Defreese Jessica L, Hwang Son-Jong, Parra-Vasquez A Nicholas G, Katz Alexander
Department of Chemical Engineering, University of California, Berkeley, Berkeley, California 94720-1462, USA.
J Am Chem Soc. 2006 May 3;128(17):5687-94. doi: 10.1021/ja0556474.
Achieving high degrees of molecular confinement in materials is a difficult synthetic challenge that is critical for understanding supramolecular chemistry on solid surfaces and control of host-guest complexation for selective adsorption and heterogeneous catalysis. In this Article, using 2H MAS NMR spectroscopy of tethered carbamates as a molecular probe, we systematically investigate the degree of steric confinement within three types of materials: two-dimensional silica surface, bulk amorphous microporous silica, and bulk amorphous mesoporous silica. The resulting NMR spectra are described with a simple two-site hopping model for motion and prove that the bulk silica network severely limits the molecular mobility of the immobilized carbamate at room temperature to the same degree as surface-functionalized materials at low-temperatures (approximately 210 K). Raising the temperature of the bulk materials to 413 K still demonstrates the effect of confinement, as manifested in significantly longer characteristic times for the immobilized carbamate relative to surface-functionalized materials at room temperature. The environment surrounding the carbonyl functionality of the immobilized carbamate is investigated using FT-IR spectroscopy, which shows the carbonyl stretching band to be equally shifted for all materials to lower wavenumbers relative to its noninteracting value in carbon tetrachloride solvent. These results suggest that electrostatic interactions between the carbonyl of the immobilized carbamate and silica surface may play an important role in confining the immobilized carbamate and nucleating the formation of a pore wall close to the immobilized carbamate during bulk materials synthesis.
在材料中实现高度的分子限制是一项艰巨的合成挑战，这对于理解固体表面的超分子化学以及控制主客体络合以进行选择性吸附和多相催化至关重要。在本文中，我们使用连接氨基甲酸盐的2H MAS NMR光谱作为分子探针，系统地研究了三种类型材料中的空间限制程度：二维二氧化硅表面、块状无定形微孔二氧化硅和块状无定形介孔二氧化硅。所得的NMR光谱用一个简单的双位点跳跃运动模型进行描述，结果表明块状二氧化硅网络在室温下将固定化氨基甲酸盐的分子迁移率严重限制到与低温（约210 K）下的表面功能化材料相同的程度。将块状材料的温度升高到413 K时，仍然显示出限制效应，表现为相对于室温下的表面功能化材料，固定化氨基甲酸盐的特征时间明显更长。使用FT-IR光谱研究了固定化氨基甲酸盐羰基官能团周围的环境，结果表明相对于其在四氯化碳溶剂中的非相互作用值，所有材料的羰基伸缩带均同等地向较低波数移动。这些结果表明，固定化氨基甲酸盐的羰基与二氧化硅表面之间的静电相互作用可能在限制固定化氨基甲酸盐以及在块状材料合成过程中使靠近固定化氨基甲酸盐的孔壁成核方面发挥重要作用。
