Journal article
The Nature of Halogen⋅⋅⋅Halogen Synthons: Crystallographic and Theoretical Studies
Chemistry : a European journal, Vol.12(35), pp.8952-8960
12/04/2006
Handle:
https://hdl.handle.net/2376/114871
PMID: 16972291
Abstract
A study of the halogen⋅⋅⋅halogen contacts in organic compounds using ab initio calculations and the results of previously reported crystallographic studies show that these interactions are controlled by electrostatics. These contacts can be represented by the geometric parameters of the CX1⋅⋅⋅X2C moieties (where θ1=CX1⋅⋅⋅X2 and θ2=X1⋅⋅⋅X2C; ri=X1⋅⋅⋅X2 distance). The distributions of the contacts within the sum of van der Waals radii (rvdW) versus θi (θ1=θ2) show a maximum at θ≈150° for X=Cl, Br, and I. This maximum is not seen in the distribution of F⋅⋅⋅F contacts. These results are in good agreement with our ab initio calculations. The theoretical results show that the position of the maximum depends on three factors: 1) The type of halogen atom, 2) the hybridization of the ipso carbon atom, and 3) the nature of the other atoms that are bonded to the ipso carbon atom apart from the halogen atom. Calculations show that the strength of these contacts decreases in the following order: I⋅⋅⋅I>Br⋅⋅⋅Br>Cl⋅⋅⋅Cl. Their relative strengths decrease as a function of the hybridization of the ipso carbon atom in the following order: sp2>sp>sp3. Attaching an electronegative atom to the carbon atom strengthens the halogen⋅⋅⋅halogen contacts. An electrostatic model is proposed based on two assumptions: 1) The presence of a positive electrostatic end cap on the halogen atom (except for fluorine) and 2) the electronic charge is anisotropically distributed around the halogen atom.
Halogen⋅⋅⋅halogen interactions governed by electrostatics: The two observed geometries of halogen⋅⋅⋅halogen contacts, a) θ1=θ2 and b) θ1=180° and θ2=90° (see figure), can be explained by using the electrostatic model. The optimum geometry found for the type (a) interaction is θ1=θ2≈150° and the interaction strength decreases in the order I⋅⋅⋅I>Br⋅⋅⋅Br>Cl⋅⋅⋅Cl.
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Details
- Title
- The Nature of Halogen⋅⋅⋅Halogen Synthons: Crystallographic and Theoretical Studies
- Creators
- Firas F AwwadiRoger D WillettKirk A PetersonBrendan Twamley
- Publication Details
- Chemistry : a European journal, Vol.12(35), pp.8952-8960
- Academic Unit
- Chemistry, Department of
- Publisher
- WILEY‐VCH Verlag; Weinheim
- Number of pages
- 9
- Grant note
- American Chemical Society Petroleum Research Fund (ACS-PRF 34779-AC)
- Identifiers
- 99900548080701842
- Language
- English
- Resource Type
- Journal article