Subject: Light induced torsional movement

From: <[email-masked]>
Date: Thu May 19 14:25:39 1994

Hi everyone

I have a very interesting system in which an apparently torsional movement 
(by 180 degrees), A-->B, is induced by UV light, and the reverse 
transformation by heat. I want to eludidate the precise path and mechanism 
for this by the semi-empirical methods AM1 and PM3.
 
I feel sure that this molecule is leading me into murky waters, and would 
appreciate your comments.

The relevant part of the molecule is shown below :
    
        H                   C=N
        |                      \
        N      ------>          N-Ph
  \    / \                      |
   C=N   Ph                     H
  /
      (A)                      (B)

Hyperchem's CIS (with 5 filled and 5 unfilled orbitals) calculation indicated 
that an excited singlet is produced by UV light, so I optimized this with RHF 
PM3 (MOPAC 6) from the xtal structure with the keyword EXCITED (i.e. CI 
with the homo and lumo)

Lo, a large torsional movement is indeed produced (about 90 deg), but 
around the N-N bond (not what I expected).  The program stops at a high 
gradient (about 17 kcal/mol/A) and says the HOF is constant.  I'm unable 
to optimize this any further..

The CI states reported are very close together now (eV):
STATE ENERGIES  EXPECTATION VALUE OF S**2  S FROM S**2=S(S+1)
   1   -3.306163   TRIPLET         2.00000   1.00000
   2   -3.218167   SINGLET         0.00000   0.00000
   3   -3.202571   SINGLET         0.00000   0.00000
   4    0.898535   SINGLET         0.00000   0.00000

(Simple ground state RHF or UHF optimization of this structure returns it to 
the starting structure A.)

** Should I be optimizing a triplet state from here on ?
(This does produce a large torsional movement around C=N, the one I was 
looking for, but again stops at high gradient)

** What if there is no clear indication of how many frontier orbitals to 
include in the CI ?

** Is the overall 'plan' to follow the movement sound ?

Your help is much appreciated.

Charles Marais

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