Molecular Insights into the Role of Electronic Substituents on the Chemical Environment of the -CH₃ and >c=O Groups of Neat Liquid Monomers Using Sum Frequency Generation Spectroscopy

In this work, the substitution of ethyl methacrylate monomers is used as an approach to investigate the effects of electron withdrawing group substituents to their conformation at the two interfaces using sum frequency generation (SFG) spectroscopy. Cyano (-CN), hydroxyl (-OH), chloro (-Cl), and bromo (-Br) groups are substituted at the ethyl end of the methacrylate backbone to replace one H atom. Then, these neat substituted monomers are monitored at both the air-liquid (AL) and solid-liquid (SL) interfaces. The SFG spectra were recorded at different polarization combinations and infrared regions to probe specific different vibrational modes. The spectral results show relative changes in the orientation of the α-methyl (α-CH₃) group with respect to variations in the substituents. This conformational change can be subsequently correlated to the carbonyl (>C=O) group, which is structurally positioned close to the α-CH₃ group. The resulting intermolecular interactions in a condensed phase, especially between the α-CH₃ group and the substituent in close proximity, caused spectral changes obtained at the AL interface. These spectral changes revealed variations in (1) the intensity of methyl Fermi resonance mode at ∼2935 cm^-1 relative to the α-CH₃ symmetric stretch, (2) the tilt angle of the α-CH₃ group relative to the carbonyl group, and (3) the intensity of the C=O stretch at ∼1720 cm^-1. The changes in the oscillator strengths of these vibrational modes suggested that these intermolecular interactions were triggered by the presence of these substituents in space. In addition, the overall conformation was driven by the strength and direction of the dipole moment. When Si-OH oscillator is introduced through hydrogen bonding interaction at the hydrophilic SL interface, a change in the C=O stretch SFG signal clarifies the significant contribution of the dipole moment in the changes observed at the AL interface. The key insight shows the importance of SFG spectroscopy as a tool to probe the small structural modifications of neat compounds.