Very basic xtallography questions

Rob W. W. Hooft rob at hooft.net
Wed Jan 12 02:16:03 EST 2000


>>>>> "JO" == John Osborn <j.c.osborn at br.ac.uk> writes:

 JO> Rob W. W. Hooft wrote:
 >>  "BV" == Bev Vincent <MaxDevore at hotmail.com> writes:

 BV> To build a crystal in theory, you need the unit cell dimensions,
 BV> space group and the contents of the asymmetric unit of the unit
 BV> cell (i.e. tell where all the atoms are). However, there is no
 BV> way to determine the crystal faces from this information. A
 BV> crystal's habit (description of the crystal shape) is as much a
 BV> characteristic of the substance as its color or unit cell, though
 BV> the habit may be altered by changing the crystallization method
 BV> (different solvent, for example). The only way to determine the
 BV> faces is through experimental methods.

 >>  True, but I'd like to add that using energy considerations it is
 >> possible to "guess" what the crystal habit might become knowing
 >> only the unit cell and the space group extinctions. These methods
 >> assume that the crystal grows one layer at a time, and that the
 >> attachment energy is isotropic.

 JO> If you try to guess the habit knowing only the cell dimensions
 JO> and space group (i.e. no atomic coordinates) then you cannot make
 JO> use of energy considerations.

I was very inaccurate in wording. I meant to say that these theories
assume that the energy of layer attachment is only dependent of 
the contact surface area. Of course there are "real" energy methods
which require much more information. Sorry for being confusing.

 JO> I am not sure what you mean by saying that the attachment energy
 JO> is assumed to be isotropic.

The models I was referring to fail horribly if an organic crystal
structure contains a one-dimensional infinite hydrogen bond network:
Faces on the predicted crystal might disappear or grow much faster
since the attachment energy in the direction of the hydrogen bond is
much higher than in the other directions.

Thank you for correcting me,

Rob Hooft
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