Structure of the calix[4]arene-(H2O) cluster: the world's smallest cup of water.

The structure of the calix[4]arene(C4A)-(H(2)O) cluster formed in a supersonic beam has been investigated by mass-selected resonant two-photon ionization (R2PI) spectroscopy, IR-UV double resonance spectroscopy, IR photodissociation (IRPD) spectroscopy and by high-level quantum chemical calculations. The IR-UV double resonance spectrum of C4A-(H(2)O) exhibits a broad and strong hydrogen-bonded OH stretching band at 3160 cm(-1) and a weak asymmetric OH stretching band at 3700 cm(-1). The IRPD measurement of the cluster produced a value of 3140 cm(-1) for the C4A-(H(2)O) --> C4A + H(2)O dissociation energy. High-level electronic structure calculations at the MP2 level of theory with basis sets up to quadruple-zeta quality suggest that the endo-isomer (water inside the C4A cavity) is approximately 1100 cm(-1) more stable than the exo-isomer (water hydrogen bonded to the rim of C4A). The endo-isomer has a best-computed (at the MP2/aug-cc-pVQZ level) value of 3127 cm(-1) for the binding energy, just approximately 15 cm(-1) shy of the experimentally determined threshold and an IR spectrum in excellent agreement with the experimentally observed one. In contrast, the B3LYP density functional fails to even predict a stable structure for the endo-isomer demonstrating the inability of that level of theory to describe the delicate balance between structures exhibiting cumulative OH-pi H-bonding and dipole-dipole interactions (endo-isomer) when compared to the ones emanating from maximizing the cooperative effects associated with the formation of hydrogen bonded homodromic networks (exo-isomer). The comparison of the experimental results with the ones from high-level electronic structure calculations therefore unambiguously assign the endo-isomer as the global minimum of the C4A-(H(2)O) cluster, world's smallest cup of water.