Optimization and application of lithium parameters for PM3

Abstract

Lithium parameters have been optimized for Stewart's standard PM3 method. The average deviation of the heats of formation calculated for 18 reference compounds is 6.2 kcal/mol from the experimental or high-level ab initzo data; the average deviation with Li/MNDO is 18.9 kcal/mol. The average error in bond lengths is also reduced by a factor of two to three. Ionization potentials and dipole moments are reproduced with comparable accuracy than Li/MNDO. However, the mean deviation for the heats of formation of both methods increases when being applied to other systems, especially to small inorganic molecules. The applicability of the new parameter set is demonstrated further for various compounds not included in the reference set, for the calculation of the activation barriers of several lithiation reactions, as well as for the estimation of oligomerization energies of methyl lithium (including the tetramer). Li/PM3 gives reliable results even for large dimeric complexes, like [{4-(CH3CR)C&N}LiI2, containing TMEDA or THF as coligands and reproduces the haptotropic interaction between Li' and n-systems (e.g., in benzyl lithium) as well as the relative energies and structural features of compounds with "hypervalent" atoms (e.g., in lithiated sulfones). 0 1993 by John Wiley & Sons, Inc.

Studies on the Bisoxazoline‐ and (−)‐Sparteine‐Mediated Enantioselective Addition of Organolithium Reagents to Imines

“…Although the Li-N distance in LiNH 2 is computed equally accurately by either method, multicoordinated lithium exhibits considerably longer bonds (0.09 -0.13 Å) to sp 2 -hybridized nitrogen atoms in MNDO than in PM3. [68] As a consequence, the imine substrate could readily be attached to 12ac•MeLi in PM3 whereas in MNDO, the approach of the imine ultimately leads to the opening of the bidentate coordination mode of the bisoxazoline. [69] This complication precluded further use of MNDO and all of the following arguments are made on the basis of PM3-optimized structures.…”

?-Caprolactone/ZnCl2 complex formation: Characterization and ring-opening polymerization mechanism

“…The optimization was stopped when Herbert or Peter tests were satisfied in the BroydenFletcher-Goldfarb-Shanno (BFGS) method. 17 Ab initio 18 calculations were made with the Gaussian 98 package 19 using as data and graphical interface the Cerius 2 program. 15 Through the Zmatrix input data from a semiempirical AM1 calculation, the geometry and the total electronic energy were calculated by the RHF method with 6-31G(d) (RHF/6-31G*) basis set.…”

A theoretical study of the addition of silyl radicals to olefinic monomers

“…These semiempirical methods are commonly accepted to allow a good description of the lone-pair/lone-pair repulsion in several compounds 20) . Thus, the calculation of hypervalent molecules and hetero-compounds has been improved with respect to other semiempirical calculations 21) . The MOPAC v.6.0 program was run on a Silicon Grahics Indigo2 R10000 workstation.…”