Subject: HyperChem: Incorrect geometries in amines

From: Paddy Kane <[email-masked]>
Date: Mon, 1 Dec 1997 22:03:35 +0000 (GMT)

Hi,

 I am a user of HyperChem, version 4. Whenever I use molecular mechanics
with the MM+ force field, to geometry optimise molecules such as
Phenylamine (C6H5NH2), the geometry about the N atom is planar rather than
pyramidal, i.e., an atom type of N2 is used for the nitrogen atom, rather
than N3 as would be expected. I get the same result with molecules such as
H2C=CHNH2 or H2C=NNH2. I also get incorrect geometries if I initially use
the model builder to obtain an approximate geometry for such molecule or
if I use any of the other three force fields available within HyperChem.
If I initially set the atom type for the nitrogen atom to be N3, I still
get an planar geometry about the nitrogen atom. 

 However, for molecules such as methylamine and cyclohexamine, i.e., where
the atom bonded to the nitrogen atom is saturated, the correct geometry
and atom type are obtained for the nitrogen atom. 

 Has anyone else experienced this problem?

 I would be very grateful if someone could advise me of how to solve the 
problem. 

 Kind rgds,
 Paddy.

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