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Crystal structure of a three-arm model compound of poly(m-phenylene isophthalamide) (PMIA), N,N',N' '-triphenyl trimesamide Phi(CONHPhi)(3), has been analyzed by the X-ray diffraction method. The torsional angles around the bonds connecting the amide group and the central benzene ring are 24-34 degrees , almost the same as those observed for many kinds of aromatic amide compounds, reflecting mainly the intramolecular energetic balance between the amide and benzene groups. On the other hand, the torsional angles around the bonds connecting the amide group and the outer benzene ring were found to distribute over a wide range of 2-51 degrees due to the additional effect of intermolecular interactions. This is the first example to show experimentally clearly the role of intra- and intermolecular interactions in the control of torsional angle around the benzene-amide linkage. The hydrogen bonds are formed between the amide groups of the neighboring molecules, resulting in the construction of three-dimensional network structure. The local packing structure of the three-arm compound was found to be essentially the same as those observed for PMIA and the linear model compounds, indicating a characteristic structural feature of the meta-linkage-type aromatic amide compounds. The energy calculation was made using the software Polymorph Predictor to extract the energetically most stable crystal structure, which was compared successfully with the X-ray analyzed structure.

To clarify the factors that govern the complicated three-dimensional hydrogen-bond network structure of poly(m-phenylene isophthalamide) and its model compounds, the packing energy calculations were performed with computer simulation software (Polymorph Predictor) and the important energy terms were extracted successfully. The crystal structures were predicted with and without the various types of interaction terms being taken into consideration and were compared with the X-ray-analyzed structures. Initially, the conformation analysis was made for a single molecule. However, the structure with the lowest energy did not always correspond to the actually observed structure. This finding suggested the importance of intermolecular, as well as intramolecular, interactions. By performing the lattice-energy calculations with and without various types of intermolecular interactions being taken into consideration, and by comparing the results with the observed structures, it has been found that the van der Waals interaction was a primarily important factor in the prediction of the molecular packing structures; however, the electrostatic (and hydrogen-bond) interaction could not be ignored at all. In other words, a good and sensitive balance between these interaction terms was quite important for obtaining a successful reproduction of the observed molecular packing structures of the model compounds.

Crystal structures have been predicted by using a software Polymorph Predictor and compared with those analyzed by X-ray method for a series of low-molecular-weight aromatic amide compounds as models of poly(m-phenylene isophthalamide) and poly(p-phenylene terephthalamide). For most of the compounds investigated here, the predicted crystal structure of the lowest or the next lowest packing energy has been found to be in good agreement with the X-ray analyzed structure. However, the energy difference was not very large between these energetically most plausible structures and the other less stable candidates, indicating a difficulty of unique prediction of 3D molecular packing structure and a possibility of existence of various types of crystal modification.

Molecular and crystal structures of a series of model compounds of poly(m-phenylene isophthalamide) have been analyzed by the X-ray diffraction method and the various types of 3-D hydrogen bond network structures have been clarified. The twisting angles between benzene and amide groups are in the range of 25−40°, which is common to all of the analyzed model compounds and the parent polymer itself and could be reproduced well by the energy calculation with the nonbonded interatomic interactions between the benzene and amide groups taken into consideration. The molecular conformation, the packing mode of molecules, and the intermolecular hydrogen bond network structure were found to have good correlation with each other and could be classified systematically into several groups. This classification should be important for the energetic interpretation of the formation mechanism of the 3-D hydrogen bond network structure.