Encapsulation of Arn complexes by calix[4]arene: endo- vs. exo-complexes

Abstract

The structure of the calix[4]arene(C4A)–Ar_n complexes has been investigated by laser induced fluorescence spectroscopy, mass-selected resonant two-color two-photon ionization (2C-R2PI) spectroscopy, fragment detected IR photodissociation (FDIRPD) spectroscopy, and high level first principles electronic structure calculations at the MP2 and CCSD(T) levels of theory. C4A has a very high ability to form van der Waals complexes with rare gas atoms. For the C4A–Ar dimer two isomers are observed. A major species shows a 45 cm⁻¹ red-shift of its band origin with respect to the monomer, while that of a minor species is 60 cm⁻¹. The binding energy of the major species is determined to be in the range of 350–2250 cm⁻¹ from 2C-R2PI spectroscopy and FDIRPD spectroscopy. Two isomers are also identified in the quantum chemical calculations, depending on whether the Ar atom resides inside (endo) or outside (exo) the C4A. We propose a scheme to derive CCSD(T)/Complete Basis Set (CBS) quality binding energies for the C4A–Ar complex based on CCSD(T) calculations with smaller basis sets and the ratio of CCSD(T)/MP2 energies for the smaller model systems benzene–Ar and phenol–Ar, for which the CCSD(T) level of theory converges to the experimentally determined binding energies. Our best computed estimates for the binding energies of the C4A–Ar endo- and endo-complexes at the CCSD(T)/CBS level of theory are 1560 cm⁻¹ and 510 cm⁻¹, respectively. For the C4A–Ar₂ trimer the calculations support the existence of two nearly isoenergetic isomers: one is the {2 : 0} endo-complex, in which the Ar₂ dimer is encapsulated inside the C4A cavity, and the other is the {1 : 1} endo–exo-complex, in which one Ar resides inside and the other outside the C4A cavity. However, the experimental evidence strongly suggests that the observed species is the {2 : 0} endo-complex. The endo structural motif is also suggested for the larger C4A–Ar_n complexes because of the observed systematic red-shifts of the complexes with the number of bound Ar atoms suggesting that the Ar_n complex is encapsulated inside the C4A cavity. The formation of the endo-complex structures is attributed to the anisotropy of the interaction with C4A during the complex formation in the expansion region.