STRUCTURES AND STABILIZATION ENERGIES OF METHYL ANIONS WITH MAIN-GROUP SUBSTITUENTS FROM THE FIRST 5 PERIODS

The effects of substituents (X) on the structures and stabilities of CH(2)X-nions for groups comprised of fourth- and fifth-period main group elements (X = K, CaH, GaH2, GeH3, AsH2, SeH, Br, Rb, SrH, InH2, SnH3, SbH2 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 rr acceptor interactions (e.g., with BH2, AIH(2), GaH2, and InH2) 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 pi-stabilizing effects for the same substituents. With the exception of CH3 and NH2, all substituents stabilize methyl anions, although the sigma stabilization by OH and F is small. The SiH3-PH2-SH-Cl, GeH3-AsH2-SeH-Br, and SnH3-SbH2-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 pi accepters (BH2, AIH(2), GaH2, and InH2) the stability decreases down a group in line with their pi-accepting ability. (C) 1994 by John Wiley & Sons, Inc.