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

[Image: see text] 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(2v), (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(2v);, (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.