State of the art theoretical study and comparison to experiment for the phenol•••argon complex

The phenol center dot center dot center dot argon complex was studied by means of various high level ab initio quantum mechanics methods and high resolution threshold ionization spectroscopy. The structure and stabilization energy of different conformers were determined. Stabilization energy of van der Waals bonded and H-bonded PhOH center dot center dot center dot Ar complex determined at CCSD(T) complete basis set (CBS) level for CP-RI-MP2/cc-pVTZ/Ar aug-cc-pVTZ geometries amount to 434 and 285 cm(-1). The CCSD(T)/CBS were constructed either as a sum of MP2/CBS interaction energy and CCSD(T) correction term [difference between CCSD(T) and MP2 correlation energies determined with medium basis set] or directly from CCSD(T)/aug-cc-pVDZ and aug-cc-pVTZ energies. Both schemes provide very similar values. Harmonic vibrational analysis revealed that the H-bonded structure does not represent energy minimum but first order transition structure. The respective imaginary vibrational mode (16 cm(-1)) connects two possible argon locations-above and below the phenol aromatic ring. Including the Delta ZPVE, we obtained stabilization enthalpy at 0 K of 389 cm(-1). This value is marginally higher (25-35 cm(-1), 0.07-0.10 kcal/mol) than the experimental value. The determination of Delta ZPVE constitutes the most significant error and possible improvements should come from more accurate evaluation of the (nonharmonic) vibrational frequencies. (c) 2008 American Institute of Physics.