Solid-state NMR studies of alkali metal ion complexes of p-tertbutyl-calixarenes

Conformation of alkali metal ion-calix[4]arene complexes investigated by IR spectroscopy in the gas phase

The OH bands of M⁺·C4A shift to the red from K⁺ to Cs⁺, indicating reduction of conformer distortion.

Electronic structure and conformational conversion of calix[4]arene complexes with alkali metal ions

The UV spectra of M⁺·(calix[4]arene) complexes under cold gas-phase conditions suggest an interaction between two of the four benzene rings in the M = Na and K complexes.

Complexation of solvents and conformational equilibria in solutions of the simplest calix[4]arenes

Structure optimization and calculation of electronic adsorption spectra of 25,26,27,28-tetrahydroxycalix[4]arene and 25,27-dimethoxy-26,28-dihydroxycalix[4]arene conformers have been performed by density-functional theory using hybrid B3LYP functional in cc-p VTZ and cc-p VDZ basis sets in Gaussian 09 package. Analysis of experimental UV-Vis spectra of solutions of 25,26,27,28-tetrahydroxycalix[4]arene, 4-tert-butylcalix[4]arene and 25,27-dimethoxy-26,28-dihydroxycalix[4]arene in various solvents has been carried out. It was shown that the ratio of absorption maxima at characteristic wavelengths at ca. 274 and 283 nm may be used to assess the extent of calix[4]arene/solvent interactions. The conclusion is drawn that spectral characteristics of calix[4]arenes are strongly affected by acid-base interactions.

Gas-phase and solution studies of three resorcin[4]arene derivatives using electrospray time-of-flight mass spectrometry

Electrospray ionisation mass spectrometry (ESI-MS) has been used to study the relative gas-phase proton and alkali metal (Li, Na, K and Cs) binding affinities of three different resorcin[4]arenes using the kinetic method. Collision-induced dissociation (CID) was used to study the fragmentation of resorcin[4]arene heterodimer sandwich complexes, allowing the relative binding affinity order to be established. All the alkali metal cations have the same gas-phase binding affinity order with the resorcin[4]arene host molecules. At collision energies of > or = 13eV, one of the [resorcin[4]arene+Metal]+, (Metal = Li, Na, K) ions fragmented through break-up of the resorcin[4]arene, whilst the other host resorcin[4]arene remained intact, causing an apparent change in binding affinity at high collision energy. This effect was not observed with caesium, since all complex ions dissociated readily under CID by displacement of the caesium cation. The binding affinity for the protonated resorcin[4]arenes was found to be different from the alkali metal cation binding affinity because of the higher proton affinity of the nitrogen-containing resorcin[4]arenes. It is shown that resorcin[4]arenes containing an oxazine ring can be converted into a ring-opened derivative via an Eschweiler-CLarke reaction in the presence of formic acid. A second ring-opening process also occurs, including a hydrolysis reaction that results in apparent Losses of 12 mass units from the intact resorcin[4]arene. Both these reactions occur in solution before mass spectrometric investigation and cannot be achieved by CID. This observation was confirmed by inducing the Eschweiter-CLarke reaction in a model benzoxazine compound.

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

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.

Electronic spectra of jet-cooled calix[4]arene and its van der Waals clusters: Encapsulation of a neutral atom in a molecular bowl

The encapsulation of neutral guest has been studied for calix[4]arene (C4A) by forming van der Waals clusters with Ar and Ne in supersonic jets. The electronic transitions of these clusters suggest that the first Ar (Ne) is encapsulated inside the C4A cavity, while the next atoms are bound outside.