low-lying isomers, magnesium-substituted cyclopropenylidene ( 2 , C s , 2 A '), 1-magnesabut-2,3-dien-1-yl-4-ylidene ( 3 , C s , 2 A ″), 1-magnesabut-2-yn-1-yl-4-ylidene ( 4 , C s , 2 A ″), 2λ 3 -magnesabicyclo[1.1.0]but-1,3-diyl-4-ylidene ( 5 , C 2 v ;, 2 A 1 ), and 1-magnesabut-2,3-dien-2-yl-4-ylidene ( 6 , C ∞v , 2 Σ + ), were also identified. The doublet linear isomer of MgC 3 H, 1-magnesabutatrienyl ( 10 , C ∞v , 2 Σ + ) turns out to be a minimum but lies 54.1 kcal mol -1 above 1 at the ROCCSD(T)/cc-pVTZ level. The quartet ( 4 Σ + ) electronic state of 10 was also found to be a minimum, but it lies 8.0 kcal mol -1 above 1 at the same level. Among quartets, isomer 10 is the most stable molecule. The next quartet electronic state (of isomer 11 ) is 34.4 kcal mol -1 above 10 , and all other quartet electronic states of other isomers are not energetically close to low-lying doublet isomers 2 to 6 . Overall, the chemical space of MgC 3 H contains more cyclic isomers ( 1 , 2 , and 3 ) on the low-energy side unlike their even-numbered MgC n H counterparts ( n = 2, 4, and 6). Though the quartet electronic state of 10 is linear, it is not the global minimum geometry on the MgC 3 H potential energy surface. Isomerization pathways among the low-lying isomers (doublets of 1 - 4 and a quartet of 10 ) reveal that these molecules are kinetically stable. For the cation, MgC 3 H + , the cyclic isomers ( 1 + , 2 + , and 3 + ) are on the low-energy side. The singlet linear isomer, 10 + , is a fourth-order saddle point. The low-lying cations are quite polar, with dipole moment values of >7.00 D. The current theoretical data would be helpful to both laboratory astrophysicists and radioastronomers for further studies on the MgC 3 H 0/+ isomers.