Subject: MM with transition metal cpds

From: [email-masked] (Bersuker)
Date: Thu, 10 Feb 94 12:27:16 CST

In reply to many inquiries I present my Abstract to the 15th Austin Symposium
on Molecular Structure(Austin, March 21-23, 1994)
 I am open to cooperation on this and related topics.

PRINCIPLES OF MOLECULAR MODELING FOR TRANSITION METAL SYSTEMS
The well known methods of molecular mechanics (MM) useful for modeling organic
compounds are, in general, invalid for transition metal compounds including
organometallics. The success of the MM methods in application to organic
compounds is due mainly to the electron homogeneity produced by similar snpm
(mostly 2sn2pm) atoms. Because of their d electron participation, transition
metals introduce strong heterogeneity into the system which makes the idea of
force fields with constant parameters inapplicable. The most important effects
of this heterogeneity are: nontransferability, ligand excitation, and
electron-coformational effects  [1].

Nontransferability  of the metal-ligand bond parameters is caused by the
three-dimensional delocalization of the bonding density due to the participation
of d electrons. This results in the strong dependence of any given M-L1 bond on
the other bonds M-L2, M-L3, etc., formed by the same atom M. It means that, in
general, there are no constant characteristics of the M-L bonds to be used in
the force field of the MM methods.

Ligand excitation  is due to the orbital charge transfer from the ligand to the
metal and subsequent back donation of electron density from the metal to the
excited states of the ligand. The partially excited ligands cannot be accurately
modeled in terms of force field parameters or molecular orbital indices derived
for the ground state.

The electron-conformational effects in organometallics are changes in the
conformation of the organic environment due to charge rearrangements in the
metal center. These effects can be of great significance within biological
systems but are not addressed by standard MM methods.

We suggest a method of modeling organometallic and other transition metal
compounds which takes into account all three of the effects described above. In
this method, the electronic structure calculation and geometry optimization are
performed on molecular fragments (including self-consistent inter-fragmentary
interactions). The main fragment (I) consists of the metal and its near-neighbor
environment; the remaining organic groups comprise the secondary fragments (II).
The method includes vibronic coupling effects for fragment I and modified MM for
fragments II. The results are used to introduce a special index of atomic
reactivity in orbital-controlled interactions and then to describe the molecular
system in terms of a 3-dimensional matrix which contains  information about the
electronic structure and geometry of one or more conformations. Comparison of a
series of molecular systems described in this fashion with their corresponding
activities allows one to determine both the electronic parameters and
conformation required for favorable interaction with a receptor site (e.g., to
determine the pharmacophore within a series of bioactive compounds).

1. I.B.Bersuker, Electronic Structure and Properties of Transition Metal
Compounds. Introduction to the Theory, in press.

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Isaac B. Bersuker		 | E-mail: 
Dept. of Chemistry	         | [email-masked]
B
Univeristy of Texas at Austin    | Vox: (512) 471-4671
Austin, TX 78712 	         | Fax: (512) 471-8696
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