Melting of rodlike molecules on Pt(111). Infrared spectroscopic studies of isotopically labeled n-alkanes

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 v_CH 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 δ_CH overtone and the methylene v_CH 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) v_CH 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 v_CH 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.