Influence of Substituent Type and Position on the Adsorption Mechanism of Phenylboronic Acids: Infrared, Raman, and Surface-Enhanced Raman Spectroscopy Studies

This paper shows systematic spectroscopic studies using Fourier-transform infrared absorption (FT-IR), Fourier-transform Raman (FT-Raman), and surface-enhanced Raman (SERS) in an aqueous silver sol of fluoro and formyl analogues of phenylboronic acids: 2-fluorophenylboronic acid (2-F-PhB­(OH)₂), 3-fluorophenylboronic acid (3-F-PhB­(OH)₂), 4-fluorophenylboronic acid (4-F-PhB­(OH)₂), 2-formylphenylboronic acid (2-CHO-PhB­(OH)₂), 3-formylphenylboronic acid (3-CHO-PhB­(OH)₂), and 4-formylphenylboronic acid (4-CHO-PhB­(OH)₂). To produce an extensive table of vibrational spectra, density functional theory (DFT) calculations with the B3LYP method at the 6-311++G­(d,p) level of theory were performed for the ground state geometry of the most stable species, dimers in cis–trans conformation. On the basis of the SERS spectral profile, the adsorption modes of the phenylboronic acid isomers were proposed. The type of substituent and its position in the phenyl ring have a strong influence on the geometry of isomers on the silver nanoparticle’s surface. This effect was especially evident in the case of 4-CH-PhB­(OH)₂, for which dearomatization of the phenyl ring took place upon adsorption.