Conformational Isomerism in the Isoreticular Metal Organic Framework Family: A Force Field Investigation

For the family of isoreticular metal organic frameworks (IRMOFs), a supramolecular conformational isomer, where the carboxylic planes in one linker are orthogonal to each other, is in principle possible. This polymorphic form has been investigated by means of molecular mechanics calculations using an accurate MM3 type force field. In the case of the parent structure IRMOF-1, the regular form is more stable by 17.8 kcal/mol (per formula unit) than this isomer. However, due to steric repulsions, in the case of the naphthalene and anthracene based IRMOF-7 and 993, this is reduced to only 2.0 and 5.6 kcal/mol, respectively. Due to a complex network of interlinker interactions, a broad variety of different rotational isomers is predicted, and only by using a genetic algorithm search algorithm the global minimum structure could be located. The low energy difference between the isomeric forms explains the disorder observed for IRMOF-7 experimentally. For IRMOF-10 and 16 with a flexible biphenyl bond in the linker, an even lower energy separation of 2.7 and 0.4 kcal/mol, respectively, is observed. The study demonstrates that theoretical calculations can be a viable tool in order to determine the actual atomistic structure of more complex porous functional materials.