Direct observation of hydrogen molecules adsorbed onto a microporous coordination polymer.

design and synthesis of high-performance hydrogenstorage materials. Although the weakest X-ray scattering amplitude of hydrogen made it difficult to determine their structure, we have succeeded in the first direct observation of elusive H2 molecules adsorbed in the nano-channels of a metal-organic porous material by the in situ synchrotron powder diffraction experiment of gas adsorption and by MEM (maximum entropy method)/Rietveld charge-density analysis [4]. The sample used in this study is microporous coordination polymer 1 with a pillared layer structure (CPL-1) with uniform ordered nano-channels of 4 Å × 6 Å. The in situ synchrotron powder diffraction experiment for hydrogen gas adsorption was carried out using the large Debye-Scherrer camera installed at beamline BL02B2. The hydrogen gas was dosed into the capillary sample through a stainless steel tube connected to the gas-handling system. The amount of adsorbed gas was controlled by adjusting the sample temperature under a constant hydrogen gas pressure of 102 kPa. The structure was investigated by MEM/Rietveld analysis [5]. The amount of H2 molecules adsorbed in CPL-1 at 90 K at 102 kPa was determined to be 0.3 molecules per Cu atom from the hydrogen adsorption isotherm. The reliability (R) factors based on the powder profile RWP and the Bragg integrated intensities RI were 2.45% and 3.33%, respectively. The cell parameters for CPL-1 with H2 gas were determined as a = 4.7101(1) Å, b = 20.0289(2) Å, c = 10.7704(1) Å, and β = 95.472(2)° (space group P21/c). The R factor based on the structure factors in the final MEM analysis RF was 1.86%. In the MEM charge density of CPL-1 without H2 molecules shown in Fig. 1(a), only the nano-channel structure was clearly seen and no electron density was observed in the nano-channels, even at lower electron-density levels. On the other hand, in the MEM charge density of CPL-1 with H2 molecules (Fig. 1(b)), the small peak maxima of the electron densities with an elongated shape distribution, which are due to the adsorbed H2 molecules, were observed in the nano-channels. It should be noted that the posit ion and orientation of the H2 molecules are mean values obtained by statistical analysis. The