[katatonic]

the nitrogen atoms are trigonal planar, while the Arsenic atoms are trigonal pyramidal..the structure of this molecule shows that two arsenic atoms are equivalent by symmetry but the other is not. how is this possible?

[BetaAmyloid]

<img src=http://img258.imageshack.us/img258/9613/ringsystem.gif> <img src=http://img20.imageshack.us/img20/5888/ringsystem2.gif>

If you look at this, the arsenics and nitrogens are in a boat shape structure.

If you compare arsenics to nitrogens, arsenics are trigonal planar while the nitrogens are trigonal pyramidal (causing one of them to offset from the other two), while on the other hand, if you compare nitrogens to arsenics, nitrogens are trigonal planar while the arsenics are trigonal pyramidal (causing one of them to offset from the other two).

I'm assuming that this program works correctly, and I am assuming that they are referring to the structures of the atoms relative to one another (one as a focal point).

This is the only explanation I have - if this isn't correct, then I do not see how the arsenics are offset. 

[aurophilic]

Are you certain that the N are trigonal planar or is that given to you in the problem. One would normaly expect that they also be trigonal pyramidal. The molecule itself should be in a chair conformation.

[BetaAmyloid]

Are you certain that the N are trigonal planar or is that given to you in the problem. One would normaly expect that they also be trigonal pyramidal. The molecule itself should be in a chair conformation.

That's what I was saying, unless they are comparing the N and As molecules relative to one another.