Encapsulation of Arn complexes by Calix[4]arene : Endo- vs. exo-complexes

Ebata Takayuki

Hontama Naoya

Inokuchi Yoshiya

Haino Takeharu

Aprà Edoardo

Xantheas Sotiris S.

Physical Chemistry Chemical Physics

The structure of the calix[4]arene(C4A)-Arn 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 of forming van der Waals complexes with rare gas atoms. For the C4A-Ar dimer, two isomers are found. 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 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 calculation, depending on whether the Ar atom resides inside (endo) or outside (exo) the C4A. We propose a novel scheme to derive CCSD(T)-quality binding energies for the C4A-Ar complex based on the ratio of CCSD(T)/MP2 energies for the smaller model systems Benzene-Ar and Phenol-Ar. Our best computed estimates for the binding energies of the C4A-Ar endo- and endo-complexes are 1560 cm-1 and 510 cm-1, respectively, at the CCSD(T)/Complete Basis Set (CBS) level of theory. For the C4A-Ar2 trimer, we again identified two isomers in the calculation: 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. The calculated binding energies for the two C4A-Ar2 isomers are nearly identical. 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-Arn complexes because of the systematic red-shifts of the complexes with the number of bound Ar atoms, that is the Arn 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.

Chemistry [ 430 ]

eng

Royal Society of Chemistry

Copyright (c) the Owner Societies 2010

[ISSN] 1463-9076

[DOI] 10.1039/B927441C

[DOI] http://dx.doi.org/10.1039/B927441C isVersionOf