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The adsorption of molecular hydrogen (H 2 ) in the graphite intercalation compound KC 24 is studied both experimentally and theoretically. High-resolution inelastic neutron data show spectral features consistent with a strong pinning of H 2 along a single axis. First-principles calculations provide novel insight into the nature of H 2 binding in intercalates but fail to account for the symmetry of the H 2 orientational potential deduced from experiment. The above discrepancy disappears once the H 2 center of mass is allowed to delocalize in the quantum-mechanical sense across three vicinal adsorption sites, naturally leading to the well-known saturation coverage of $2H 2 per metal atom in this material. Our results demonstrate that H 2 storage in metal-doped carbon substrates can be severely affected by hitherto unexplored quantummechanical effects. [12,13] and CsC 24 [14] has been investigated by inelastic neutron scattering (INS). INS features were attributed to the presence of two distinct adsorption sites. Our recent work on KC 24 ðH 2 Þ x shows no preferential site occupancy for x ¼ 1:0 and 1.5 [15,16], where x represents the amount of adsorbed H 2 . This finding strongly suggests the existence of a single adsorption site in KC 24 , making it a superb candidate for more detailed studies. The aim of this Letter is to probe the H 2 -KC 24 potential energy landscape via a detailed analysis of high-resolution INS data. The experimental results are supplemented by plane-wave densityfunctional-theory (PW-DFT) calculations so as to provide novel insight into the nature of H 2 -GIC complexes.High-purity KC 24 was synthesized from nuclear-grade Papyex [17] using a modified one-zone method with a 20 wt % stoichiometric K excess at 300 C [16]. The sample was annealed for 3 days until the appearance of a uniform blue color signaling charge transfer from the alkali to graphite, and then stored under argon. Neutron measurements were carried out under in situ H 2 loading on the IRIS [18] and TOSCA [19] spectrometers. Data were collected at 1.5 K (IRIS) and 12.5 K (TOSCA) for x ¼ 0, 0.25, 0.5, 1.0, and 2.0, as measured volumetrically during dosing. An additional over-saturation concentration of x ¼ 6:25 was measured on IRIS only. The x ¼ 0 data were used as background spectrum for subsequent subtraction.Figure 1(a) shows the dependence of the (003) Bragg reflection with x. These data were collected following the relaxation of normal-H 2 to the para ground state over a period of several hours. The intensity of the KC 24 peak at 2.90 Å diminishes with x while a second feature appears above 2.97 Å , with its peak maximum shifting steadily towards higher d-spacings up to x ¼ 2:0. Both peaks remain discrete and coexist at all explored x, indicating the PRL 101, 126101 (2008)

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Summary In a randomised, placebo-controlled trial we have compared the eflcacy of ondansetron and droperidol in reducing postoperative