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Internet Electronic Journal of Molecular Design - IEJMD, ISSN 1538-6414, CODEN IEJMAT
| ABSTRACT - Internet Electron. J. Mol. Des. May 2002, Volume 1, Number 5, 242-251 |
Semiempirical Quantum Mechanics and the Quantification of Ligand Electronic Parameters
Aaron M. Gillespie, Karl A. Pittard, Thomas R. Cundari, and David P. White
Internet Electron. J. Mol. Des. 2002, 1, 242-251
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Abstract:
Quantification of electronic effects in organometallic chemistry has received little
attention in the literature despite their importance in the control of catalytic processes. It
is well recognized that the A1 CO vibration for Ni(CO)3L is a good
measure of electronic effects. Recently, Crabtree has used density functional theory to compute
the A1 CO vibration for Ni(CO)3L complexes and found good agreement
with experiment. In this paper, a more rapid computational method for the derivation of a general
quantitative measure of ligand electronic effects is presented. With semiempirical quantum mechanics
(PM3(tm) Hamiltonian in Spartan 5.0 or 02), the A1 CO vibrations for
Mo(CO)5L, W(CO)5L, and [CpRh(CO)(L)] are computed, which is termed the
semiempirical quantum mechanics electronic parameter, SEP. The SEP values are compared with
measures of electronic effects in the experimental and computational literature and are
found to be robust, general measures of the electronic nature of a ligand. Semiempirical
quantum mechanics was used to geometry optimize the Mo(CO)5L, W(CO)5L, and
[CpRh(CO)(L)] complexes. Spartan 5.0 or Spartan 02 was used to compute the
vibrational frequencies of Mo(CO)5L, W(CO)5L, and [CpRh(CO)(L)] with the PM3(tm)
Hamiltonian. The newly defined SEP was found to correlate well with Tolman's
experimental νCO parameter, Lever's electrochemical parameter, as well as Crabtree's
computed parameter. The SEP values are a robust, easy to compute computational
measure of the electronic nature of a wide variety of different ligands.
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