Structures and stabilization energies of methyl anions with main group substituents from the first five periods

The effects of substituents ( X ) on the structures and stabilities of CH 2 X − anions for groups comprised of fourth‐ and fifth‐period main group elements ( X = K, CaH, GaH 2 , GeH 3 , AsH 2 , SeH, Br, Rb, SrH, InH 2 , SnH 3 , SbH 2 , TeH, and I) have been investigated by ab initio pseudopotential calculations. Full geometry optimizations have been carried out on the CH 2 X − anions and the corresponding neutral parent molecules, CH 3 X , at HF/DZP + and MP2/DZP + levels. Results for substituents from the second ( X = LiF) and third ( X = NaCl) periods provide comparisons of substituent effects of the main group elements of the first four rows of the periodic table on methyl anions. Frequency calculations characterize the nature of stationary points and show pyramidal CH 2 X − anion structures to be the most stable unless π acceptor interactions (e.g., with BH 2 , AlH 2 , GaH 2 , and InH 2 favor planar geometries. The CH 2 X − stabilization energies [at QCISD(T)/DZP + /MP2/DZP + + ZPE level for X = KI and QCISD(T)/6−31 + G*/MP2/6−31 + G* + ZPE level] for X = LiCl) also show strong π‐stabilizing effects for the same substituents. With the exception of CH 3 and NH 2 , all substituents stabilize methyl anions, although the σ stabilization by OH and F is small. The SiH 3 PH 2 SHCl, GeH 3 AsH 2 SeHBr, and SnH 3 SbH 2 TeHI sets of substituents give stabilization energies between 19 and 30 kcal/mol. The stability of methyl anions substituted by the halogens and the chalcogens ( X = OH, SH, SeH, and TeH) increases down a group in accord with the increasing substituent polarizability, while for π acceptors (BH 2 , AlH 2 , GaH 2 , and InH 2 ) the stability decreases down a group in line with their π‐accepting ability.