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 CH₃ 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.