Pre-reactive complexes in mixtures of water vapour with halogens: Characterisation of H2O center dot center dot center dot ClF and H2O center dot center dot center dot F-2 by a combination of rotational spectroscopy and ab initio calculations

Complexes H2O . . . ClF and H2O . . . F-2 were detected by means of their g round-state rotational spectra in mixtures of water vapour with chlorine mo nofluoride and difluorine, respectively. A fast-mixing nozzle was used in c onjunction with a pulsed-jet, Fourier-transform microwave spectrometer to p reclude the vigorous chemical reaction that these dihalogen species undergo with water. The ground-state spectra of seven isotopomers ((H2O)-O-16 . . . (ClF)-Cl-35, (H2O)-O-16 . . . (ClF)-Cl-37, (H2O)-O-18 . . . (ClF)-Cl-35, (D2O)-O-16 . . . (ClF)-Cl-35, (D2O)-O-36 . . . (ClF)-Cl-37, HDO . . . (CIF) -C-35 and HDO . . . (ClF)-Cl-37) of the ClF complex and five isotopomers (H 2O . . . F-2, (H2O)-O-18 . . . F-2, D2O . . . F-2,F- (D2O)-O-18 . . . F-2 a nd HDO . . . F-2) of the F-2 complex were analysed to yield rotational cons tants, quartic centrifugal distortion constants and nuclear hyperfine coupl ing constants. spectroscopic constants were interpreted with the aid of sim ple models of the complexes to give effective geometries and intermolecular stretching force constants. Isotopic substitution showed that in each comp lex the H2O molecule acts as the electron donor and either ClF or F-2 acts as the electron acceptor, with nuclei in the order H2O . . . ClF or H2O . . . F-2. For H2O ClF, the angle phi between the bisector of the HOH angle an d the O . . . Cl internuclear line has the value 58.9(16)degrees, while the distance r(O . . . Cl) = 2.6081(23) Angstrom. The corresponding quantities for H2O . . . F-2 are phi = 48.5(21)degrees and r(O . . . F-i) = 2.7180(27 ) Angstrom, where F-i indicates the inner F atom. The potential energy V(ph i) as a function of the angle phi was obtained from ab initio calculations at the aug-cc-pVDZ/MP2. level of theory for each complex by carrying out ge ometry optimisations at fixed values of phi in the range +/-80 degrees. The global minimum corresponded to a complex of C-s symmetry with a pyramidal configuration at O in each. The function V(phi) was of the double-minimum t ype in each case with equilibrium values phi (c) = +/- 55.8 degrees and +/- 40.5 degrees for H2O . . . ClF and H2O . . . F-2, respectively. The barrie r at the planar C-2v conformation was V-upsilon = 174cm(-1) for H2O . . . C lF and 7cm(-1) for H2O . . . F-2. For the latter complex, the zero-point en ergy level lies above the top of the barrier.