Abstract
Anharmonicity plays a crucial role in hydrogen transfer reactions in hydrogen-bonding systems, which leads to a peculiar spectral line shape of the hydrogen stretching mode as well as highly complex intra/intermolecular vibrational energy relaxation. Single-molecule study with a well-defined model is necessary to elucidate a fundamental mechanism. Recent low-temperature scanning tunnelling microscopy (STM) experiments revealed that the cis↔cis tautomerization in a single porphycene molecule on Cu(110) at 5 K can be induced by vibrational excitation via an inelastic electron tunnelling process and the N-H(D) stretching mode couples with the tautomerization coordinate [Kumagai et al. Phys. Rev. Lett. 2013, 111, 246101]. Here we discuss a pronounced anharmonicity of the N-H stretching mode observed in the STM action spectra and the conductance spectra. Density functional theory calculations find a strong intermode coupling of the N-H stretching with an in-plane bending mode within porphycene on Cu(110).
Highlights
Anharmonicity plays a crucial role in hydrogen transfer reactions in hydrogen-bonding systems, which leads to a peculiar spectral line shape of the hydrogen stretching mode as well as highly complex intra/ intermolecular vibrational energy relaxation
Porphycene has been investigated as a model of a double H transfer that occurs in a shallow and strongly anharmonic potential energy surface resulting from strong intramolecular hydrogen bonds.[5,6,7,8]
Recent low-temperature scanning tunnelling microscopy (STM) experiments revealed that the cis 2 cis tautomerization of a single porphycene molecule on Cu(110) can be induced by vibrational excitation via inelastic electron tunnelling and that the N–H stretching mode couples with the tautomerization coordinate.[9]
Summary
Anharmonicity plays a crucial role in hydrogen transfer reactions in hydrogen-bonding systems, which leads to a peculiar spectral line shape of the hydrogen stretching mode as well as highly complex intra/ intermolecular vibrational energy relaxation. We do not take into account any influence of the tip–molecule interaction that may modify the potential energy surface of the tautomerization.[11] Note that this normalization was not carried out in our previous report.[9] As discussed later (cf Fig. 2c), N may be varied depending on the measured current range when one- and two-electron processes are simultaneously involved.
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