First Rule of Crystallography

1
First Rule of Crystallography
The Quality of the final structure is ultimately
determined by the quality of the crystal used!!
Corollary 1 Generally, the bigger the crystal
the better the quality Corollary 2 Small
crystals usually suffer from problems beyond
dwarfism Robert Schiedt, Notre Dame
2
Some Comments on Crystals

• Crystals should be single.
• The opposite of single is twinned.
• Needles may not be bundles
• Plates may not be layered
• Crystals can be too big gt 0.5mm
• The measure of crystal quality is mosaicity

3
Energy of Crystallization

• DGDH TDS
• For a process to occur DG must be negative
• The entropy of crystallization is always
  negative!
• Therefore the enthalpy must be more negative.
• Must maximize energy by packing

4
Crystal Forces

• Covalent BondingDiamond
• Ionic InteractionsNaCl
• Hydrogen Bonding
• Dipole-Dipole Interactions
• Pi-Pi interactions
• Van der Wals Interactions

5
Consequences

• Conformation in the crystal may be very different
  from solution, gas phase.
• Acidic protons may not be where expected from pKa
  data.
• Inversion centers are always desirable.
• Minor products may form the crystal

6
Macro Energy
Generally the strongest interactions occur
along the shortest crystal dimensions One very
short axisneedles One very long axis--plates