This work describes the combined use of synchrotron X-ray diffraction and density functional theory (DFT) calculations to understand the cocrystal formation or phase separation in 2D monolayers capable of halogen bonding. The solid monolayer structure of 1,4-diiodobenzene (DIB) has been determined by X-ray synchrotron diffraction. The mixing behavior of DIB with 4,4′-bipyridyl (BPY) has also been studied and interestingly is found to phase-separate rather than form a cocrystal, as observed in the bulk. DFT calculations are used to establish the underlying origin of this interesting behavior. The DFT calculations are demonstrated to agree well with the recently proposed monolayer structure for the cocrystal of BPY and 1,4-diiodotetrafluorobenzene (DITFB) (the perfluorinated analogue of DIB), where halogen bonding has also been identified by diffraction. Here we have calculated an estimate of the halogen bond strength by DFT calculations for the DITFB/BPY cocrystal monolayer, which is found to be ∼20 kJ/mol. Computationally, we find that the nonfluorinated DIB and BPY are not expected to form a halogen-bonded cocrystal in a 2D layer; for this pair of species, phase separation of the components is calculated to be lower energy, in good agreement with the diffraction results.
Physisorbed monolayers based on relatively weak noncovalent interactions can serve as excellent model systems for understanding crystallization of materials in reduced dimensionality. Here we employ a powerful combination of scanning tunneling microscopy (STM), differential scanning calorimetry (DSC), and computational modeling to reveal two-dimensional (2D) crystallization and mixing behavior of saturated and unsaturated (cis as well as trans) aliphatic primary amides. The foundation of the present work is laid by DSC measurements, which reveal characteristic adsorption and mixing behavior of aliphatic amides. These results are further supported by STM visualization of the adlayers. STM reveals, at submolecular resolution, the adsorption as well as the two-component 2D phase behavior of these molecules at the liquid-solid interface. The saturated and trans-unsaturated amides exhibit random mixing in view of their size and shape complementarity. Binary mixtures of saturated and cis-unsaturated amides, on the other hand, display unprecedented mixing behavior. The linear saturated and bent cis-unsaturated amide molecules are found to mix surprisingly better at the liquid-solid interface than might have been expected on account of the dissimilarity in their shapes. Strong, directional intermolecular hydrogen-bonding interactions as well as the relative stabilization energies of the adlayers are responsible for such unusual mixing behavior. Computational modeling provides additional insight into all the possible interactions in 2D assemblies and their impact on stabilization energies of the supramolecular networks. This study provides a model for understanding the effect of nanoscale cocrystallization on the thin film structure at interfaces and demonstrates the importance of molecular geometry and hydrogen bonding in determining the coadsorption behavior.
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 has p2 symmetry with lattice parameters a ¼ 17.77 Å , b ¼ 13.69 Å and ¼ 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.
The monolayer crystal structure of phenazine adsorbed on graphite is determined by a combination of synchrotron X-ray diffraction and DFT calculations. The molecules adopt a rectangular unit cell with lattice parameters a = 13.55 Å and b = 10.55 Å, which contains 2 molecules. The plane group of the unit cell is p2gg, and each molecule is essentially flat to the plane of the surface, with only a small amount of out-of-plane tilt. Density functional theory (DFT) calculations find a minimum energy structure with a unit cell which agrees within 7.5% with that deduced by diffraction. DFT including dispersion force corrections (DFT+D) calculations help to identify the nature of the intermolecular bonding. The overlayer interactions are principally van der Waals, with a smaller contribution from weak C-H···N hydrogen bonds. This behaviour is compared with that of 4,4′-bipyridyl
The formation of a halogen bonded self-assembled co-crystal physisorbed monolayer containing N···Br interactions is reported for the first time. The co-crystal monolayer is identified experimentally by synchrotron X-ray diffraction and the structure determined. Density functional theory (DFT) calculations are also employed to assess the magnitudes of the different interactions in the layer. Significantly, compared to other halogen bonds in physisorbed monolayers we have reported recently, the N···Br bond here is found to be non-linear. It is proposed that the increasing importance of the lateral hydrogen bond interactions, relative to the halogen bond strength, leads to the bending of the halogen bonds.
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