Knowledge Bank

Several species which are candidates for interstellar magnesium-bearing molecules have been studied with correlated ab initio methods and large correlation consistent basis sets. Equilibrium structures, rotational constants, dipole moments, and harmonic frequencies have been computed in order to assist in the search for these species in the interstellar medium. In the case of MgCCH $(2\Sigma )$, a comparison is made with the recent laboratory observations of anderson and $Ziurys,1$ who report $B0=5010.4MHz$; the computed value of Be [RCCSD(T)/cc-pCVTZ] = 4927 MHz. Cyclic $MgC2(1A1)$ was found to be slightly more stable than linear MgCC $(1\Sigma )$ by about 13 kcal/mol [CMRCl+Q/cc-pVTZ]. Also of interest are the energetics for $Mg(2S)+CCH(2\Sigma )\to MgCCH(2\Sigma )\to MgC2(1A1)+H(2S),$ which indicate that the net gas phase reaction is substantially endoergic and thus very unlikely to occur in the interstellar medium.