[Paper Review] An Isolated Water Droplet in the Aqueous Solution of a Supramolecular Tetrahedral Cage

Federico Sebastiani, Trandon A. Bender+8 more|arXiv (Cornell University)|Apr 20, 2020

Supramolecular Chemistry and Complexes78 references48 citations

TL;DR

This study reveals that a supramolecular Ga4L612- tetrahedral cage encapsulates a distinct, dynamically arrested water droplet of 9±1 molecules under ambient conditions, exhibiting a unique hydrogen-bonding network not matching any known phase of water. Using THz spectroscopy and ab initio molecular dynamics, the authors demonstrate that this confined water is structurally and dynamically distinct from bulk water or ice, creating a strong thermodynamic driving force for guest encapsulation due to favorable entropy and enthalpy changes upon release from the cavity.

ABSTRACT

Water under nanoconfinement at ambient conditions has exhibited low-dimensional ice formation and liquid-solid phase transitions, but with structural and dynamical signatures which map onto known regions of waters phase diagram. Using THz absorption spectroscopy and ab initio molecular dynamics, we have investigated the ambient water confined in a supramolecular tetrahedral assembly, and determined that a distinct network of 9-10 water molecules is present within the nanocavity of the host. The low-frequency absorption spectrum and theoretical analysis of the water in the $Ga_4$$L_6$$^{-12}$ host demonstrate that the structure and dynamics of the encapsulated droplet is distinct from any known phase of water. A further inference is that the release of the highly unusual encapsulated water droplet creates a strong thermodynamic driver for the high affinity binding of guests in aqueous solution for the $Ga_4$$L_6$$^{-12}$ supramolecular construct.

Motivation & Objective

• To determine the structural and dynamic properties of water confined within a supramolecular Ga4L612- tetrahedral cage under ambient conditions.
• To investigate how nanoconfinement alters water's hydrogen-bonding network and low-frequency vibrational modes compared to bulk water.
• To elucidate the thermodynamic driving forces behind high-affinity guest binding in aqueous solution, particularly the role of encapsulated water release.
• To establish a direct link between the unique behavior of confined water and the catalytic efficiency of the supramolecular host.

Proposed method

• THz absorption spectroscopy was used to probe low-frequency intermolecular vibrations of water in the Ga4L612- cage, with spectra recorded at 293 K in the 50–450 cm⁻¹ range.
• Double difference analysis (∆∆α(ν) = ∆α(1)(ν) − ∆α(2)(ν)) isolated the THz fingerprint of the water cluster by subtracting spectra with and without the [Et4N]+ guest.
• Ab initio molecular dynamics (AIMD) simulations were performed using the B97M-rV functional and DZVP basis set in CP2K, with 30 ps NVE trajectories and periodic boundary conditions.
• Theoretical IR spectra in the THz range were calculated via Fourier transform of velocity-velocity correlation functions modulated by Atomic Polar Tensors (APT), focusing only on water contributions.
• The time-averaged number of water molecules inside the cage was calculated from three independent AIMD simulations to confirm the 9±1 count.
• Spectra simulations included only water contributions, excluding the cage and counter-ions, to isolate the water-specific dynamics.

Experimental results

Research questions

• RQ1 What is the number and structural organization of water molecules confined within the Ga4L612- tetrahedral cage at ambient conditions?
• RQ2 How does the hydrogen-bonding network of the encapsulated water differ from that of bulk water or known ice phases?
• RQ3 What is the origin of the unique low-frequency THz absorption signature observed in the water-filled cage?
• RQ4 How does the dynamics of the confined water droplet compare to bulk water, and what does this imply for its thermodynamic stability?
• RQ5 What is the thermodynamic driving force for guest encapsulation, and how is it related to the release of the confined water cluster?

Key findings

• The Ga4L612- cage encapsulates a stable water droplet of 9±1 molecules under ambient conditions, as confirmed by both experiment and ab initio molecular dynamics.
• The confined water exhibits a dynamically arrested state with a disrupted hydrogen bond network on its outer layer and intact bonding in the core, distinct from any known phase of water.
• The THz absorption spectrum of the encapsulated water shows a unique signature in the 100–270 cm⁻¹ range, indicating altered intermolecular hydrogen bond stretching dynamics.
• The linewidth narrowing of the intermolecular stretching mode indicates reduced translational and rotational mobility, consistent with a rigid, low-dynamics environment.
• The release of the confined water is entropically and enthalpically favorable, as the water is
• The thermodynamic driving force for guest binding arises from the high free energy of the confined water, which is destabilized due to incomplete hydrogen bonding and restricted dynamics, making its release favorable upon guest entry.