Direct Observation of Double Hydrogen Transfer via Quantum Tunneling in a Single Porphycene Molecule on a Ag(110) Surface.

Matthias Koch,Mats Persson,Takashi Kumagai,Mark Pagan,Jacek Waluk,Sylwester Gawinkowski

doi:10.1021/jacs.7b06905

Abstract

Quantum tunneling of hydrogen atoms (or protons) plays a crucial role in many chemical and biological reactions. Although tunneling of a single particle has been examined extensively in various one-dimensional potentials, many-particle tunneling in high-dimensional potential energy surfaces remains poorly understood. Here we present a direct observation of a double hydrogen atom transfer (tautomerization) within a single porphycene molecule on a Ag(110) surface using a cryogenic scanning tunneling microscope (STM). The tautomerization rates are temperature independent below ∼10 K, and a large kinetic isotope effect (KIE) is observed upon substituting the transferred hydrogen atoms by deuterium, indicating that the process is governed by tunneling. The observed KIE for three isotopologues and density functional theory calculations reveal that a stepwise transfer mechanism is dominant in the tautomerization. It is also found that the tautomerization rate is increased by vibrational excitation via an inelastic electron tunneling process. Moreover, the STM tip can be used to manipulate the tunneling dynamics through modification of the potential landscape.

Highlights

Hydrogen-transfer reactions are involved in a variety of chemical and biological processes.[1]
Right panels show magnified Z traces in the region marked by the dashed lines in the left, where a short-lived trans state is observed. (e) Current (It) dependence of the tautomerization rates for the “high” to “low” transition (RH→L) and the opposite process (RL→H) for HH-porphycene measured at 5 K
Porphycene, the isotope ratio of the tautomerization rate between different isotopologues, and the density functional theory (DFT) calculations indicated that the tautomerization occurs by the stepwise mechanism rather than the concerted one

Summary

Introduction

Hydrogen-transfer reactions are involved in a variety of chemical and biological processes.[1]. Already in 1927 the importance of quantum tunneling was pointed out by Hund for an intramolecular hydrogen rearrangement such as flipping of ammonia (umbrella motion).[6] Tunneling is of fundamental interest in quantum physics/chemistry and plays a crucial role in important chemical[7,8] and enzymatic[9] reactions. Low-temperature STM has been proven to be a novel and powerful tool to investigate NQEs of hydrogen on surfaces in real space, whereby hydrogen diffusion,[11] hydrogen-bond rearrangement in small water clusters[12,15] and flipping motion of hydrogen atom[14] via tunneling, and the zero-point energy contribution in the hydrogen bond[13,16] were directly observed. Theoretical calculations highlighted considerable impacts of NQEs on the structure and dynamics of adsorbed molecules.[17−19] Interestingly, tunneling was found to be an important contribution to hopping (diffusion) of heavy atoms and molecules such as Cu,[20] Co, and CO22 on a Cu(111) surface at cryogenic temperatures

Methods

Results

Conclusion