Subject: CCL:planar pi systems

From: "Jens Spanget-Larsen" <[email-masked]>
Date: Tue, 1 Apr 1997 08:51:39 +0100

Rudy Gostowski:

> As an organic chemist I tend to expect planar pi systems to dominate other
> structural influences.
> However, when trying to model dimers having severe steric strain between the
> monomers I find that the pi systems will be bent.
> This is a way to reduce repulsions between the monomers.
> My question is should I constrain the pi systems to be planar by setting
> the relevant dihedrals ?

No, don't constrain the pi-systems to be planar.  Pi-systems are 
quite flexible and can bend and twist without loosing their 
characteristic conjugative properties.  See f.ex. Thulstrup et al., 
Theor. Chim. Acta 89, 301 (1994). 

> Does a program such as MOPAC find the true minimum in this case ?

This depends on what you mean by "true minimum".  MOPAC will find the 
equilibrium structure consistent with the model Hamiltonian applied.

Yours, Jens >--<

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JENS SPANGET-LARSEN  
Department of Chemistry      Phone:  +45 46757781 + 2710
Roskilde University (RUC)    Fax:    +45 46757721 
P.O.Box 260                  E-Mail: [email-masked]
DK-4000 Roskilde, Denmark    http://frederik.ruc.dk/dis/chem/psos
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