The quantum sieving effect between D2 and H2 is examined for a series of metal-organic frameworks (MOFs) over the temperature range 77-150 K. Isothermal adsorption measurements demonstrate a consistently larger isosteric heat of adsorption for D2 vs H2, with the largest difference being 1.4 kJ/mol in the case of Ni-MOF-74. This leads to a low-pressure selectivity for this material that increases from 1.5 at 150 K to 5.0 at 77 K. Idealized adsorption solution theory indicates that the selectivity decreases with increasing pressure, but remains well above unity at ambient pressure. Infrared measurements on different MOF materials show a strong correlation between selectivity and the frequency of the adsorbed H2 translational band. This confirms that the separation is predominantly due to the difference in the zero-point energies of the adsorbed isotopologues.

Diffuse reflectance infrared (IR) spectroscopy performed over a wide temperature range (35-298 K) is used to study the dynamics of H(2) adsorbed within the isostructural metal-organic frameworks M(2)L (M = Mg, Mn, Co, Ni and Zn; L = 2,5-dioxidobenzene-1,4-dicarboxylate) referred to as MOF-74 and CPO-27. Spectra collected at H(2) concentrations ranging from 0.1 to 3.0 H(2) per metal cation reveal that strongly red-shifted vibrational modes arise from isolated H(2) bound to the available metal coordination site. The red shift of the bands associated with this site correlate with reported isosteric enthalpies of adsorption (at small surface coverage), which in turn depend on the identity of M. In contrast, the bands assigned to H(2) adsorbed at positions >3 Å from the metal site exhibit only minor differences among the five materials. Our results are consistent with previous models based on neutron diffraction data and independent IR studies, but they do not support a recently proposed adsorption mechanism that invokes strong H(2)···H(2) interactions (Nijem et al. J. Am. Chem. Soc.2010, 132, 14834-14848). Room temperature IR spectra comparable to those on which the recently proposed adsorption mechanism was based were only reproduced after contaminating the adsorbent with ambient air. Our interpretation that the uncontaminated spectral features result from stepwise adsorption at discrete framework sites is reinforced by systematic red shifts of adsorbed H(2) isotopologues and consistencies among overtone bands that are well-described by the Buckingham model of molecular interactions in vibrational spectroscopy.

Microporous metalÀorganic frameworks possessing exposed metal cation sites on the pore surface are of particular interest for high-density H 2 storage at ambient temperatures, owing to the potential for H 2 binding at the appropriate isosteric heat of adsorption for reversible storage at room temperature (ca. À20 kJ/mol). The structure of Cr 3 (BTC) 2 (BTC 3À = 1,3,5-benzenetricarboxylate) consists of dinuclear paddlewheel secondary building units connected by triangular BTC 3À bridging ligands to form a three-dimensional, cubic framework. The fully desolvated form of the compound exhibits BET and Langmuir surface areas of 1810 and 2040 m 2 /g, respectively, with open axial Cr 2þ coordination sites on the paddlewheel units. Its relatively high surface area facilitates H 2 uptakes (1 bar) of 1.9 wt % at 77 K and 1.3 wt % at 87 K, and a virial-type fitting to the data yields a zero-coverage isosteric heat of adsorption of À7.4(1) kJ/mol. The detailed hydrogen loading characteristics of Cr 3 (BTC) 2 have been probed using both neutron powder diffraction and inelastic neutron scattering experiments, revealing that the Cr 2þ site is only partially populated until a marked elongation of the CrÀCr internuclear distance occurs at a loading of greater than 1.0 D 2 per Cr 2þ site. Below this loading, the D 2 is adsorbed primarily at the apertures of the octahedral cages. The HÀH stretching frequency corresponding to H 2 molecules bound to the primary site is observed in the form of an orthoÀpara pair at 4110 and 4116 cm À1 , respectively, which is significantly shifted compared to the frequencies for free H 2 of 4155 and 4161 cm À1 . The infrared data have been used to compute a site-specific binding enthalpy for H 2 of À6.7(5) kJ/mol, which is in agreement with the zero-coverage isosteric heat of adsorption derived from gas sorption isotherm data.

Magnetotactic bacteria are a group of motile prokaryotes that synthesize chains of lipid-bound, magnetic nanoparticles called magnetosomes. This study exploits their innate magnetism to investigate previously unexplored facets of bacterial hydrodynamics at surfaces. Through use of weak, uniform, external magnetic fields and local, micromagnetic surface patterns, the relative strength of hydrodynamic, magnetic, and flagellar force components is tuned through magnetic control of the bacteria's orientation. The resulting swimming behaviors provide a means to experimentally determine hydrodynamic parameters and offer a high degree of control over large numbers of living microscopic entities. The implications of this controlled motion for studies of bacterial motility near surfaces and for micro- and nanotechnology are discussed.

Diffuse reflectance infrared (IR) spectroscopy was used to study the structure and dynamics of H 2 and CO 2 adsorbed within the isostructural metal−organic frameworks M 2 L (M = Mg, Mn, Fe, Co, Zn; L = 2,5-dioxidobenzene-1,4-dicarboxylate) referred to as M-MOF-74 and CPO-27-M. Cluster models of the primary adsorption site were excised from periodic models that were optimized using plane-wave density functional theory at the Perdew−Burke−Ernzerhof (PBE) level. Models incorporating an adsorbed H 2 or CO 2 were optimized using dispersion-corrected density functional theory (DFT), and the anharmonic vibrational frequencies of the adsorbate were calculated using the discrete variable representation method. The calculated vibrational frequency shifts reveal the same trend among the M 2 L materials as those observed experimentally and provide insight into the origins of these shifts. Our experimental spectra of adsorbed CO 2 confirm a unique blue shift of the ν 3 mode for molecules adsorbed in Mg 2 L, while the frameworks assembled from transition metals induce a red shift. By shifting the focus to the CO 2 local vibrational modes, a deeper insight into the influence of "back bonding" (metal d-electron density donation into CO 2 π* orbitals) is revealed; for Mg 2 L there is a nearcomplete cancellation of the opposing local mode contributions to the observed frequency shift. Additional spectral features in the CO 2 ν 3 region are assigned to (1) the ν 3 mode of the 13 CO 2 isotopologue, (2) a combination mode involving a ν 2 excitation, and (3) librational sidebands arising from center-of-mass motion of the adsorbed molecule on the surface. Interestingly, below 100 K we observe the appearance of a new band that is distinct from the primary ν 3 band observed at room temperature. This band is attributed to an alternate, localized orientation of CO 2 adsorbed to the metal site, which is supported by the DFT model.