Laser spectroscopic and theoretical studies of encapsulation complexes of calix[4]arene.

The complexes between the host calix[4]arene (C4A) and various guest molecules such as NH(3), N(2), CH(4), and C(2)H(2) have been investigated via experimental and theoretical methods. The S(1)-S(0) electronic spectra of these guest-host complexes are observed by mass-selected resonant two-photon ionization (R2PI) and laser-induced fluorescence (LIF) spectroscopy. The IR spectra of the complexes formed in molecular beams are obtained by IR-UV double resonance (IR-UV DR) and IR photodissociation (IRPD) spectroscopy. The supramolecular structures of the complexes are investigated by electronic structure methods (density functional and second order perturbation theory). The current results for the various molecular guests are put in perspective with the previously reported ones for the C4A-rare gas (Rg) (Phys. Chem. Chem. Phys. 2007, 126, 141101) and C4A-H(2)O complexes (J. Phys. Chem. A, 2010, 114, 2967). The electronic spectra of the complexes of C4A with N(2), CH(4), and C(2)H(2) exhibit red-shifts of similar magnitudes with the ones observed for the C4A-Rg complexes, whereas the complexes of C4A with H(2)O and NH(3) show much larger red-shifts. Most of the IR-UV DR spectra of the complexes, except for C4A-C(2)H(2), show a broad hydrogen-bonded OH stretching band with a peak at ~3160 cm(-1). The analysis of the experimental results, in agreement with the ones resulting from the electronic structure calculations, suggest that C4A preferentially forms endo-complexes (guests inside the host calizarene cavity) with all the guest species reported in this study. We discuss the similarities and differences of the structures, binding energies, and the nature of the interaction between the C4A host and the various guest species.