The air-free reaction between FeCl 2 and H 4 dobdc (dobdc 4À = 2,5-dioxido-1,4-benzenedicarboxylate) in a mixture of N,N-dimethylformamide (DMF) and methanol affords Fe 2 (dobdc) 3 4DMF, a metalÀorganic framework adopting the MOF-74 (or CPO-27) structure type. The desolvated form of this material displays a BrunauerÀEmmettÀTeller (BET) surface area of 1360 m 2 /g and features a hexagonal array of onedimensional channels lined with coordinatively unsaturated Fe II centers. Gas adsorption isotherms at 298 K indicate that Fe 2 (dobdc) binds O 2 preferentially over N 2 , with an irreversible capacity of 9.3 wt %, corresponding to the adsorption of one O 2 molecule per two iron centers. Remarkably, at 211 K, O 2 uptake is fully reversible and the capacity increases to 18.2 wt %, corresponding to the adsorption of one O 2 molecule per iron center. M€ ossbauer and infrared spectra are consistent with partial charge transfer from iron(II) to O 2 at low temperature and complete charge transfer to form iron(III) and O 2 2À at room temperature. The results of Rietveld analyses of powder neutron diffraction data (4 K) confirm this interpretation, revealing O 2 bound to iron in a symmetric sideon mode with d OÀO = 1.25(1) Å at low temperature and in a slipped side-on mode with d OÀO = 1.6(1) Å when oxidized at room temperature. Application of ideal adsorbed solution theory in simulating breakthrough curves shows Fe 2 (dobdc) to be a promising material for the separation of O 2 from air at temperatures well above those currently employed in industrial settings. ' INTRODUCTIONWith over 100 million tons produced annually, O 2 is one of the most widely used commodity chemicals in the world. 1 Its potential utility in processes associated with the reduction of carbon dioxide emissions from fossil fuel-burning power plants, however, means that the demand for pure O 2 could grow enormously. For implementation of precombustion CO 2 capture, pure O 2 is needed for the gasification of coal, which produces the feedstock for the waterÀgas shift reaction used to produce CO 2 and H 2 . 2 In addition, oxyfuel combustion is receiving considerable attention for its potential utility as an alternative to postcombustion CO 2 capture. Here, pure O 2 is diluted to 0.21 bar with CO 2 and fed into a power plant for fuel combustion. Since N 2 is absent from the resulting flue gas, the requirement for postcombustion separation of CO 2 from N 2 is eliminated. 3 The separation of O 2 from air is currently carried out on a large scale using an energy-intensive cryogenic distillation process. 4 Zeolites are also used for O 2 /N 2 separation, 5 both industrially and in portable medical devices; however, this process is inherently inefficient as the materials used adsorb N 2 over O 2 with poor selectivity. By employing materials that selectively adsorb

During the formation of metal-organic frameworks (MOFs), metal centres can coordinate with the intended organic linkers, but also with solvent molecules. In this case, subsequent activation by removal of the solvent molecules creates unsaturated 'open' metal sites known to have a strong affinity for CO 2 molecules, but their interactions are still poorly understood. Common force fields typically underestimate by as much as two orders of magnitude the adsorption of CO 2 in open-site Mg-MOF-74, which has emerged as a promising MOF for CO 2 capture. Here we present a systematic procedure to generate force fields using high-level quantum chemical calculations. Monte Carlo simulations based on an ab initio force field generated for CO 2 in Mg-MOF-74 shed some light on the interpretation of thermodynamic data from flue gas in this material. The force field describes accurately the chemistry of the open metal sites, and is transferable to other structures. This approach may serve in molecular simulations in general and in the study of fluid-solid interactions. Most energy scenarios project a significant increase in the role of renewable energy sources 1 . These scenarios also predict an even higher increase in our energy needs. As a consequence, although the relative consumption of fossil fuels will be decreasing, in absolute terms we will continue to burn more coal. In such a scenario, carbon capture and sequestration will be one of the only viable technologies to mitigate CO 2 emissions 1,2 . At present the cost associated with the capture of CO 2 from flue gas is one of the bottlenecks in the large-scale deployment of this technology. Of particular concern is that the efficiency of a coal-fired power plant decreases by as much as 30-40% (ref. 3) because of the energy required to separate and compress CO 2 . The aim of decreasing this parasitic load has motivated the search for novel materials 4,5 .A promising class of materials is metal-organic frameworks (MOFs) 4,6 . MOFs are crystalline materials that consist of metal centres connected by organic linkers. These materials have an extremely large internal surface area and, compared to other common adsorbents, promise very specific customization of their chemistry. By changing the metal and the linker, one can in principle generate many millions of possible materials. In practice, however, we can synthesize only a very small fraction of these materials, and it is important to develop a theoretical method that supports the experimental efforts to identify an ideal MOF for carbon capture. A key aspect is the ability to predict the properties of a MOF before the material is synthesized. At present it is possible to carry out accurate quantum chemical calculations on these types of systems 7 . State-of-the-art density functional theory (DFT) calculations provide important insights into the energetics and siting of CO 2 at zero Kelvin 7 . The separation of flue gas, however, requires thermodynamic information (for example, adsorption isotherms) at flue-gas conditions (40...

We benchmark the performance of 20 approximate density functionals for the calculation of one-bond carbon-hydrogen NMR spin-spin coupling constants (SSCCs). These functionals range from the simplest local-spin density approximation to novel meta-generalized gradient approximation and hybrid density functionals. Our testing set consists of 72 diverse molecules that represent multiple types of hybridization of the carbon atom corresponding to 96 experimentally measured one-bond carbon-hydrogen SSCCs. Our results indicate that generalized gradient approximations perform best for this type of coupling.

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The probability current density is used in addition to the electron density and its gradient as a variable in the construction of an exchange-correlation functional. Starting from the Perdew-Burke-Ernzerhof generalized gradient approximation, we employ exact conditions to build a nonempirical exchange functional. Matching the correlation functional to that for exchange yields a current-dependent approximation for correlation. The resulting functional is given in a simple closed form. Application of this approximation to open shell atoms eliminates the artificial level splitting of formally degenerate states observed with generalized gradient approximations.