The Union Carbide (UC) ethylene polymerization catalyst, based on silica-supported chromocene, is one of the first industrial catalysts prepared by surface organometallic chemistry, though the structure of the surface sites remains elusive. Recently, our group reported that monomeric and dimeric Cr(II) sites, as well as Cr(III) hydride sites, are present and that their proportion varies as a function of the Cr loading. While ¹H chemical shifts extracted from solid-state ¹H NMR spectra should be diagnostic of the structure of such surface sites, unpaired electrons centered on Cr atoms induce large paramagnetic ¹H shifts that complicate their NMR analysis. Here, we implement a cost-efficient DFT methodology to calculate ¹H chemical shifts for antiferromagnetically coupled metal dimeric sites using a Boltzmann-averaged Fermi contact term over the population of the different spin states. This method allowed us to assign the ¹H chemical shifts observed for the industrial-like UC catalyst. The presence of monomeric and dimeric Cr(II) sites, as well as a dimeric Cr(III)-hydride sites, was confirmed and their structure was clarified.

中文翻译：

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使用 DFT 分配顺磁性单金属和双金属表面位点的 1H 化学位移：联合碳化物聚合催化剂的案例研究

联合碳化物 (UC) 乙烯聚合催化剂基于二氧化硅负载的茂铬，是通过表面有机金属化学制备的首批工业催化剂之一，尽管表面位点的结构仍然难以捉摸。最近，我们小组报道了单体和二聚体 Cr( II ) 位点以及 Cr( III ) 氢化物位点的存在，并且它们的比例随着 Cr 负载量的变化而变化。虽然从固态^{1 H NMR 光谱中提取的1} H 化学位移应该可以诊断此类表面位点的结构，但以 Cr 原子为中心的未成对电子会诱导大的顺磁性¹使核磁共振分析复杂化的 H 位移。在这里，我们实施了一种经济高效的 DFT 方法，使用玻尔兹曼平均费米接触项对不同自旋态的总体计算反铁磁耦合金属二聚体位点的^{1 H 化学位移。}这种方法使我们能够分配观察到的工业类 UC 催化剂的^{1 H 化学位移。}确认了单体和二聚体 Cr( II ) 位点以及二聚体 Cr( III ) 氢化物位点的存在，并阐明了它们的结构。