Quantum bottlenecks and unidirectional energy flow in molecules

Quantum mechanical effects can enable energy to flow more efficiently in one direction along a molecule than in others. Ultrafast spectroscopic experiments on substituted benzenes [J. Phys. Chem. B 117, 10898 (2013)] reveal such an asymmetry in the flow of vibrational energy between the two chemical groups of the molecule, i.e., between the phenyl and the substituent. We examine theoretically energy flow in toluene, one of the substituted benzenes probed in the recent experiments, and show that quantum mechanical bottlenecks give rise to a preferred direction of energy flow in this molecule.