Hydrogen-bond donor–acceptor imbalance in low-frequency terahertz water spectra

The low-frequency dielectric response of liquid water is commonly described by a dominant Debye relaxation together with additional faster contributions whose microscopic origin remains debated. Here, we show that the dielectric function of water between 0.14 and 1.21 THz can be represented by a collective Debye relaxation plus a Drude-Smith term constrained to the zero-dc-conductivity limit. The Drude-Smith spectral weight increases upon heating pure liquid water from 20 to 50 °C and decreases upon isotopic substitution (D2O at 20 °C vs H2O at 20 °C). Among several microscopic measures of electronic polarization and its fluctuations extracted from molecular-dynamics simulations, including path-integral simulations with nuclear quantum effects, the population of water molecules with unequal numbers of donated and accepted hydrogen bonds best tracks the trends of the Drude-Smith spectral weight, changing with the same sign and comparable magnitude. Because such donor-acceptor imbalance is the local condition for net intermolecular charge transfer, it points to fluctuating molecular charges, a degree of freedom that fixed-charge representations lack by construction. Consistently, ab-initio-based dielectric spectra show that the ∼0.1-1 THz response cannot be solely described within a fixed-charge representation. These results support a plausible interpretation of parts of the excess low-frequency THz response of water as electronic-polarization fluctuations associated with transient donor-acceptor imbalance within the aqueous hydrogen-bond network.