We have optimized the three possible structures of methyllithium
hexamer based on an octhaderal array of he lithium atoms. Previous
studies have shown that the alkyllithium hexamers exist as distorted octahedera
of lithium atoms surrounded by the six alkyl groups wherein the empty faces
are located trans to each other. We have also considered the other two
arrangements of the empty faces and optimized the three different isomers.
Optimizations, subject to local and overall symmetry constraints, were
done using the Gaussian series of programs at both
the 3-21G and 4-31G levels. Additional calculations were done on the 4-31G
optimized structures utilizing better basis sets . The structures can be
displayed dynamically (requires Java and frames
capability) using the Sun java MoleculeViewer
or statically. Our best energy results, obtained
at the 6-31G* level, show the trans to be more stable than the apical (8.8
kcal/mol) or the edge isomer (26.3 kcal/mol).
The calculations described were done on the computers of the City University
of New York and Long Island University.
Comments may be sent to the authors:
L. J. Kirschenbaum.
J. M. Howell
Notes
Methyllithium is used as a model for alkyllithiums. Experimental studies have shown that alkyllithiums exist as oligomers in solid state, gas phase, and in solution. Methyl-, ethyl-, and t-butyl- lithium polymers are found to be tetramers in the crystalline solids. On the other hand, cyclohexyl-, isopropyl-, n-butyl-, (tetramethylcypropyl)methyl- lithium exist as hexameric solids.
Gaussian 90, Revision I, M. J. Frisch, M. Head-Gordon, G. W. Trucks, J. B. Foresman, H.B. Schlegel, K. Raghavachari, M. Robb, J. S. Binkley, C. Gonzalez, D. J. Defrees, D. J. Fox, R. A. Whiteside, R. Seeger, C. F. Melius, J. Baker, R. L. Martin, L. R. Kahn, J. J. P. Stewart, S. Topiol, and J. A. Pople, Gaussian Inc., Pittsburgh PA, 1990.