The PASADENA Effect at a Solid Surface: High-Sensitivity Nuclear Magnetic Resonance of Hydrogen Chemisorption

Abstract

The use of parahydrogen as a high-sensitivity spin label for nuclear magnetic resonance (NMR) was first proposed and then demonstrated to provide orders-of-magnitude signal enhancement in species which are formed from the molecular addition of hydrogen. The phenomenon known as the PASADENA effect (parahydrogen and synthesis allow dramatically enhanced nuclear alignment) derives from the fact that deviation of the parahydrogen mole fraction in a sample of H_2 from its statistical high-temperature limit of 1/4 is associated with an inherent form of spin order. Upon molecular addition of the two protons from a single H_2 molecule into coupled, magnetically inequivalent environments, this order is manifested as large nonequilibrium spin population differences across allowed NMR transitions.

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