Department of Physics and Astronomy, Oberlin College, Oberlin, Ohio 44074, USA.
J Am Chem Soc. 2011 Dec 21;133(50):20310-8. doi: 10.1021/ja2071384. Epub 2011 Nov 28.
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.
采用宽温度范围（35-298 K）的漫反射红外（IR）光谱研究了同构金属-有机骨架 M(2)L（M = Mg、Mn、Co、Ni 和 Zn；L = 2,5-二氧代苯-1,4-二羧酸酯）中吸附的 H(2)的动力学，这些骨架被称为 MOF-74 和 CPO-27。在 H(2)浓度范围为 0.1 至 3.0 H(2)/金属阳离子的情况下收集的光谱表明，强烈红移的振动模式源于与可用金属配位位点结合的孤立 H(2)。与该位点相关的带的红移与报道的等吸附焓（在小表面覆盖度下）相关，而这又取决于 M 的身份。相比之下，分配给距离金属位点>3 Å 位置吸附的 H(2)的带在五种材料之间仅表现出微小差异。我们的结果与基于中子衍射数据和独立 IR 研究的先前模型一致，但它们不支持最近提出的吸附机制，该机制涉及强烈的 H(2)···H(2)相互作用（Nijem 等人，J. Am. Chem. Soc.2010, 132, 14834-14848）。只有在吸附剂被环境空气污染后，才能重现与最近提出的吸附机制相关的室温 IR 光谱。我们的解释是，未污染的光谱特征源自离散骨架位点的逐步吸附，这一点通过吸附的 H(2)同位素的系统红移以及泛音带之间的一致性得到加强，这些一致性很好地描述了振动光谱中分子相互作用的 Buckingham 模型。
