Melting of Rodlike Molecules on Pt(111). Infrared Spectroscopic Studies of Isotopically Labeled n-Alkanes

Abstract

The melting of monolayers of isotopically labeled n-alkanes on Pt(111) surfaces has been followed by reflection−absorption infrared (RAIR) spectroscopy, and the results are compared with those of an earlier study of the unlabeled molecules (J. Phys. Chem. 1995, 99, 15629−15278). Temperature-dependent studies show that monolayers of the n-octane isotopolog, CD3(CH2)6CD3, melt on Pt(111) from a two-dimensionally ordered phase to one having one-dimensional order (“hexatic”) near 220 K, a temperature essentially identical to that seen for unlabeled n-octane. The RAIR spectra of the labeled molecules generally confirm (but in one case clarify an overly simplistic interpretation of) the assignments of the bands made in the earlier study. Specifically, the assignment of a low-frequency “softened” mode near 2760 cm-1 to a νCH stretch for proximal (i.e., surface-contacting) methylene C−H bonds has been verified. A feature near 2900 cm-1 had previously been assigned to a distal methylene C−H stretch, and the line width of this band was thought to increase as a result of the 2D → 1D melting transition. The present study shows that this latter proposal is not entirely correct: the 2900 cm-1 feature actually consists of two bands separated by 5−15 cm-1 (for adsorbed n-octane) due to a Fermi resonance between a distal methylene C−H stretch and the first overtone of a methyl C−H bending mode. Owing to the small frequency difference between the split components of the Fermi resonance, these two bands partially overlap, and thus the appearance of the combined feature is largely determined by small changes in the relative frequencies of the methyl δCH overtone and the methylene νCH fundamental. The studies of the labeled n-alkanes confirm, however, that melting to the 1D phase is accompanied by distinct changes in the frequency of the low-frequency (softened) νCH band near 2760 cm-1. More generally, these results establish unambiguously that the normal modes of an adsorbed n-alkane are very different from the normal modes of the free molecule because the low frequencies of the proximal C−H oscillators cause them to be decoupled from the distal C−H oscillators. An analysis of the temperature dependence of the softened νCH mode using a critical temperature scaling model shows a correlation which can be rationalized in the context of the amplitude of the frustrated translational mode (the true low-frequency soft mode) that drives the (2D → 1D) order−order transition in this system.

Hydrogenation and dehydrogenation of cyclohexene on Pt(100): A sum frequency generation vibrational spectroscopic and kinetic study

“…The observed mode is assigned to the symmetric-symmetric C-H stretch of the CH 2 group. This assignment is in accordance with RAIRS and SFG results of 1,4-cyclohexadiene adsorption on Pt(1 1 1) in UHV and high pressure environments [14,23,24]. The red shift from the typical CH 2 stretching range (2840-2940 cm À1 ) is due in part to the strong electron withdrawal effect from the surface platinum atoms [20].…”

Ultraviolet photoproduction of ISM dust

“…Such a resonance has been discussed in the litterature, both theoretically and experimentally (e.g. Snyder & Scherer 1979;Manner et al 1998;Ratajczak & Yaremko 1999;Jordanov et al 2003) in other cases, and is expected to contribute significantly for alkyl chains around ∼2900 cm −1 . This Fermi resonance indicates the difficulty in analysing the spectral profile and band intensities of complex organic materials using the simple view of independent vibrations.…”

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

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14] These features were first observed by Demuth et al using high-resolution electron energy loss spectroscopy in the cases of cyclohexane on Ni͑111͒ and Pt͑111͒, 1 and called as the "soft mode." The soft mode has been attributed to the C-H bonds directly interacting with metal surfaces.…”