AB-INITIO PREDICTIONS OF NEW CARBON HYPERMAGNESIUM SPECIES - MG2C ANDMG3C

The ground and very low-lying excited states of new Mg2C and Mg3C mole cules have been studied using high-level ab initio techniques. Four st ructures of Mg2C, (C-2 upsilon, (1)A(1)), (C-2 upsilon, B-3(1)), (D-in finity h, (3) Sigma(g)(-)), and (D-infinity h, (5) Sigma(u)(-)), were found to lie within 4 kcal/mol (at QCISD(T)/6-311+G(2df)) of one anoth er. A C-3 upsilon ((1)A(1)) structure was found to be the only low-ene rgy structure for Mg3C. Both Mg2C and Mg3C were found to be thermodyna mically stable with respect to all dissociation channels. Dissociation energies are found to be, for Mg2C (C-2 upsilon, (1)A(1)) --> MgC ((3 ) Sigma(-))+Mg (S-1), 24.6 kcal/mol and, for Mg3C (C-3 upsilon, (1)A(1 )) --> Mg2C (C-2 upsilon, (1)A(1))+Mg (S-1), 41 kcal/mol at the QCISD( T)/6-311+G(2df)+ZPE level.