The compounds in question, which Yaghi calls IRMOFs (short for "isoreticular metal organic frameworks"), have OZn₄ as their basic construction unit, the oxygen being at the center of a tetrahedron of zinc atoms. The zinc atoms are then interconnected through a variety of organic compounds that consist of benzene rings with diametrically opposed carboxylic acid groups and these define the sides of a cubic lattice. Paper #2 shows 16 such compounds of which the one with a cyclobutyl ring fused to the benzene group, labelled IRMOF-6, was best at storing methane. Compounds were made by heating solutions of zinc nitrate dissolved in the solvent dimethylformamide with the dicarboxylic acid, at around 100° C, in a closed vessel for several hours. The structures of all of the compounds were determined by X-ray crystallography.

The IRMOF with the largest cavity had a chain of three benzene rings separating the two carboxylic acid groups, and this could in theory accommodate a sphere of diameter 29 Å. Indeed, so voluminous are the voids in this compound that it has the lowest density of any crystalline solid ever recorded: 0.21 g/cm³. Nor do IRMOFs rely on their voids being packed with molecules to support the framework; they retain their shape even when their pores are empty, and are robust enough to be heated to 400° C. Similar compounds have been made in the past but their molecular scaffolding crumpled once their supporting molecules were removed.

Not surprisingly, the IRMOFs are being actively tested and developed by chemical, petrol, and auto companies as a means of storing gases such as hydrogen and methane. The compound most investigated for paper #2 was IRMOF-6, whose ability to absorb various vapors was assessed. For two of these, carbon tetrachloride and methylene chloride, the weight of vapor absorbed even exceeds the weight of the compound itself, reaching 1.2 g/g, whereas for other vapors, such as methane, benzene, and cyclohexane, the limit is 0.8 g/g. Under atmospheric pressure, the methane storing capacity of IRMOF-6 is 155 ccs/g but this can be increased to a maximum of 240 ccs/g by increasing the pressure to 36 atmospheres. This is well below the pressure of methane in conventional methane cylinders (200 atmospheres).

It is the uniqueness of Yaghi’s compounds that is attracting attention, and he speculates that they are only the first examples of a series of microporous compounds: "We are systematically changing the metrics and chemical functionality in periodic, crystalline, networks without destroying the original structure topology, and this is what makes IRMOFs unique," Yaghi tells Science Watch.

Currently his group is working on materials with even larger pores, and he sees what they are doing as only the beginning: "We are laying the conceptual foundations for reticular chemistry, which, as exemplified by IRMOFs, represents an approach to logical synthesis that has led to materials with dramatically enhanced porosity and specific surface area. We are now beginning to see its applicability to linking together other building blocks such as proteins and other biomolecules."