Experimental and theoretical characterization of a lone pair-π complex: water-hexafluorobenzene.

The lone pair-π interaction between H(2)O and C(6)F(6) was studied using matrix isolation infrared spectroscopy and quantum chemical calculations. Co-deposition of H(2)O with C(6)F(6) in a nitrogen matrix at 17 K followed by annealing to 30 K, results in the appearance of multiple new peaks in the infrared spectrum that are shifted from the H(2)O and C(6)F(6) parent absorptions. These peaks only appear when both the H(2)O and C(6)F(6) are present and have been assigned to distinct structures of a 1:1 H(2)O·C(6)F(6) complex. Similar experiments were performed with D(2)O and HDO and the corresponding infrared peaks for the structures of the D(2)O·C(6)F(6) and HDO·C(6)F(6) complexes have also been observed. Theoretical calculations were performed for the H(2)O·C(6)F(6) complex using the B3LYP, MP2, and CCSD(T) methods. Geometry optimizations at the B3LYP/aug-cc-pVTZ and MP2/aug-cc-pVDZ levels of theory located three structural minima, all of which involve the lone pair-π interaction between the H(2)O and the C(6)F(6) ring, but with different relative orientations of the H(2)O and C(6)F(6) subunits. BSSE corrected interaction energies were estimated at the CCSD(T)/aug-cc-pVTZ level and found to be between -11.2 and -12.3 kJ/mol for the three H(2)O·C(6)F(6) structures. Vibrational frequencies for the each of the structures were calculated at the B3LYP/aug-cc-pVTZ and MP2/aug-cc-pVDZ levels. The frequencies calculated with both methods support the assignments of the observed new peaks in the infrared spectra to the structures of the H(2)O·C(6)F(6) complex; however, the B3LYP calculated frequency shifts were found to be in better quantitative agreement with the experimentally observed frequency shifts.