Optimization and application of lithium parameters for PM3

✍ Scribed by Ernst Anders; Rainer Koch; Peter Freunscht

• Publisher: John Wiley and Sons
• Year: 1993
• Tongue: English
• Weight: 889 KB
• Volume: 14
• Category: Article
• ISSN: 0192-8651
• DOI: 10.1002/jcc.540141106

✦ Synopsis

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.

*This work has never been published in full. None of the five parameter sets originally derived were entirely satisfactory. The one chosen for use had known deficiencies that have been mentioned many times in the literature-the C-Li and H-Li interactions are overestimated.