Imaging covalent bond formation by H atom scattering from graphene

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Science  26 Apr 2019:
Vol. 364, Issue 6438, pp. 379-382
DOI: 10.1126/science.aaw6378

Atom scattering reveals bond formation

When molecules collide, they can form an addition complex in which new chemical bonds can form. However, if energy does not flow out of this complex and into the rest of the molecule, the new bond will usually simply dissociate. Jiang et al. observed the scattering of hydrogen atoms from graphene and interpreted their results with a first-principles potential energy surface and a dynamical simulation (see the Perspective by Hornekaer). At near-normal incidence, these experiments probe transient carbon-hydrogen bond formation when the hydrogen atoms collide with the centers of the six-atom carbon rings. Rapid intramolecular vibrational relaxation results from orbital rehybridization and structural deformations that occur during bond formation.

Science, this issue p. 379; see also p. 331


Viewing the atomic-scale motion and energy dissipation pathways involved in forming a covalent bond is a longstanding challenge for chemistry. We performed scattering experiments of H atoms from graphene and observed a bimodal translational energy loss distribution. Using accurate first-principles dynamics simulations, we show that the quasi-elastic channel involves scattering through the physisorption well where collision sites are near the centers of the six-membered C-rings. The second channel results from transient C–H bond formation, where H atoms lose 1 to 2 electron volts of energy within a 10-femtosecond interaction time. This remarkably rapid form of intramolecular vibrational relaxation results from the C atom’s rehybridization during bond formation and is responsible for an unexpectedly high sticking probability of H on graphene.

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