Encapsulation of Ar(n) complexes by calix[4]arene: endo- vs. exo-complexes

Area of Science:

• Physical Chemistry
• Supramolecular Chemistry
• Computational Chemistry

Background:

• Calix[4]arene (C4A) is known for its ability to form van der Waals complexes with rare gas atoms.
• Understanding the structural and energetic properties of these complexes is crucial for applications in host-guest chemistry and materials science.

Purpose of the Study:

• To investigate the structure and binding energies of calix[4]arene-argon (C4A-Ar(n)) complexes.
• To elucidate the preferred binding sites (endo vs. exo) of argon atoms within the calix[4]arene cavity.
• To develop a reliable computational method for predicting binding energies of such complexes.

Main Methods:

• Experimental techniques: Laser-induced fluorescence spectroscopy, mass-selected resonant two-color two-photon ionization (2C-R2PI) spectroscopy, and fragment detected IR photodissociation (FDIRPD) spectroscopy.
• Computational methods: High-level ab initio electronic structure calculations (MP2 and CCSD(T)) with extrapolation to the Complete Basis Set (CBS) limit.
• Development of a scheme to derive accurate binding energies using CCSD(T)/MP2 energy ratios from smaller model systems.

Main Results:

• Two isomers of the C4A-Ar dimer were experimentally observed, with distinct red-shifts in their band origins.
• Binding energies for the major and minor C4A-Ar isomers were experimentally determined and computationally estimated.
• Calculations predicted two nearly isoenergetic isomers for the C4A-Ar(2) trimer: a {2:0} endo-complex and a {1:1} endo-exo-complex, with experimental evidence favoring the endo-complex.
• The endo structural motif was suggested for larger C4A-Ar(n) complexes, supported by systematic red-shifts with increasing argon atoms.

Conclusions:

• The calix[4]arene cavity preferentially encapsulates argon atoms, forming stable endo-complexes.
• The anisotropic interactions between C4A and argon during complex formation drive the preference for endo structures.
• The developed computational scheme provides accurate binding energy predictions for C4A-Ar complexes, valuable for future supramolecular studies.