[jeffmoonchop]

Anyone know anything about the kinetics of π-π stacking in crystals?

I have a new cocrystal which I've done some UNI potentials analysis looking at the growth rate of faces and found that the fastest faces grow primarily by pi stacking. I know that the strength of these intermolecular bonds is weaker than H-bonds.

The second fastest growing face is the one with the strongest total interaction energy (by H-bonding). This face also grows with the ease of integration due to easy sterics.

The slowest faces are due to low interaction energy but mostly the difficulty of integrating due to sterics. The slower faces need to rely on surface nucleation because the molecules are oreinted perpendicular to the face.

Do the sterics play such a large part of the growth rate of faces to the extent that flat molecules will just stack first rather than H-bond end to end? My question is mostly about why pi stacking is found to be fastest. It seems that interaction potentials are not the only factor to consider.

[Enthalpy]

Hi jeffmoonchop, take my comment here with caution, as I only make an analogy with semiconductors, which may not apply to organics.

In semiconductor crystals, growth does not result primarily from isolated atoms that stick on a surface. Much more, new atoms stick at steps between superimposed crystal planes, where the atoms can make bonds to more atoms already in the crystal. So, the growth results from the sidewise propagation of the steps. In this situation, the strength of bonds that are not perpendicular to the growing face determine the speed.

Beware this can be completely off in your situation.

[Corribus]

I think you may have to consider the number of interactions (or, the available surface area for interaction, or the probability that two faces will come into contact) in addition to the strength per interaction. Recall, kinetics and thermodynamics are related but the former is not necessarily a slave to the latter.

[jeffmoonchop]

In semiconductor crystals, growth does not result primarily from isolated atoms that stick on a surface. Much more, new atoms stick at steps between superimposed crystal planes, where the atoms can make bonds to more atoms already in the crystal. So, the growth results from the sidewise propagation of the steps. In this situation, the strength of bonds that are not perpendicular to the growing face determine the speed.

Hi thanks for the answer. Yes normally growth does occur like this, however, the direction of growth of these steps are normally towards a corner which is itself a face which is normally the fastest growing face. The face growing perpendicular to the sideways growing step is normally slower because it needs to wait until the sideways steps have been occupied more fully before another layer can begin. How do these new layers begin? Isnt it normally surface nucleation?  For example for a needle morphology, the fastest faces by far are the ends of the needle but the slower large faces grow so slowly because of the unlikely event of surface nucleation to produce a new layer. Else how would the plate get fatter?

Sorry for going on a bit but the slowest face of one of my samples doesnt seem to have any interactions between layers (or very little) and only grows due to the growth of other faces. Is this what you mean at the end of your comment?

I think you may have to consider the number of interactions (or, the available surface area for interaction, or the probability that two faces will come into contact) in addition to the strength per interaction. Recall, kinetics and thermodynamics are related but the former is not necessarily a slave to the latter.

Yes thats what I was thinking. Just needed to get my explanations correct for when I publish.

[Enthalpy]

Yes, that's what makes me wary of my comparison. When making a 1µm thick epitaxy on a wafer several inches wide, and possibly cut a bit tilted to favour the epitaxy, nucleation sites are so abundant that they don't limit the speed. But on small crystals, all fast-growing directions are quickly complete, and then the crystal shows only its slow-growing faces.

[Enthalpy]

I think you may have to consider the number of interactions (or, the available surface area for interaction, or the probability that two faces will come into contact) in addition to the strength per interaction.

I strongly suppose that symmetry, in addition to flat faces, makes the abnormally high melting points of benzene and cubane. A liquid molecule arriving at the solid surface fits with many possible orientations, and this favours the growth of the solid. It makes the estimation of the melting point no easier.