1. OSU International Symposium on Molecular Spectroscopy
Halogen bonds and hydrogen bonds in the gas phase:
Similarity revealed through rotational spectroscopy
A C Legon
24 June 2009

2. (Matrix isolation studies of BHX and BXY by Bruce Ault
1. How are properties of isolated hydrogen- and halogen-bonded complexes BHX and BXY obtained? What can they tell us?
Properties of isolated weakly bound complexes are intrinsic properties, free of solvent, lattice, etc. perturbations
 Rotational spectra of pairs BHX isolated in the gas phase.
(Matrix isolation studies of BHX and BXY by Bruce Ault
and co-workers and Lester Andrews and co-workers)

3. Analysis of rotational spectra gives:
 Radial and angular geometry (separation and relative orientation of B and HX in space)
 Strength of the intermolecular binding (as measured by kσ, obtained from centrifugal distortion constants)
 Extent of electric charge rearrangement on formation of BHX (from nuclear quadrupole coupling constants).
 In some cases, details of the potential energy function

4. J = 2←1 transitions in the rotational spectrum of H2OHF
recorded by the Stark-modulation technique at T =200 K
Jim Millen
Stark effects

5. Vibrational satellites in low-frequency, intermolecular bending modes
 Nuclear spin statistical weight alternations.
 Variation of rotational constants with v(o)
 Relative intensity of vibrational satellites in the mode (o)
gives vibrational spacings v(o) = 1←0, 2←0, etc.
 Variation of electric dipole moment with v(o)
 For D2ODF also.
 Fit all data to potential energy function V( ) =A 4 +B2

6. Potential energy
function for the
bending mode νβ(o)
in H2OHF

9. Assumptions:
(i) unperturbed geometry for each component
(ii) linear OH─F system (linear H bond)

10. Use jet expansion of B and HX to give very low
What if the equilibrium amount of BHX is too small
to allow detection of its rotational spectrum?
Use jet expansion of B and HX to give very low
effective T and record BHX rotational spectrum
in a microwave Fabry-Perot cavity (i.e. a pulsed-jet,
F-T microwave spectrometer)
Bill Flygare

12. 2. Rationalisation of angular geometries of BHX
Some rules
The angular geometry of a hydrogen bonded complex BHX can be predicted by assuming that, in the equilibrium arrangement, the axis of the HX molecules lies:
Along the axis of a non-bonding electron pair of B,
Along the local symmetry axis of a  orbital,
if B carries no n pairs.
3) If B carries both n- and  -electron pairs,
rule 1) is definitive

13. Halogens XY react with simple Lewis bases
How to investigate halogen-bonded complexes BXY?
Halogens XY react with simple Lewis bases
e.g. ClF + ethyne → explosion
To avoid this, take advantage of the properties of the supersonic expansion

17. 3. Angular geometries of BHCl and BClF:
A preliminary look

27. Flygare et al.

28. Are hydrogen bonds always linear?

29. Difficulties of locating H in complexes BHX (X=F,Cl or Br)
 H makes very small contribution to principal moments of inertia. (Small mass, close to centre of mass).
 Large changes in zero-point motion when H of hydrogen bond is substituted by D leads to large errors in locating H by substitution method.
 Is there an alternative approach?

30. A way of determining H-bond nonlinearity

31. A way of determining H-bond nonlinearity

32. Ab initio calculations at the
CCSD(T)/pVQZ level θ = 17.7° and φ = 73.9°

33. J L Alonso et al.

34. A more refined comparison
Angular geometries of BHCl and BClF:
A more refined comparison

37. Ab initio calculations at
the CCSD(T)/pVQZ level
θ = 17.7°
θ = 4.3°

40. Conclusions Angular geometries of BClF and BHCl are
isomorphous for a given B but, when symmetry allows,
the hydrogen bond in B HCl has a greater propensity
to be nonlinear.
Why?

42. 6. Are complexes BHCl and BClF always isomorphous?

43. (Shea, Kukolich, JCP 1981)

47. 7. Proton transfer in the gas phase:
Is there a halogen bond equivalent?

53. Does X+ transfer occur in halogen-bonded complexes?
F─ F bond is weak, so (CH3)3N with F2 is system most
likely to exhibit F+ transfer

58. *
* A Karpfen, Theo. Chem. Acc., 110,1, Also gives μ=8.0 Debye and implies kσ ~80 N m-1

59. Acknowledgements Hydrogen bonds Halogen bonds Funding EPSRC
Equilibrium gas mixtures
UCL
Pulsed-jet spectroscopy
University of Exeter
John Bevan
Stephen Rogers
Andreas Georgiou
Zybszek Kisiel
Hugh Warner
Joanna Thorn
David Lister
Kelvin Hinds
Christopher Evans
Hannelore Bloemink
Stephen Cooke
Gina Cotti
Gary Corlett
Eric Waclawik
Jenny Thumwood
James Davey
Pulsed-jet spectroscopy
UCL/University of Exeter
Charles Willoughby
Elizabeth Goodwin
Annette Travis
Nigel Howard
Andrew Wallwork
Christopher Rego
George Cole
Sean Peebles
Funding
EPSRC
Leverhulme Trust
Ruth King Trust

62.  /deg (18) (14) (1) (4)
 /deg (32) (6) (1) (2)

66. NB kσ /(N m-1) ICl NH3 ICl ClF ClF BrCl BrCl Br2 Br2 PH3 Cl2 Cl2 H2S
HCN
H2O
kσ /(N m-1)
C2H4
C2H2 CO
B = N2 NB