[scienceguy]

If you had say 2 molecules of benzene and they were parallel to each other and you brought them close together, would the conjugated pi bonds repel each other or would they form bonds?

I'm thinking of benzene molecules being stacked like plastic chips. Would they bond or repel?

[orgopete]

I hope someone contributes more data on this. I seem to recall there is some kind of experimental data (UV?) for this parallel interaction, yet I also seem to recall that they would be perpendicular, my guess.

The parallel interactions that occur with various tethers. I'm sure donor-acceptor types are probably known. What about phenyl-phenyl?

[Corribus]

Simple answer: there would be a bonding and antibonding interaction.

Not so simple answer: There are lots of possible interactions and the nature of the interaction can be modeled to some degree using exciton coupling theory. Obviously the strength of the interaction will depend significantly on, among other things, the mutual orientations of the pi-systems and their distance apart. In the case that pi-systems would be brought together in a mutually perpendicular orientation, there'd be no interaction because of symmetry.

Though not benzene, a classic example of these kinds of interactions is pyrene, which forms efficient excimer interactions. For this reason the fluorescence properties of pyrene are extremely sensitive to concentration. An example that may be important in biology is the through-space coupling of chromophores in the antenna complex of the photoreaction system responsible for photosynthesis. The interactions, which behave a little like J- and H-aggregates, are thought to play a role in the fast transfer of energy and electrons in these systems. Porphyrin complexes and conducting polymers are also well known to exhibit these kinds of interactions.

Of the top of my head I don't recall how these interactions manifest in benzene. I'd have to look through my considerable library of papers on this topic. If you're interested...
But, you may be interested in looking into the enormous volume of work that has been done on conjugated oligomers attached to para-2.2-cyclophanes, which are basically benzenes forced on top of each other. I can't for the life of me remember the guys name who started all of it; if I can think of it, I'll post it here. A quick google search for "cyclophane and 'conjugated oligomer'" reveals a lot of reviews on the topic, though.

[Enthalpy]

Solid benzene makes a complex pattern where each molecule is perpendicular to all neighbours, hence parallel molecules don't attract much if at all.

Graphite planes are more or less parallel but the atoms are offset, not facing an other, and the planes attract an other so little that adhesive tape separates a graphene foil. Graphite is full of stacking faults, so much that its density never matches the theory, and by much. So, even with molecules much bigger than benzene, the bonds perpendicular to the aromatic rings are very weak.

[scienceguy]

Thanks for the responses! Enthalpy's answer is what I was looking for. I found this wikipedia page also relevant:

https://en.wikipedia.org/wiki/Pi_interaction

[clarkstill]

Solid benzene makes a complex pattern where each molecule is perpendicular to all neighbours, hence parallel molecules don't attract much if at all.

I'm not sure you can draw that conclusion. Firstly, crystal structures don't always represent a thermodynamic minimum, so it could just be kinetically more favourable to form this structure. Secondly, even if this is the thermodynamic minimum it doesn't mean that the face/face interaction isn't attractive, merely that the edge/face one is moreso.

[Babcock_Hall]

I found this abstract from 1998 helpful (and the full text is available):  "A representative set of high resolution x-ray crystal structures of nonhomologous proteins have been examined to determine the preferred positions and orientations of noncovalent interactions between the aromatic side chains of the amino acids phenylalanine, tyrosine, histidine, and tryptophan. To study the primary interactions between aromatic amino acids, care has been taken to examine only isolated pairs (dimers) of amino acids because trimers and higher order clusters of aromatic amino acids behave differently than their dimer counterparts. We find that pairs (dimers) of aromatic side chain amino acids preferentially align their respective aromatic rings in an off-centered parallel orientation. Further, we find that this parallel-displaced structure is 0.5-0.75 kcal/mol more stable than a T-shaped structure for phenylalanine interactions and 1 kcal/mol more stable than a T-shaped structure for the full set of aromatic side chain amino acids. This experimentally determined structure and energy difference is consistent with ab initio and molecular mechanics calculations of benzene dimer, however, the results are not in agreement with previously published analyses of aromatic amino acids in proteins. The preferred orientation is referred to as parallel displaced pi-stacking."
http://www.ncbi.nlm.nih.gov/pubmed/9624131

It sounds as if the relative stability depends upon whether one has a dimer or a higher order complex.

[kriggy]

I think you might find some of the answers in some of the works of Pavel Hobza, as far as I remember he did lots of work in field on non-covalent interactions
HOBZA, Pavel and ZAHRADNÍK, Rudolf. Intermolecular Complexes: the Role of van der Waals Systems in Physical Chemistry and in the Biodisciplines. 1. ed. Praha: Academia, 1988.
HOBZA, Pavel and MULLER-DETHLEFS, Klaus. Non-Covalent Interactions. Theory and Experiment. Book Series: RSC Theoretical and Computational Chemistry Series, 2010.

[Enthalpy]

[...] it doesn't mean that the face/face interaction isn't attractive, merely that the edge/face one is moreso.

Yes, that's what I had written too.

[Enthalpy]

[...] interactions between aromatic amino acids [...]

But amino acids interact through hydrogen brigdes which tend to be strong. Wouldn't these overshadow the interactions of the aromatic rings?

[Babcock_Hall]

I am not sure what you mean by hydrogen bridges.  Do you mean hydrogen bonds?

[Enthalpy]

Yes, hydrogen bonds, my bad.

Between the oxygen atoms and the hydrogens located on nitrogen. These bonds give the intermolecular force between polyamide macromolecules for instance, and are said to overshadow other forces between aramides.

[Babcock_Hall]

Although hydrogen bonds are important in the secondary structure of proteins, there are any number of important forces, such as the hydrophobic force that leads to packing the nonpolar side chains into the core of a globular, water-soluble protein.  And the folded state of ribonuclease, for example, is not much more stable than the unfolded state.  So, pi-stacking or other effects could still be important.