Heterogeneity of Free O–H Groups at the Air/Water Interface

Free O–H groups at the air/water interface produce a sharp peak in vibrational sum-frequency generation (vSFG) spectra. Consequently, their orientational anisotropy is often overlooked, since a sharp feature is assumed to indicate spectral homogeneity. Using polarization-resolved vSFG experiments and path-integral quantum dynamics with the MB-pol potential, we show that the orientational anisotropy in free O–H groups is significant. The vSFG measurements at different polarization combinations show a ∼15 cm –1 frequency shift in the free O–H peak. Simulations show that, despite being non-hydrogen-bonded, the free O–H frequency depends on orientation: O–H bonds tilted toward the interfacial plane are red-shifted compared to the more upright O–H. This anisotropy originates from orientation-dependent local electric fields generated by neighboring water molecules, as demonstrated by a clear anticorrelation between the projected field along the O–H bond and the fundamental frequency. Combining polarization-resolved vSFG with quantum data-driven many-body simulations enables direct spectral interpretation in terms of orientation-dependent local electric fields, yielding a molecular-level picture of interfacial free O–H heterogeneity.