Assigning 1H Chemical Shifts in Paramagnetic Mono and Bimetallic Surface Sites using DFT: a Case Study on the Union Carbide Polymerization Catalyst

09 December 2022, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

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 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 site was confirmed and their structure was clarified.

Supplementary materials

Title
Description
Actions
Title
Supporting Information
Description
Experimental procedure and additional data
Actions

Comments

Comments are not moderated before they are posted, but they can be removed by the site moderators if they are found to be in contravention of our Commenting Policy [opens in a new tab] - please read this policy before you post. Comments should be used for scholarly discussion of the content in question. You can find more information about how to use the commenting feature here [opens in a new tab] .
This site is protected by reCAPTCHA and the Google Privacy Policy [opens in a new tab] and Terms of Service [opens in a new tab] apply.