Ionization and protonation of MgC3: A theoretical study

A theoretical study of the MgC3+ and MgC3H+ species has been carried out. P redictions for their geometries and vibrational frequencies have been made at both second-order Moller-Plesset (MP2) and B3LYP levels, whereas electro nic energies have been computed at G2 and coupled cluster single and double excitation model augmented with a noniterative triple excitation conection (CCSD(T)) levels. The predicted global minimum for MgC3+ is a rhombic stru cture ((2)A(1) electronic state), whereas a T-shaped structure and an open- chain isomer lie about 10 and 12 kcal/mol, respectively, higher in energy. In the case of MgC3H+ the predicted global minimum is also a four-membered ring obtained upon protonation of the most stable neutral isomer. Low ioniz ation potentials and high proton affinities are generally obtained, especia lly for the most stable MgC3+ isomer. The estimated values at the CCSD(T) l evel for the predicted global minimum are 7.20 eV [ionization potential (IP )] and 256.5 kcal/mol [proton affinities (PA)]. Therefore, if present in th e interstellar medium, MgC3 should be easily ionized and would react quite easily to give the protonated species. (C) 2002 John Wiley & Sons, Inc.