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

(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)

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

J = 2←1 transitions in the rotational spectrum of H2OHF recorded by the Stark-modulation technique at T =200 K Jim Millen 1923-1999 Stark effects

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

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

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

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 1936-1981

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

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

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

Flygare et al.

Are hydrogen bonds always linear?

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?

A way of determining H-bond nonlinearity

A way of determining H-bond nonlinearity

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

J L Alonso et al.

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

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

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?

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

(Shea, Kukolich, JCP 1981)

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

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

* * A Karpfen, Theo. Chem. Acc., 110,1, 2003 Also gives μ=8.0 Debye and implies kσ ~80 N m-1

Acknowledgements Hydrogen bonds Halogen bonds Funding EPSRC Equilibrium gas mixtures UCL 1974-1983 Pulsed-jet spectroscopy University of Exeter 1993-2005 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 1981-2005 Charles Willoughby Elizabeth Goodwin Annette Travis Nigel Howard Andrew Wallwork Christopher Rego George Cole Sean Peebles Funding EPSRC Leverhulme Trust Ruth King Trust

 /deg 57.8(18) 90.7(14) 88(1) 122.6(4)  /deg 16.2(32) 30.0(6) 40(1) 36.5(2)

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