Dihalogens halogen bonding

Keywords Lewis bases Dihalogens Halogen bond Angular geometry ... 

Different types of halogen bonding as they occur in the interaction of small molecules have been reviewed. In all cases studied so far, the halogen bond turned out to have a number of characteristic properties reminiscent of the well-known hydrogen bond. The best investigated cases are those in which a dihalogen XY interacts with a Lewis base B. 

This chapter is restricted to a discussion of halogen-bonded complexes B XY that involve a homo- or hetero-dihalogen molecule XY as the electron acceptor and one of a series of simple Lewis bases B, which are chosen for their simplicity and to provide a range of electron-donating abilities. Moreover, we shall restrict attention to the gas phase so that the experimental properties determined refer to the isolated complex. Comparisons with the results of electronic structure calculations are then appropriate. All of the experimental properties of isolated complexes B- XY considered here result from interpreting spectroscopic constants obtained by analysis of rotational spectra. 

The second conclusion concerns the difference Ar = rB...Hx(Z - X)-rB. -xy(Z X) between the Z to X distances in the two series B- HX and B- XY. Ar is positive and nearly constant for a given B and X, when XY is CI2, Br2, BrCl or ClF. Since the order of the internuclear distances is r(XY) > r(HX) for any given atom X, this result means the outer atom Y of the dihalogen molecule XY is always more distant from a given point in B for the complex B- XY than is the atom X from the same reference point in B for the complex B- HX. This second general result is relevant to the discussion of linear versus non-linear hydrogen and halogen bonds in Sect. 6. 

Much of the current research into halogen bonding involves organohalogen or inorganic halide acceptors. However, the dihalogens (X2) and inter halogens (XY) continue to attract attention. This review will focus specifically on structural and theoretical studies of such systems over the past decade. While polyhalide anions are closely related to the neutral donor-acceptor concepts that are the focus here, they will be discussed only when necessary to elaborate on the central theme of the review. 

Formation of a halogen bond at one atom of a dihalogen does not preclude the second atom from also acting as an electron acceptor. The stronger the electron donor, however, the less likely that a D X-X D complex will be formed. 

The reactivity of the initial halogen bonded complex has also received considerable attention. Husebye and coworkers have suggested a process whereby the dihalogen bond is cleaved to give a key cation intermediate and a halide anion (Eq. 4) [182]. This mechanism is consistent with that often proposed for nitrogen electron donor systems. 

Legon AC (2007) The Interaction of Dihalogens and Hydrogen Halides with Lewis Bases in the Gas Phase An Experimental Comparison of the Halogen Bond and the Hydrogen Bond. 126 17-64... 

Pennington WT, Hanks TW, Arman HD (2007) Halogen Bonding with Dihalogens and Interhalogens. 126 65-104... 

Mez+)Me...H-C agostic bond (hydride bond) C-Hal+8... 8Hal-Y dihalogen bond (halogen bond)... 

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