Computational Chemistry and Molecular Modeling
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
The effects of substituents (X) on the structures and stabilities of CH2X- anions 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 CH2X- anions and the corresponding neutral parent molecules, CH3X, 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 CH2X- anion structures to be the most stable unless π acceptor interactions (e.g., with BH2, AlH2, GaH2, and InH2 favor planar geometries. The CH2X- 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 CH3 and NH2, all substituents stabilize methyl anions, although the σ 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 π acceptors (BH2, AlH2, GaH2, and InH2) the stability decreases down a group in line with their π-accepting ability. © 1994 by John Wiley & Sons, Inc.
Type of Medium: