Low-temperature observation of the softened C-H stretching vibrations of cyclohexane on Rh(111)

Abstract

The C-H stretching vibrations of cyclohexane on Rh͑111͒ were investigated using infrared reflection absorption spectroscopy between 20 and 89 K. At 20 K, the softened C-H stretching band consists of several sharp peaks, ranging from 2500 to 2700 cm −1 . The wide-range distribution of the softened C-H stretching peaks results from inhomogeneity of adsorption environments. With increasing the substrate temperature, each softened C-H stretching peak becomes significantly broadened, but the normal C-H stretching peaks are little changed. These results indicate that the local interaction between the softened C-H species and the Rh͑111͒ surface is sensitive to a thermally excited low-energy mode. The temperature-dependent broadening of a soft mode at low temperature is analyzed using a vibrational dephasing model, where the softened C-H stretching mode is anharmonically coupled with a thermally excited frustrated translation mode.

The Quantum Nature of C–H···Metal Interaction: Vibrational Spectra and Kinetic and Geometric Isotope Effects of Adsorbed Cyclohexane

“…The soft mode consists of several peaks that are significantly sharp compared with the reported results on other metal surfaces. In the present study, sharp peaks of the soft mode were observed at 20 K, whereas the previous experiments on other metal surfaces were performed above 90 K. Recently, we have measured temperature‐dependent IRAS spectra as shown in Figure . The peaks of the softened CH stretching mode are significantly broadened with increasing substrate temperature.…”

“…The peaks of the softened CH stretching mode are significantly broadened with increasing substrate temperature. We have concluded that molecular frustrated modes are thermally excited at higher temperatures, and the anharmonic coupling between the softened ν CH and a frustrated translational mode (most probably parallel to the surface, the F // mode) causes vibrational dephasing in the case of cyclohexane on the Rh(111) surface …”

“…Recently, we have reported that the soft ν CH (ν CD ) mode of cyclohexane on Rh(111) actually consists of sharp peaks at low temperature (20 K) . Furthermore, the width of each peak strongly depends on the substrate temperature .…”

“…The soft ν CH (ν CD ) bands of cyclohexane on the clean and hydrogen‐preadsorbed Rh(111) surfaces were not a broad single band, but consisted of several sharp peaks when the substrate temperature was very low (∼20 K). Figure a and b show a series of IRAS spectra of normal and soft ν CH (ν CD ) modes for cyclohexane C 6 H 12 (C 6 D 12 ), respectively, on clean and hydrogen‐preadsorbed Rh(111) surfaces . Hydrogen and cyclohexane were adsorbed on the surface through a pulse valve, and exposure was precisely controlled by changing pulse duration and the number of pulses.…”

“…Since the C–H···M interaction shows a similarity to the conventional H‐bond from the vibrational point of view, we expected that the geometric isotope effects could be observed in adsorption systems. In this chemical record, we review the previous studies of cyclohexane adsorbed on metal surfaces and summarize a series of our recent studies concerning adsorbed cyclohexane on clean and hydrogen‐preadsorbed Rh(111) surfaces …”