Both Inter- and Intramolecular Coupling of O-H Groups Determine the Vibrational Response of the Water/Air Interface

TL;DR

This study combines experimental and simulation approaches to reveal how both inter- and intramolecular couplings influence the vibrational spectra of water at the air interface, highlighting their distinct spectral effects.

Contribution

It provides the first comprehensive analysis of inter- and intramolecular coupling effects on interfacial water vibrational spectra using sum-frequency generation and isotopic dilution.

Findings

Intramolecular coupling causes a double-peak feature in spectra.

Intermolecular coupling leads to a red shift in OH stretch response.

Both couplings significantly affect the vibrational response of interfacial water.

Abstract

Vibrational coupling is relevant not only for dissipation of excess energy after chemical reactions but also for elucidating molecular structure and dynamics. It is particularly important for OH stretch vibrational spectra of water, for which it is known that in bulk both intra- and intermolecular coupling alter the intensity and line shape of the spectra. In contrast with bulk, the unified picture of the inter/intra-molecular coupling of OH groups at the water-air interface has been lacking. Here, combining sum-frequency generation experiments and simulation for isotopically diluted water and alcohols, we unveil effects of inter- and intramolecular coupling on the vibrational spectra of interfacial water. Our results show that both inter- and intramolecular coupling contribute to the OH stretch vibrational response of the neat H2O surface, with intramolecular coupling generating a double-peak feature, while the intermolecular coupling induces a significant red shift in the OH stretch response.