Does the Sum-Frequency Generation Signal of Aromatic C–H Vibrations Reflect Molecular Orientation?

Abstract

Organic molecules with aromatic groups at the aqueous interfaces play a central role in atmospheric chemistry, green chemistry, and on-water synthesis. Insights into the organization of interfacial organic molecules can be obtained using surface-specific vibrational sum-frequency generation (SFG) spectroscopy. However, the origin of the aromatic C−H stretching mode peak is unknown, prohibiting us from connecting the SFG signal to the interfacial molecular structure. Here, we explore the origin of the aromatic C−H stretching response by heterodyne-detected SFG (HD-SFG) at the liquid/vapor interface of benzene derivatives and find that, irrespective of the molecular orientation, the sign of the aromatic C−H stretching signals is negative for all the studied solvents. Together with density functional theory (DFT) calculations, we reveal that the interfacial quadrupole contribution dominates, even for the symmetry-broken benzene derivatives, although the dipole contribution is non-negligible. We propose a simple evaluation of the molecular orientation based on the aromatic C−H peak area.

Sensitive Detection of Biomolecular Adsorption by a Low-Density Surfactant Layer Using Sum-Frequency Vibrational Spectroscopy

“…The presence of the CH 2,s peak indicates that DPTAP molecules are not well-ordered in larger molecular areas. A new peak around ∼3060 cm –1 appeared, which can be assigned to the CH of the aromatic ring of indole, − and can be fitted by the same sign as those reported in HD-SFG experiments. , Interestingly, increasing the amount of indole molecules (0–5 mM) did not change the OH intensity appreciably, contrary to the case of salts where further stabilization of the monolayer was observed. ,, The cation−π interaction between the choline group of DPTAP and indole molecules makes the surfactant more hydrophobic against the laser heating effect. Counterion adsorption to a headgroup of the surfactant has been known to increase the surface activity of the surfactant; for example, changing the counterion from Cl – to I – decreased the cmc (critical micelle concentration) of the ionic liquid cation by two orders as I – binds more strongly .…”

Adsorption, Orientation, and Speciation of Amino Acids at Air–Aqueous Interfaces for the Direct Air Capture of CO2

“…Here, the hyperpolarizability ratio was taken to be in the range 0.8–0.9 . Using these parameters, and neglecting quadrupole contributions, we find that the phenyl ring is tilted at an angle 60 ± 6° when the twist angles are averaged as revealed by simulations . Overall, the experimental and simulation results show close correspondence and, therefore, successfully benchmark the results reported in this study.…”

Agglomeration Drives the Reversed Fractionation of Aqueous Carbonate and Bicarbonate at the Air–Water Interface

“…We note that these vSFG measurements monitor solute-induced changes in the OH spectrum of water and are, therefore, indirect measures of the interfacial ion population. Moreover, there is still some debate in the literature regarding the interpretation of these measurements. − …”