Cartesian Decomposition of Infrared Spectra Reveals the Structure of Solution-Deposited, Self-Assembled Benzoate and Alkanoate Monolayers on Rutile (110)

• Published in: Journal of physical chemistry. C Vol. 120; no. 43; pp. 24866 - 24876
• Main Authors: DeBenedetti, William J. I, Skibinski, Erik S, Hinckley, Joshua A, Nedessa, Sara B, Hines, Melissa A
• Format: Journal Article
• Language: English
• Published: American Chemical Society 03-11-2016
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.6b08450

Cartesian polarization analysis transforms a set of surface infrared spectra obtained in different geometries into their Cartesian components using a mathematical transform, providing direct insight into the bonding geometry of adsorbed molecules. This technique was extended to uniaxial substrates and used to analyze solution-deposited, self-assembled benzoate and alkanoate monolayers on rutile (110). This analysis resolved a long-standing controversy regarding the existence of paired molecules in benzoate monolayers, showing that two distinct isomers exist within the monolayer: a tilted tetramer, which is paired, and a twisted monomer, which is not. The two isomers are nearly isoenergetic, as shown by analysis of STM images and complementary DFT simulations. Infrared and XPS spectra as well as STM images of heptanoate and octanoate monolayers showed the formation of complete monolayers (as opposed to sparse layers or multilayers); however, the alkyl chains in the monolayer are disordered and loosely packed with a significant density of conformational defectsa stark contrast to the near-crystalline, all-trans alkyl monolayers typically formed on Au and Si surfaces. The high disorder in the alkanoate monolayers was attributed to geometry, as the density of alkanoate binding sites on rutile (110) is 30% less than the density of alkyl monolayers on Si. The high density of gauche defects in alkanoate monolayers was attributed to the small energy difference between the all-trans and single-gauche-defect conformers in isolated alkyl chains. In contrast, strong intermolecular interactions in tight-packed alkyl monolayers on Au and Si surfaces suppress gauche defect formation.