The structure of the calix[4]arene(C4A)-(H2O) 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-(H2O) 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-(H2O) → C4A + H2O 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 ~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 ~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-π 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-(H2O) cluster, world’s smallest cup of water.

This is a preprint of an article published by American Chemical Society in Journal of Physical Chemistry A, 2010, available online: http://pubs.acs.org/doi/abs/10.1021/jp902967q