Abstract
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.
Original language | English |
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Pages (from-to) | 28201-28209 |
Number of pages | 9 |
Journal | Journal of Physical Chemistry C |
Volume | 123 |
Issue number | 46 |
DOIs | |
State | Published - Nov 21 2019 |
Externally published | Yes |
Funding
The authors would like to thank Drs. Hugh Richardson, Eric Masson, Andrew Tangonan, Benjamin Bythell, and Michael Jensen for assistance and in-depth discussions. Also, the authors thank Ahmed Aboelenen for IR characterization, Ali Rafiei for Raman characterization, Gabrielle Chiong and Tharushi Ambagaspitiya for assistance in the artwork. The current work was supported by NSF (Grants CHE-0947031 and CHE-1338000) for the acquisition of the femtosecond laser and nuclear magnetic spectrometer, the start-up fund of the Department of Chemistry and Biochemistry, the College of Arts and Sciences, and the Vice President for Research at Ohio University. Additionally, the authors would like to thank the Nanoscale and Quantum Phenomena Institute for their financial support. U.I.P. was also partially supported by the NSF (Grant CBET-1705817).
Funders | Funder number |
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Nanoscale and Quantum Phenomena Institute | |
U.I.P. | CBET-1705817 |
National Science Foundation | 1705817 |
National Sleep Foundation | CHE-1338000, CHE-0947031 |
Ohio University | |
Department of Chemistry and Biochemistry, Brigham Young University | |
College of Arts and Sciences, Boston University |