Surface Structures of Amine-Bearing Siloxane Self-Assembled Monolayers: Influence of Conformation on Methylene Vibrational Frequencies

Abstract

Functionalized self-assembled monolayers (SAMs) are of interest for numerous applications including materials for national security and defense, chemical and biological sensing, and electronic device components. Understanding the surface structures of SAMs is important for determining the functionality and efficiency of these systems and aiding in the design of advanced materials. In this study, the surface structures of amine-bearing siloxane SAMs on fused silica were investigated by vibrational sum frequency generation (VSFG) spectroscopy and density functional theory (DFT). VSFG spectra of the SAMs were collected in the C–H and N–H stretching regions, and observed spectral features were assigned to specific vibrational modes that provided information regarding the surface structure of the deposited SAM. The molecular conformation of secondary amine moieties was determined to have a significant effect on the vibrational frequencies of methylene vibrational modes. Red-shifting of the methylene symmetric stretching mode by greater than 50 cm–1 was observed experimentally in systems containing secondary amines. This effect was confirmed by DFT calculations and found to be a consequence of molecular conformation that allowed the formation of resonance structures depending on the position of the nitrogen lone-pair electrons with respect to C–N and C–H bonding and antibonding orbitals, respectively.