Subject: SHAPE OF CYCLOHEXATRIENE: MY CALCS

From: "E. Lewars" <[email-masked]>
Date: Tue, 7 May 1996 12:56:07 -0400 (EDT)

1996 May 7

  Hello,  There has been a discussion of the conformation of 1,3-cyclohexadiene
as calculated by MM and AM1. I got these results, using Spartan:

MM jobs:
1  C2 (nonplanar ring) input from the builder, opt with MM3-->C2, 9.12 kcal/mol

2  Cs (planar ring) input from the builder, opt with MM3-->C2v, 10.38 kcal/mol

AM1 jobs:
3  C2 input from MM3, opt with AM1--> C2, 17.49 kcal/mol, no imaginary freqs

4  C2v (planar ring) input from MM3, opt with AM1--> C2v, 17.64 kcal/mol,
                                     1 imag. freq, 74i cm-1 ("-74" cm-1)

Now, an MM energy cannot be compared with an AM1 energy, since they refer
to different things, but within each class of computation the C2 (nonplanar
ring) geometry has the lower energy, and the AM1 C2 geom is a relative minimum
on the potential energy surface, since it has no imaginary frequencies. The
"small" value of the imaginary freq of the AM1 C2v structure shows that this AM1
transition state is "flat" (not steeply curved along the reaction coordinate),
as might have been suspected from the very small activation energy, and
animating the vibration reveals that the planar C2v molecule wants to twist
to the C2 geometry.
  This little exercise shows (1) the importance of the _input_ geom; an MM
job can throw an input structure into a higher sym, but a semiempirical
or ab initio calc will keep the input symmetry  (2) the importance of
doing a freq job on an optimized structure, to see if it is a relative
minimum or some kind of saddle point (e.g. a transition state=1st-order
saddle point) ON THAT PARTICULAR PES, which is not necessarily the _true_ PES.
Most MM programs do not yet do freq calcs, altho' I think the latest version of
MM3 does.
  Does any one know what _experiment_ tells us about the shape of 1,3-hexadiene?

E. Lewars  Chemistry Dept Trent U, Ontario, CANADA