BITNET mail follows
GEOMETRY OF NITROGEN MOLECULES
A number of reports of problems with calculating the geometry of
amines using mol mech and semiempirical and small basis set ab initio
calc have recently been aired on the network. I would like to refer
interested workers to a paper by Boggs and NIU. J.of COMP CHEM. vol
SIX pages 46-55 (1985) AB initio calc of amine out of plane Angles.
This paper shows that very large basis sets with very siffuse s and
p functions are needed on ALL atoms to solve this problem. I would
infer from this that one should not be too upset if one gets incorrect
results from cruder methods which are incapable of describing the
" delicate balance between lone pair effects and delocalization"
These methods do have severe limitations and this is clearly one
of them. Something we have to learn to live with, despite the apparent
early succsses of thirty years ago by Coulson, Hameka and Stephen Mason
and others on the hybridization of amino compounds.
On another topic The recently raised problem of why G90 would not
give a planar (d3h)structure even though to all intents and purposes
the trans state exists and could be gotten from a calc in C3v
symm. It looks like another case of the need for extreme accuracy
in inputting coordinates in this program. TRY using the NOSYMM option
and see what it gives. This is a common problem formolecules with
tetrahedral symmetry. Using bond dist and angles as input it is
common for the program to end in errors because of apparent loss in
symm.This problem was discussed a few months ago on the network.
John E. Bloor(PA13808 at UTKVM1)