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Thiswork describes the combined use of synchrotron X-ray diffraction anddensity functional theory (DFT) calculations to understand the cocrystalformation or phase separation in 2D monolayers capable of halogenbonding. The solid monolayer structure of 1,4-diiodobenzene (DIB)has been determined by X-ray synchrotron diffraction. The mixing behaviorof DIB with 4,4′-bipyridyl (BPY) has also been studied andinterestingly is found to phase-separate rather than form a cocrystal,as observed in the bulk. DFT calculations are used to establish theunderlying origin of this interesting behavior. The DFT calculationsare demonstrated to agree well with the recently proposed monolayerstructure for the cocrystal of BPY and 1,4-diiodotetrafluorobenzene(DITFB) (the perfluorinated analogue of DIB), where halogen bondinghas also been identified by diffraction. Here we have calculated anestimate of the halogen bond strength by DFT calculations for theDITFB/BPY cocrystal monolayer, which is found to be ∼20 kJ/mol.Computationally, we find that the nonfluorinated DIB and BPY are notexpected to form a halogen-bonded cocrystal in a 2D layer; for thispair of species, phase separation of the components is calculatedto be lower energy, in good agreement with the diffraction results. [ABSTRACT FROM AUTHOR]

The crystalline monolayer of 1,2-bis(4-pyridyl)ethylene physisorbed on a graphite surface at 0.44 monolayers coverage has been observed and characterized by synchrotron X-ray diffraction and differential scanning calorimetry. The experimentally determined monolayer structure hasp2 symmetry with lattice parametersa = 17.77 Å,b = 13.69 Å andν = 39.7°. The unit cell contains two molecules, which are oriented in a plane parallel to the surface. It is proposed that the molecules are arranged such that they are able to form a weak C–H ··· N hydrogen bond between pyridine groups. The monolayer melts at 414 K, which is unusually close to the bulk melting point for a sub-monolayer coverage system. This molecule is chiral when adsorbed on the surface, but both isomers appear in the unit cell leading to no overall chirality in the monolayer. [ABSTRACT FROM AUTHOR]