Why Are MgCH Isomers Missing in the Interstellar Medium?

Considering the recent findings of linear doublet (Σ) MgCH isomers ( = 2, 4, and 6) in the evolved carbon star IRC+10216, various structural isomers of MgCH and MgCH are theoretically investigated here. For MgCH, 11 doublet and 8 quartet stationary points ranging from 0.0 to 71.8 and 0.0 to 110.1 kcal mol, 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 MgCH, the global minimum geometry turns out to be a cyclic isomer, 2-magnesabicyclo[1.1.0]but-1,3,4-triyl (, , ). In addition, five low-lying isomers, magnesium-substituted cyclopropenylidene (, , '), 1-magnesabut-2,3-dien-1-yl-4-ylidene (, , ″), 1-magnesabut-2-yn-1-yl-4-ylidene (, , ″), 2λ-magnesabicyclo[1.1.0]but-1,3-diyl-4-ylidene (, ;, ), and 1-magnesabut-2,3-dien-2-yl-4-ylidene (, , Σ), were also identified. The doublet linear isomer of MgCH, 1-magnesabutatrienyl (, , Σ) turns out to be a minimum but lies 54.1 kcal mol above at the ROCCSD(T)/cc-pVTZ level. The quartet (Σ) electronic state of was also found to be a minimum, but it lies 8.0 kcal mol above at the same level. Among quartets, isomer is the most stable molecule. The next quartet electronic state (of isomer ) is 34.4 kcal mol above , and all other quartet electronic states of other isomers are not energetically close to low-lying doublet isomers to . Overall, the chemical space of MgCH contains more cyclic isomers (, , and ) on the low-energy side unlike their even-numbered MgCH counterparts ( = 2, 4, and 6). Though the quartet electronic state of is linear, it is not the global minimum geometry on the MgCH potential energy surface. Isomerization pathways among the low-lying isomers (doublets of - and a quartet of ) reveal that these molecules are kinetically stable. For the cation, MgCH, the cyclic isomers (, , and ) are on the low-energy side. The singlet linear isomer, , 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 MgCH isomers.