title: Structure of the calix[4]arene-(H2O) cluster: the world's smallest cup of water.
abstract: 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.
authors.affiliation: Department of Chemistry, Graduate School of Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan.
authors.forename: Naoya
authors.initials: N
authors.lastname: Hontama
authors.forename: Yoshiya
authors.initials: Y
authors.lastname: Inokuchi
authors.forename: Takayuki
authors.initials: T
authors.lastname: Ebata
authors.forename: Claude
authors.initials: C
authors.lastname: Dedonder-Lardeux
authors.forename: Christophe
authors.initials: C
authors.lastname: Jouvet
authors.forename: Sotiris S
authors.initials: SS
authors.lastname: Xantheas
mesh_terms: D047250:Calixarenes / Q000737:chemistry*; D003198:Computer Simulation; D006860:Hydrogen Bonding; D007536:Isomerism; D008956:Models, Chemical; D015394:Molecular Structure; D010636:Phenols / Q000737:chemistry*; D014867:Water / Q000032:analysis* / Q000737:chemistry
chemical_list: D010636:Phenols; C121325:calix(4)arene; D014867:Water; D047250:Calixarenes