Subject: CCL:G03 Structure Optimization Problem

From: Wayne Fisher <wfisher~at~utdallas.edu>
Date: Thu, 03 Jun 2004 16:13:08 -0500

Ladies and Gentlemen,

I am using Gaussian 03 to look for energy minima and first order saddle
points in a small molecule undergoing an Sn2 reaction with or without
the presence of an associated lithium ion.  When the lithium ion is not
present I have no trouble finding stable reactants and products, their
conformational isomers, and transition states between them, both in the
gas phase or in a pcm-model solvent.  When the lithium ion is present I
can find energy minima and first order saddle points only in the gas
phase.  With a pcm-model solvent present the best I have been able to do
is second order saddle points.

In the second order saddle point structures the lithium is associated
with the oxygen of a carbonyl group.  The imaginary modes are fairly low
frequency, around -80 to -130.  The corresponding motions are primarily
the lithium moving at right angles to the O-Li axis and the two modes
are at right angles to each other.

In attempts to find minima and first order saddle points I have tried
the following.

A variety of input structures including those that optimize in the gas
phase with the lithium and those with lithium added to a structure that
optimizes without the lithium.

Distorting the input structure in the direction indicated by one of the
imaginary modes, or their vector sum.

Various options in the route section including
maxstep=1 or the default maxstep=30 option to the opt keyword
opt to a minimum or to a transition state with the calcfc or calcall
option for force constants (opt=(ts, calcfc) or opt=(ts,calcall))
using the maxcycle=100 option to the opt keyword
Including the nosymmetry keyword or not.

Can anyone give me any other ideas?

Thanks,

Wayne Fisher
wfisher-at-utdallas.edu
The University of Texas at Dallas