Subject: Diphenylamine and rules of thumbs

From: [email-masked] (Alexandre Hocquet)
Date: Tue, 7 Jan 1997 19:05:46 +0400

>>I would be interested if there is a 'rule of thumb' about differences
>>in bond length (and angles) between solid and gase phase organic
>>molecules.
>
>        We can actually do better than a rule of thumb.  MM3 (that is,
>Allingers original MM3, none of the copies, this is the Mac version) can
>calculate the difference between bond lengths from different determination
>methods
> (I'm not entirely sure how much
>the nitrogen lone pair will affect the position in the X-ray though).

>>Specifically, we are interested in diphenylamine and would
>>like to know if one can use the solid state (x-ray) data and somehow
>>scale it to a gas phase value.
>
>        One problem here, the usual with MM3.  We don't have all
>parameters.  However, using an "amide" (sp2) nitrogen and guessing a single
>parameter, I calculated the following table for diphenyl amine:
>
>        Per-Ola Norrby
>

Dear Gert and Per-Ola, dear cclers,

The problem with diphenylamine is the status of the nitrogen. Should we
consider it as trigonal or tetrahedral ? Per-Ola took an amide N because it
was the case 
that minimised parameter guessing.

I suppose a tetrahedral amine would have been more convenient to this kind
of molecule, guessing the potential energy surface would look like the
diphenylmethane or phenoxybenzene ones. Obviously, the lack of parameters
did
not allow that.

Why is it so annoying ? As Per-Ola pointed out, the position of the N lone pair
could have influence upon the the measurement of bonds distances based on 
Xray techniques.

Actually, Allinger et al. (Theochem, 312 (1994), p69-83) do give a rule of thumb
 (Aaaah ! at last, he comes to the point) for converting bonds from electron
diffraction into microwave or Xray .

It is quite different for a C-Nsp3 and a C-Nsp2 bond :

         Xray (298K)  Xray (100K)     MW
C-Nsp3   +0,002         -0,002      +0,004
C-Nsp2   +0,007         +0,003      +0,004   (in Angstroms)

Hope this helps...

Alexandre Hocquet

Laboratorio de Cristalografia
Facultad de Ciencias Fisicas
Universidad de Chile
Blanco Encalada, 2008
Santiago
CHILE
fax : 56 2 696 73 59