Hierarchies of Intermolecular Potentials and Forces: Progress Towards a Quantitative Evaluation

X-ray diffraction and theoretical studies for the quantitative assessment of intermolecular arene-perfluoroarene stacking interactions

The arene-perfluoroarene stacking interaction was studied by experimental and theoretical methods. A series of compounds with different possibilities for formation of this recognition motif in the solid state were synthesized, and their crystal structures determined by single-crystal X-ray diffraction. The crystal packing of these compounds, as well as the packing of related compounds retrieved from crystallographic databases, were analyzed with quantitative crystal potentials: total lattice energies and the cohesive energies of closest molecular pairs in the crystals were calculated. The arene-perfluoroarene recognition motif emerges as a dominant interaction in the non-hydrogen-bonding compounds studied here, to the point that asymmetric dimers formed over the stacking motif carry over to asymmetric units made of two molecules in the crystal both for pure compounds and for molecular complexes; however, inter-ring distances and angles range from 3.70 to 4.85 A and from 5 to 21 degrees , respectively. Pixel energy partitioning reveals that whenever aromatic rings stack, the largest cohesive energy contribution comes from dispersion, which roughly amounts to 20 kJ mol(-1) per phenyl ring, while the coulombic term is minor but significant enough to make a difference between the arene-arene or perfluoroarene-perfluoroarene interactions on the one hand, and arene-perfluoroarene interactions on the other, whereby the latter are favored by about 10 kJ mol(-1) per phenyl ring. No evidence of special interaction which can be attributed to HF confrontation was recognizable.

A Theoretical Study of Cohesion, Structural Deformation, Inclusion, and Dynamics in Porous Hydrogen-Bonded Molecular Networks

Molecules with multiple sites of hydrogen bonding attached to suitable cores tend to crystallize as open networks. The resulting crystals can have the following unusual properties: They can include significant amounts of guest molecules; the guests are typically located in channels and can be exchanged without loss of crystallinity; and the geometry of the networks can change in response to new guests. We have found that DFT calculations can provide accurate simulations of the unusual structure and properties of such materials, represented by crystals of prototypic tetrapyridinone 1. These calculations have yielded three key insights that cannot be obtained directly from experiments. (1) The hypothetical porous network obtained by removing guests from crystals of compound 1 is highly flexible, and its deformations are inherently anisotropic, leading to lengthening or shortening of the channels along the c axis and no significant changes along the a and b axes. (2) Quantitative analysis of the total cohesive energy has revealed that hydrogen bonding within the network makes a dominant contribution, along with interactions of guests with the network. (3) Differences in the overall stability of crystals of compound 1 as the guests are varied do not arise primarily from significant changes in the cohesive energy of the network itself; instead, differences in guest-guest interactions play a key role, resulting from the nature of the guests and constraints imposed by the surrounding network. These insights, together with the results of ab initio molecular dynamics, help explain how hydrogen-bonded networks can be robust yet permit molecular movement that underlies the exchange of guests and adaptive porosity. These insights promise to be of general value to scientists studying ordered molecular materials in which strong directional interactions are prominent.