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Pre‐reactive Complexes in Mixtures of Water Vapour with Halogens: Characterisation of H 2 O⋅⋅⋅ClF and H 2 O⋅⋅⋅F 2 by a Combination of Rotational Spectroscopy and Ab initio Calculations
Author(s) -
Cooke Stephen A.,
Cotti Gina,
Evans Christopher M.,
Holloway John H.,
Kisiel Zbigniew,
Legon Anthony C.,
Thumwood Jennifer M. A.
Publication year - 2001
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20010601)7:11<2295::aid-chem22950>3.0.co;2-t
Subject(s) - chemistry , isotopomers , hyperfine structure , rotational spectroscopy , halogen , molecule , analytical chemistry (journal) , atomic physics , alkyl , organic chemistry , chromatography , physics
Complexes H 2 O⋅⋅⋅ClF and H 2 O⋅⋅⋅F 2 were detected by means of their ground‐state rotational spectra in mixtures of water vapour with chlorine monofluoride and difluorine, respectively. A fast‐mixing nozzle was used in conjunction with a pulsed‐jet, Fourier‐transform microwave spectrometer to preclude the vigorous chemical reaction that these dihalogen species undergo with water. The ground‐state spectra of seven isotopomers (H 2 16 O⋅⋅⋅ 35 ClF, H 2 16 O⋅⋅⋅ 37 ClF, H 2 18 O⋅⋅⋅ 35 ClF, D 2 16 O⋅⋅⋅ 35 ClF, D 2 16 O⋅⋅⋅ 37 ClF, HDO⋅⋅⋅ 35 ClF and HDO⋅⋅⋅ 37 ClF) of the ClF complex and five isotopomers (H 2 O⋅⋅⋅F 2 , H 2 18 O⋅⋅⋅F 2 , D 2 O⋅⋅⋅F 2 , D 2 18 O⋅⋅⋅F 2 and HDO⋅⋅⋅F 2 ) of the F 2 complex were analysed to yield rotational constants, quartic centrifugal distortion constants and nuclear hyperfine coupling constants. These spectroscopic constants were interpreted with the aid of simple models of the complexes to give effective geometries and intermolecular stretching force constants. Isotopic substitution showed that in each complex the H 2 O molecule acts as the electron donor and either ClF or F 2 acts as the electron acceptor, with nuclei in the order H 2 O⋅⋅⋅ClF or H 2 O⋅⋅⋅F 2 . For H 2 O⋅⋅⋅ClF, the angle ϕ between the bisector of the HOH angle and the O⋅⋅⋅Cl internuclear line has the value 58.9(16)°, while the distance r (O⋅⋅⋅Cl)= 2.6081(23) Å. The corresponding quantities for H 2 O⋅⋅⋅F 2 are ϕ =48.5(21)° and r (O⋅⋅⋅F i )=2.7480(27) Å, where F i indicates the inner F atom. The potential energy V ( ϕ ) as a function of the angle ϕ was obtained from ab initio calculations at the aug‐cc‐pVDZ/MP2 level of theory for each complex by carrying out geometry optimisations at fixed values of ϕ in the range ±80°. The global minimum corresponded to a complex of C s symmetry with a pyramidal configuration at O in each. The function V ( ϕ ) was of the double‐minimum type in each case with equilibrium values ϕ e =±55.8° and ±40.5° for H 2 O⋅⋅⋅ClF and H 2 O⋅⋅⋅F 2 , respectively. The barrier at the planar C 2v conformation was V 0 =174 cm −1 for H 2 O⋅⋅⋅ClF and 7 cm −1 for H 2 O⋅⋅⋅F 2 . For the latter complex, the zero‐point energy level lies above the top of the barrier.