Clifford Russell Bowers

Department of Chemistry, University of Florida

(Huang)

Nuclear spin singlet-states are attracting a lot of attention nowadays because they can significantly extend the useable lifetime of hyperpolarization. Singlet order decays more slowly than ordinary magnetization because the former is protected from the intra-pair dipole-dipole relaxation mechanism. Singlet-triplet imbalance (STI), which refers to the population differential between the singlet and triplet manifolds for a pair of strongly coupled spin-1/2 nuclei, can be prepared directly from parahydrogen, the singlet spin isomer of dihydrogen that is easily generated by cooling the gas in the presence of an ortho-para conversion catalyst. With perfect magnetic equivalence and isolation, the parahydrogen singlet is the quintessential long-lived state with a lifetime of many weeks. The transformation of singlet order into observable Zeeman order can be achieved by symmetry breaking chemistry.¹ The key requirement is pairwise addition with retention of inter-pair spin-spin coupling. This talk will survey some highlights from our recent research utilizing heterogeneous catalysis to convert dihydrogen STI into magnetization on various target molecules in gases and solution. One of the main challenges that needs to be overcome is the fast diffusion of H ad-atoms that is typically observed on the catalytically active metal surfaces. We show that through rational design of heterogeneous catalysts, significant improvements can be made in the efficiency of the singlet-to-magnetization transformation.^2-3 Three examples are presented: ultra-low loadings of Pt on shaped cerium oxide nanocrystal supports, PtSn intermetallic nanoparticles for hyperpolarization of water and alcohols, and certain other bimetallic combinations that have delivered substantially higher performance than monometallic nanoparticle catalysts. Lastly, a new approach utilizing heterogeneous catalysis for the continuous-flow synthesis of symmetric and pseudo-symmetric parahydrogen adducts hosting long-lived states will be presented.

Acknowledgements: NSF grants CHE-1808239 (CRB and WH), CBET-1933723 (HH-W and CRB) and the NHMFL-UCGP which is supported by the National Science Foundation Cooperative Agreement No. DMR-1644779 and the State of Florida.

1. CR Bowers, DP Weitekamp, PRL, 1986; CR Bowers, DP Weitekamp, JACS, 1987

2. Evan Wenbo Zhao, Raghu Maligal-Ganesh, Yong Du, Tommy Yunpu Zhao, James Collins, Tao Ma, Lin Zhou, Tian-Wei Goh, Wenyu Huang, Clifford R Bowers - Chem, 2018

3. Evan W Zhao, Raghu Maligal‐Ganesh, Chaoxian Xiao, Tian‐Wei Goh, Zhiyuan Qi, Yuchen Pei, Helena E Hagelin‐Weaver, Wenyu Huang, Clifford R Bowers -Angew. Chem. 2017