Hydrogen bonding to halogens

For a review of hydrogen bonding to halogens, see Smith, in Patai The Chemistry of the Carbon-Halogen Bond, pt. 1 Wilev New York. 1973. pp. 265-300. See also Bastiansen Femholt Hedberg Scip J Am. Chem. Soc. 1985, 107. 7836. ... 

In the two structures just described the odd structural water molecule is linked to other water molecules and to an oxygen atom of the S04 groups. It is interesting to note, however, that halides rarely crystallize with an odd number of water molecules because hydrogen bonds to halogen atoms (except fluorine) are not, in general, formed and structural water cannot therefore be held in the crystal. 

The effect of a substituent may be substantially modified by fast, concurrent, reversible addition of the nucleophile to an electrophilic center in the substituent. Ortho- and para-CS.0 and pam-CN groups have been found by Miller and co-workers to have a much reduced activating effect on the displacement of halogen in 2-nitrohaloben-zenes with methoxide ion [reversible formation of hemiacetal (143) and imido ester anions (144)] than with azide ion (less interaction) or thiocyanate (little, if any, interaction). Formation of 0-acyl derivatives of 0x0 derivatives or of A-oxides, hydrogen bonding to these moieties, and ionization of substituents are other examples of reversible and often relatively complete modifications under reaction conditions. If the interaction is irreversible, such as hydrolysis of a... 

The main difference between hydrogen bond and the halogen bond lies in the propensity of the hydrogen bond to be non-linear [28,29], when symmetry of the complex is appropriate (molecular point group Cs or Ci). In so far as complexes B- ClF are concerned, the nuclei Z Cl - F, where Z is the acceptor atom/centre in B, appear to be nearly collinear in all cases, while the nuclei Z- H - Cl in complexes B- HC1 of appropriate symmetry often show significant deviations from collinearity. This propensity for the hydrogen-bonded species B- HC1 to exhibit non-linear hydrogen bonds can be understood as follows. 

Br, and I (but not F) for example, hydrochloric acid, HCl, and hydrobromic acid, HBr (HCl and HBr are hydrohalic acids acids that have hydrogen bonded to atoms of the halogen elements.)... 

The halonium structure 17 was also attributed107 to protonated bromo- and chloromethane, in agreement with theoretical predictions105,106. However, in these two ions an additional interaction (stronger for the bromo than for the chloro compound) is present between the hydrogen bonded to the halogen and the carbon atom107. Protonated iodomethane, on the contrary, structurally resembles protonated methane with the proton bonded to the carbon atom. These conclusions are supported by both MI and CID data for protonated and deuterated halomethanes and by thermochemical data. 

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