Binding
Water Clusters to an Aromatic-Rich Hydrophobic
Pocket: [2.2.2]Paracyclophane–(H<sub>2</sub>O)<sub><i>n</i></sub>, <i>n</i> = 1–5
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Abstract
[2.2.2]Paracylcophane
(tricyclophane, TCP) is a macrocycle with
three phenyl substituents linked by ethyl bridges (−CH<sub>2</sub>CH<sub>2</sub>−) in the para-position, forming an aromatic-rich
pocket capable of binding various substituents, including nature’s
solvent, water. Building on previous work [Buchanan, E. G.; et al. <i>J. Chem. Phys.</i> <b>2013</b>, <i>138</i>,
064308] that reported on the ground state conformational preferences
of TCP, the focus of the present study is on the infrared and ultraviolet
spectroscopy of TCP–(H<sub>2</sub>O)<sub><i>n</i></sub> clusters with <i>n</i> = 1–5. Resonant two-photon
ionization (R2PI) was used to interrogate the mass selected electronic
spectrum of the clusters, reporting on the perturbations imposed on
the electronic states of TCP as the size of the water clusters bound
to it vary in size from <i>n</i> = 1–5. The TCP–(H<sub>2</sub>O)<sub><i>n</i></sub> S<sub>0</sub>–S<sub>1</sub> origins are shifted to lower frequency from the monomer,
indicating an increased binding energy of the water or water network
in the excited state. Ground state resonant ion-dip infrared (RIDIR)
spectra of TCP–(H<sub>2</sub>O)<sub><i>n</i></sub> (<i>n</i> = 1–5) clusters were recorded in the
OH stretch region, which probes the H-bonded water networks present
and the perturbations imposed on them by TCP. The experimental frequencies
are compared with harmonic vibrational frequencies calculated using
density functional theory (DFT) with the dispersion-corrected functional
ωB97X-D and a 6-311+g(d,p) basis set, providing firm assignments
for their H-bonding structures. The H<sub>2</sub>O molecule in TCP–(H<sub>2</sub>O)<sub>1</sub> sits on top of the binding pocket, donating
both of its hydrogen atoms to the aromatic-rich interior of the monomer.
The antisymmetric stretch fundamental of H<sub>2</sub>O in the complex
is composed of a closely spaced set of transitions that likely reflect
contributions from both para- and ortho-forms of H<sub>2</sub>O due
to internal rotation of the H<sub>2</sub>O in the binding pocket.
TCP–(H<sub>2</sub>O)<sub>2</sub> also exists in a single conformational
isomer that retains the same double-donor binding motif for the first
water molecule, with the second H<sub>2</sub>O acting as a donor to
the first, thereby forming a water dimer. The OH stretch infrared
spectrum reflects a cooperative strengthening of both π-bound
and OH···O H-bonds due to binding to TCP. The TCP–(H<sub>2</sub>O)<sub><i>n</i></sub>, <i>n</i> = 3–5
clusters all form H-bonded cycles, retaining their preferred structures
in the absence of TCP, but distorted significantly by the presence
of the TCP molecule. TCP–(H<sub>2</sub>O)<sub>3</sub> divides
its population between two conformational isomers that differ in the
direction of the H-bonds in the cycle, either clockwise or counterclockwise,
which are distinguishable by virtue of the <i>C</i><sub>2</sub> symmetry of the TCP monomer. TCP–(H<sub>2</sub>O)<sub>4</sub> and TCP–(H<sub>2</sub>O)<sub>5</sub> have OH stretch
IR spectra that are close analogues of their benzene–(H<sub>2</sub>O)<sub><i>n</i></sub> counterparts in the H-bonded
OH stretch region, but differ somewhat in the free and π OH
stretch regions as the tetramer and pentamer cycles begin to spill
out of the pocket interior. Lastly, excited state RIDIR spectroscopy
in the OH stretch region is used to probe the response of water cluster
to ultraviolet excitation, showing how the proximity of a given water
molecule to the aromatic-rich π clouds affects the infrared
spectrum of the water network