The structures and reactivities of various cyclic C 5 and C 6 hydrocarbons (cyclopentene, cyclopentadiene, cyclohexene, 1,3-cyclohexadiene, and 1,4-cyclohexadiene) adsorbed on Pt(111) have been examined by means of reflection-absorption infrared (RAIR) spectroscopy. At temperatures below 200 K, these molecules bind intact to the Pt(111) surface by means of strong interactions with CdC double bonds. Steric interactions between the surface and certain CH 2 groups on the ring systems figure prominently in determining the conformation adopted in the bound states of several of the molecular adsorbates (e.g., cyclopentene and cyclohexene). These bonding habits are identified both by observing significant electronic interactions that weaken certain C-H bonds (so-called mode softening) and also by developing analogies with trends seen with similar ring systems and complexes. At higher temperatures above 200 K, the C 5 species are dehydrogenated in high yield to a planar, surface-bound pentahaptocyclopentadienyl species (η 5 -C 5 H 5 ) while the C 6 cyclic hydrocarbons react to give benzene; these surface-bound products, which are stable to temperatures >400 K, have been identified in earlier studies as well. The present work adds to the understanding of the nature and energetics of the sequential C-H bond activation processes involved in their formation. In addition, several intermediates lying along the reaction pathways to the respective planar intermediates have been identified and spectroscopically characterized for the first time. Most notably, we observe that 1,3cyclohexadiene loses one hydrogen between 200 and 250 K to give a stable η 5 -cyclohexadienyl intermediate. An efficient partial dehydrogenation of cyclopentene at 250 K to give the corresponding diene is also observed. Our data also demonstrate the importance of heretofore unappreciated hyperconjugation effects in the vibrational spectroscopy of the C-H stretching modes of metal-surface-bound π systems. The insights developed in this study regarding such electronic interactions are used to develop an understanding of the binding sites and conformational states adopted by the various adsorbates and intermediates formed during their decomposition.

The melting of ordered monolayers of n-hexane, n-octane, and n-decane adsorbed on Pt( 11 1) has been studied by reflection-absorption infrared spectroscopy (RAIRS). Each alkane forms an overlayer at low temperatures (< 160 K) that is characterized by both quasi-long-range translational and orientational order. Previous lowenergy electron diffraction (LEED) studies suggested that the alkanes are arranged on the surface in the all-trans conformation, and this has been confirmed by RAIRS. The low-temperature ordered state is distinguished by the presence of a sharp, resolved set of soft modes near 2760 cm-I. The perturbation of the frequencies of some of the C-H oscillators due to the presence of M***H-C interactions between the adsorbed n-alkane molecules and the surface has a dramatic effect on the RAIR spectra. Specifically, there are two RAIR-allowed methylene VC-H stretching modes: one which largely involves motion of the methylene C-H bond that projects away from the surface (the distal C-H bond) and another that largely involves motion of the near-surface @e., proximal) C-H bond. We have assigned these modes to the bands at -2909 and -2760 cm-', respectively. The assignments for the "methyl" modes seen at -2947, 2929, and 2810 cm-' follow in this same spirit: one C-H bond of the methyl groups interacts with the surface, and the resulting shift in the frequency of this oscillator to 2810 cm-' serves to decouple it from the vibrations of the other two. At higher temperatures, a transition occurs to a one-dimensionally ordered (possibly "hexatic") phase in which translational order has been lost along the long axis of the molecules but orientational ordering is maintained; the approximate 2D -1D transition temperatures are 187 K for n-hexane, 212 K for n-octane, and 225 K for n-decane. Upon transforming to this hexatic phase, the C-H stretching bands (especially the soft modes) broaden substantially and shift in frequency; these spectroscopic changes reflect the inhomogeneity generated upon translation of the chains relative to their positions in the ordered 2D phase. At higher temperatures (210 K for hexane, 240 K for octane, and 270 K for decane), the RAIR spectra suggest that the order-disorder transition noted by LEED yields a population of alkane molecules in which trans segment conformations predominate, but a small population of gauche kink defects is present as well. As expected for true phase transitions, all changes noted by RAIRS are reversed when the samples are cooled. The conformational dynamics seen on a Pt( 11 1) surface are compared with those that occur in the premelting and melting transitions of bulk n-alkane crystals. The present data also answer a long standing question about the mechanism of mode softening: the line widths of soft modes are determined largely by the degree of site homogeneity and thus are acutely sensitive to the conformational, rotational, and translational order of the adsorbate overlayer.Non-three-dimensional phase transitions continue to attract a great deal of interes...

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.

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The interaction and reactivity of n-octane adsorbed on the (1×1) and (5×20) surfaces of Pt(100) have been examined by reflection−absorption infrared (RAIRS), temperature-programmed reaction (TPRS), and Auger electron (AES) spectroscopies and by low-energy electron diffraction (LEED). A strong C−H···M interaction between the adsorbate and the metal was present on both surfaces at 100 K, as evidenced by the presence of “softened modes” in the C−H stretching region of the vibrational spectra centered at ∼2630 and 2750 cm-1 for the (1×1) and (5×20) surfaces, respectively. The softened modes observed for n-octane on the (5×20) surface of Pt(100) are reminiscent of those seen when this molecule is adsorbed on Pt(111). On both of these surfaces (Pt(100)-(5×20) and Pt(111)) the molecule adopts an all-trans conformation and is adsorbed so as to align the plane of the C−C−C framework parallel to that of the surface. This organization leads to a series of bands appearing in the 2500−2840-cm-1 region which result from the high-symmetry C−H···M contacts occurring between the n-alkane overlayer and the underlying hexagonal symmetry surfaces. The softened modes observed for an overlayer of octane on the unreconstructed (1×1) surface at low temperature, however, were broad and featureless. Relative to the case for the Pt(100)-(5×20) surface, the activity for the dehydrogenation of an overlayer of n-octane was much greater on Pt(100)-(1×1). These reactivity differences appear to be weakly correlated with the nature of the mode-softening seen in the low-temperature vibrational spectra.