Binding water clusters to an aromatic-rich hydrophobic pocket: [2.2.2]paracyclophane-(H2O)n, n = 1-5.
[2.2.2]Paracylcophane (tricyclophane, TCP) is a macrocycle with three phenyl substituents linked by ethyl bridges (-CH2CH2-) 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. J. Chem. Phys. 2013, 138, 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-(H2O)n clusters with n = 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 n = 1-5. The TCP-(H2O)n S0-S1 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-(H2O)n (n = 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 H2O molecule in TCP-(H2O)1 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 H2O in the complex is composed of a closely spaced set of transitions that likely reflect contributions from both para- and ortho-forms of H2O due to internal rotation of the H2O in the binding pocket. TCP-(H2O)2 also exists in a single conformational isomer that retains the same double-donor binding motif for the first water molecule, with the second H2O 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-(H2O)n, n = 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-(H2O)3 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 C2 symmetry of the TCP monomer. TCP-(H2O)4 and TCP-(H2O)5 have OH stretch IR spectra that are close analogues of their benzene-(H2O)n 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.
Department of Chemistry, Purdue University , West Lafayette, Indiana 47907-2084, United States.
Evan G
EG
Buchanan
Timothy S
TS
Zwier