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The aim of my research is to understand non-covalent interactions of closed-shell molecules. The focus is on factors controlling the properties (especially directionality, interaction strength and electric-charge redistribution) of complexes involving the hydrogen bond B…HX, the halogen bond B…XY and, recently, the ‘coinage-metal’ bond B…MX (B is a simple Lewis base, HX a hydrogen halide, XY a di-halogen and M is Cu or Ag). Precise values of the properties of such complexes in effective isolation are determined by rotational spectroscopy[1-3].

For the hydrogen bond, many series of complexes B…HX have been examined by systematically varying B (for example, B = N₂, CO, C₂H₂, C₂H₄, HCN, H₂S, H₂O, PH₃ or NH₃, etc.) and then HX to reveal the dependence of properties of isolated B…HX molecules on the nature of B and HX. Generalizations about the H bond have emerged. For example, a model has been proposed to account for its directionality [4], namely that the HX molecule lies along the axis of a non-bonding or π-bonding electron pair carried by B. Another generalization⁵ concerns the intermolecular stretching force constant k_σ for B…HX (one measure of the strength of the hydrogen bond). It turn out that k_σ can be reproduced by assigning a nucleophilicity and an electrophilicity to B and HX, respectively. Investigations of the ammonium and methylammonium halides (CH₃)_nH_3-nN…HX revealed⁶ that the extent of proton transfer to the base (i) is small for n = 0 but increases with n for a given X, and (ii) increases in the order F < Cl < Br < I for (CH₃)₃N…HX, until for X = I the ion pair (CH₃)₃NH⁺…I^- is the preferred description.⁷

Existence of the halogen bond in the gas-phase was established in a parallel approach for complexes B…XY (XY is F₂, Cl₂, Br₂, ClF, BrCl or ICl), using a similar, wide range of Lewis bases B.^8-12 To form B…XY but avoid the often violent reaction of B with XY, a fast-mixing technique was used to ‘freeze’ the ‘pre-reactive’ complex while its spectrum was recorded.¹¹ Striking parallels between the properties of B…XY (now extended to include XY = ICF₃)¹³ and their B…HX analogues emerged.^11,14 Evidently, the generalizations for hydrogen bonds also hold for halogen bonds. Indeed, the parallels are so strong that the non-covalent interaction in B…XY can be called a ‘halogen’ bond¹¹ to emphasize its analogy with the hydrogen bond. A significant difference between hydrogen and halogen bonds is the greater propensity for non-linearity in the former.^11,15 The non-linearity is accentuated in the weakly bound complexes B…HCCH because the primary hydrogen bond is weak and the secondary attraction responsible for the non-linearity is more significant.¹⁶

Recently, new molecules B…MCl (e.g. B = N₂, H₂O, H₂S, C₂H₄, C₂H₂, PH₃ or NH₃; M= Ag or Cu)^17-20 have been synthesized in the gas phase by laser ablation methods and have been characterized by rotational spectroscopy. They posses properties (e.g. angular geometry) parallel to those of the corresponding B…HX and B…XY, despite the larger non-covalent interaction strength that arises from the much increased polarity of MCl compared with HX or XY. This suggests the existence of a ‘silver’ or ‘copper’ bond analogue of hydrogen and halogen bonds.^17-20 Studies of other B…MX are planned to test the general applicability of the ‘coinage-metal’ bond description.

References

1. Gas-phase spectroscopy and the properties of hydrogen-bonded dimers: HCN…HF as the spectroscopic prototype, A. C. Legon and D. J. Millen, Chemical Reviews, 86, 635-57, (1986).

2. The rotational spectrum, chlorine nuclear quadrupole coupling constants, and molecular geometry of a hydrogen-bonded dimer of cyclopropane and hydrogen chloride, A. C. Legon, P. D. Aldrich and W. H. Flygare, J. Amer. Chem. Soc., 104, 1486-90, (1982).

3. Spectroscopic investigations of hydrogen bonding interactions in the gas phase. VII. The equilibrium conformation and out-of-plane bending potential energy function of the hydrogen-bonded heterodimer H₂O…HF determined from its microwave rotational spectrum, Z. Kisiel, A. C. Legon and D. J. Millen, Proc. Roy. Soc. Lond. A, 381, 419-42, (1982).

4. Angular geometries and other properties of hydrogen-bonded dimers: a simple electrostatic interpretation based on the success of the electron-pair model, A. C. Legon and D. J. Millen, Chem. Soc. Reviews, 16, 467-498, (1987).

5. Hydrogen bonding as a probe for electron densities: Limiting gas phase nucleophilicities and electrophilicities of B and HX, A. C. Legon and D. J. Millen, J. Amer. Chem. Soc., 109, 356-8, (1987).

6. The nature of the ammonium and methylammonium halides in the vapour phase: Hydrogen bonding versus proton transfer, A. C. Legon, Chemical Society Reviews, 22, 153-163, (1993).

7. Rotational spectrum of the trimethylamine-hydrogen iodide dimer: An ion pair (CH₃)₃NH⁺…I^- in the gas phase, A. C. Legon and C. A. Rego, J. Chem. Phys., 99, 1463-68, (1993).

8. Characterisation of the intermediate C₂H₄…Cl₂ in a gaseous mixture of ethene and chlorine by rotational spectroscopy: a weak ∏-type donor-acceptor complex, H.I. Bloemink, K. Hinds, A.C. Legon and J.C. Thorn, Chemistry - a European Journal, 1, 17-25, (1995).

9.. Rotational spectroscopy of mixtures of trimethylamine and fluorine: Identification of the ion pair [(CH₃)₃NF]⁺…F^- in the gas phase, H.I. Bloemink, S.A. Cooke, J.H. Holloway and A.C. Legon, Angew. Chem. Intl. Ed. Engl., 36, 1340-1342, (1997).

10. Evidence concerning the relative nucleophilicities of nonbonding and ∏-bonding electrons from the rotational spectrum of furan…ClF, S.A. Cooke, G.K. Corlett, J.H. Holloway and A.C. Legon, J. Chem. Soc. Faraday Trans., 94, 2675-2680, (1998)

11. Pre-reactive complexes of dihalogens XY with Lewis bases B in the gas phase: A systematic case for the ‘halogen’ analogue B…XY of the hydrogen bond B…HX, A.C. Legon, Angew. Chem. Int. Ed. Engl., 38, 2686-2714, (1999).

12. Pre-reactive complexes in mixtures of water vapour with halogens: Characterisation of H₂O…ClF and H₂O…F₂ by a combination of rotational spectroscopy and ab initio calculations, S.A. Cooke, G. Cotti, C.M. Evans, J.H. Holloway, Z. Kisiel, A.C. Legon and J.M.A. Thumwood, Chemistry - A European Journal, 7, 2295-2305, (2001).

13. Rotational spectra and properties of complexes B…ICF₃ (B = Kr or CO) and a comparison of the efficacy of ICl and ICF₃ as iodine donors in halogen bond formation, S. L. Stephens, N. R. Walker and A. C. Legon, J. Chem. Phys., 135, 224309, (2011).

14. The halogen bond: an interim perspective, A. C. Legon, Phys. Chem. Chem. Phys., 12, 7736-7747, (2010).

15. Nonlinear hydrogen bonds and rotational spectroscopy: Measurement and rationalisation of the deviation from linearity, A.C. Legon, Faraday Discuss. Chem. Soc., 97, 19-33, (1994).

16. Rotational spectrum, inversion and geometry of 2,5-dihydrofuran…ethyne and a generalization about Y…H-C hydrogen bonds, G.C. Cole, R.A. Hughes and A.C. Legon, J. Chem. Phys., 122, 134311, (2005).

17. Monohydrates of cuprous chloride and argentous chloride: H₂O…CuCl and H₂O…AgCl characterised by rotational spectroscopy and ab initio calculations, V. A. Mikhailov, F. J. Roberts, S. L. Stephens, S.J. Harris, D. P. Tew, J. N. Harvey, N. R. Walker and A. C. Legon, J. Chem. Phys., 134, 134305, (2011).

18. Characterisation of H₂S…CuCl and H₂S…AgCl isolated in the gas phase: A rigidly pyramidal geometry at sulphur revealed by rotational spectroscopy and ab initio calculations, N. R. Walker, D. P. Tew, S. J. Harris, D. E. Wheatley and A. C. Legon, J. Chem. Phys., 134, 014307, (2011).

19. H₃N…AgCl: Synthesis in a supersonic jet and characterisation by rotational spectroscopy, V. A. Mikhailov, D. P. Tew, N. R. Walker and A. C. Legon, Chem. Phys. Letters, 499, 16-20, (2010).

20. A prototype transition metal-olefin complex C₂H_4…AgCl synthesised by laser ablation and characterised by rotational spectroscopy and ab initio methods, S. L. Stephens, D. P. Tew, V. A. Mikhailov, N. R. Walker and A. C. Legon, J. Chem. Phys., 134, 024315, (2011).

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