Non-rotational mechanism of polarization in alcohols

TL;DR

This study reveals that quantum tunneling and proton separation, rather than molecular rotation, primarily govern the dielectric properties of alcohols across a broad frequency spectrum, challenging traditional models.

Contribution

It introduces a quantum non-rotational mechanism explaining alcohols' polarization, supported by ultrabroadband spectroscopy data across various molecular lengths.

Findings

Quantum tunneling influences dielectric response.

Proton-hole correlation length is a key parameter.

Rotational polarization is secondary, prominent only below 0.3 THz.

Abstract

Chemical polarity governs various mechanical, chemical and thermodynamic properties of dielectrics. Polar liquids have been amply studied, yet the basic mechanisms underpinning their dielectric properties remain not fully understood, as standard models following Debye's phenomenological approach do not account for quantum effects and cannot aptly reproduce the full dc-up-to-THz spectral range. Here, using the illustrative case of monohydric alcohols, we show that deep tunneling and the consequent intermolecular separation of excess protons and "proton-holes" in the polar liquids govern their static and dynamic dielectric properties on the same footing. We performed systematic ultrabroadband (0-10 THz) spectroscopy experiments with monohydric alcohols of different (0.4-1.6 nm) molecular lengths, and show that the finite lifetime of molecular species, and the proton-hole correlation length are the two principle parameters responsible for the dielectric response of all the studied alcohols across the entire frequency range. Our results demonstrate that a quantum non-rotational intermolecular mechanism drives the polarization in alcohols while the rotational mechanism of molecular polarization plays a secondary role, manifesting itself in the sub-terahertz region only.