Subject: Torsion angles

From: [email-masked]
Date: Thu, 19 Jul 2001 12:26:28 +0100

Dear CCLers,

A possible challenge of the day ...  Getting correct [acyclic, of
course] torsion angles in conjugated systems...

If I have missed the general solution to this quandry please forgive the
band width, and let me know.

There have been many methodologies put forth for computing [gas phase]
molecular geometries.  Some are expensive, and some are quite
inexpensive.  They all have parameters [To CI or not To CI, basis set,
USS,UPP, Beta, exponents, etc.]
Some get bond lengths and bond angles correct.  At nearly all levels of
theory it seems difficult to get torsion angles dependably at an
acceptable, approximate, affordable cost.  B3LYP can be an acceptable
fall back when all the semiempirical methods fail, and they seem to in
my experience over >25  years at doing chromophore design.. But B3LYP
usually gets things too flat, i.e.Ph-N=N-Ph, and when there is the
possibility of internal hydrogen bonding, even between SP2 N and SP2 or
Sp3 O, and a H attached to an aromatic ring.  MP2 is only slightly
better.

Some of the best results I have seen were done > 25 years ago when
Birner [Leipzig, I think] simply added to lowly INDO all the 2-center
<pq|rs> computed over Slater functions.....

Yes there has been a lot of progress, but I still see opportunities.

Comments?

John McKelvey