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-Ar(2) trimer the calculations support the existence of two nearly isoenergetic isomers: one is the {2 : 0} endo-complex, in which the Ar(2) 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.

Limitations and Extensions of the Lock-and-Key Principle: Differences between Gas State, Solution and Solid State Structures

“…The rather shallow cavity of small calixarenes lead particularly often to extra- (or exo-) cavity complexation, although the simple lock-and-key principle would predict an intra- (or endo-) complex. For complexes between argon and calix[4]arene in the gas state, spectroscopic and quantum-chemical calculations show both orientations, as expected with a preference for the endo-complex ( Figure 12 ) [ 47 , 48 ]. Laser spectroscopic molecular beam experiments and computations of calix[4]arene complexes with a variety of small ligands such as NH 3 , N 2 , CH 4 , and C 2 H 2 indicate also preferred endo complexes, for H 2 O and NH 3 as guest mainly by dipole–dipole interactions, for Ar, N 2 , CH 4 and C 2 H 2 mostly by dispersion forces [ 49 ].…”

Structure of the Calix[4]arene−(H2O) Cluster: The World’s Smallest Cup of Water

“…The electronic spectrum of C4A-Ar is very similar to that of the bare C4A, except that all the bands are red-shifted by 45 cm -1 with respect to those of C4A. The C4A-Ar cluster is an endo-complex, in which the Ar atom is located inside the C4A cavity, retaining overall C 4 symmetry 25. In contrast, the spectrum of C4A-(H 2 O) is quite different from that of either C4A or C4A-Ar.…”

Laser Spectroscopic and Theoretical Studies of Encapsulation Complexes of Calix[4]arene

“…In our previous work 25 we have determined that the C4A-Ar complex forms an endo-structure of C 4 symmetry, in which the Ar atom is located inside the C4A cavity lying along the C 4 axis of C4A. 25 For the C4A-N 2 complex a sharp band origin at 35292 cm -1 is observed, which is shifted by 65 cm -1 from that of bare C4A.…”

“…The CCSD(T)/aug-cc-pVDZ binding energies for M=Ar, H 2 O and NH 3 are shown in parentheses. The Table includes the results for the H 2 O and Ar hosts reported earlier 15,25 for completeness. Estimates of the Complete Basis Set (CBS) limit at the MP2 level using the interaction for the former is smaller than that of the later (as the permanent dipole moment of NH 3…”