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.