Substrate dependent structure of adsorbed aryl isocyanides studied by sum frequency generation (SFG) spectroscopy

Abstract

Effects of metal substrate on the bonding nature of isocyanide group of two aryl isocyanides, 1,4-phenylene diisocyanide (PDI) and 4-methylphenyl isocyanide (MPI), and tilt angle of MPI were examined by measuring sum frequency generation (SFG) spectra of the self-assembled monolayers (SAMs) of these molecules on Au, Pt, Ag, and Pd surfaces. The SFG peaks due to "metal bonded" and "free"-NC groups were resolved by comparing the SFG spectra of PDI with IR spectra obtained by DFT calculations and previous results of vibrational spectroscopy. Based on the peak positions of the "metal bonded"-NC, it is clarified that while PDI and MPI were adsorbed at top sites on Au, Ag, and Pt surfaces, they adsorbed at bridge sites on the Pd surface. The tilt angles of MPI were determined from the intensity ratio between the SFG peaks of C-H symmetric and asymmetric stretching vibrational modes of the CH(3) group. The tilt angles of the MPI SAMs were in the order of Pt < Pd < Ag < Au, reflecting the bonding nature between the -NC group and the substrate atoms.

A quantum chemical study of substituent effects on CN bonds in aryl isocyanide molecules adsorbed on the Pt surface

“…67−71 Actually, our DFT calculations with PBE functional predicted the adsorption site of X−C6H4−NC molecule on the Pt(111) surface as the hollow site, although the experiment indicated that the molecule adsorbs at the atop site. 22 In the present calculation, N−C bond was oriented perpendicular to the Pt(111) surface, according to the experimental report. Figure 3 shows the top and side views of the optimized geometry of H−C6H4−NC molecule adsorbed on the Pt(111) surface.…”

“…The C atom was positioned above one Pt atom, and the N−C bond was oriented perpendicular to the surface, which is consistent with the experimentally observed atop adsorption site. 22 Finally, the coordinates of the atoms in the bottom two layers were fixed while the adsorbed molecule and the upper two layers of the Pt surface were fully relaxed. Spin polarization was turned on in all calculations.…”

“…Uosaki et al reported the substrate dependence of aryl isocyanide molecules adsorbed on the metal surface by the sum frequency generation (SFG) spectroscopy. 22 The effect of a para-substituted group of aryl isocyanides and metal substrates on the SFG spectrum were also investigated. For examining a bonding nature between NC terminal group and substrate atoms, tilt angles of 4methylphenyl isocyanide (MPI) were estimated from the intensity ratio between SFG peaks of C−H symmetric and asymmetric stretching vibrational modes of CH3 group.…”

Adsorption and Oligomerization of 1,3-Phenylene Diisocyanide on Au(111)

“…More recent work 32 showed similar frequencies of 2121 (free) and 2172 (surface-bound) cm -1 . Sum-frequency generation (SFG) results 4 found corresponding vibrational frequencies of 2122 and 2195 cm -1 . An average of the vibrational frequencies of isocyanide SAM's on gold (summarized in reference 4 ) results in values of 2123 ± 1 cm -1 for a free isocyanide group and 2181 ± 4 cm -1 for the surface-bound species.…”

Interfacial Structure of Soft Matter Probed by SFG Spectroscopy

“…Since the SFG intensity is proportional to , the sign of the complex is lost in the conventional SFG spectra and, thus, difficulties are encountered in determining the absolute orientation of the molecules from the SFG spectra. As shown in Figure , by using substrates such as gold and silver, which can produce a certain nonresonant signal () under the SFG excitation conditions, one is able to obtain the phase information of by analyzing the interference between and in the SFG spectra . However, this requires a special substrate, which is not always possible for the sample of interest.…”