Origin of the X−Hal (Hal = Cl, Br) Bond-Length Change in the Halogen-Bonded Complexes

A generalized explanation is provided for the existence of the red- and blue-shifting nature of X–Z bonds (Z = H, halogens, chalcogens, pnicogens, etc.) in X–Z⋯Y complexes based on computational studies on a selected set of weakly bonded complexes and analysis of existing literature data. The additional electrons and orbitals available on Z in comparison to H make for dramatic differences between the H-bond and the rest of the Z-bonds. The nature of the X-group and its influence on the X–Z bond length in the parent X–Z molecule largely controls the change in the X–Z bond length on X–Z⋯Y bond formation; the Y-group usually influences only the magnitude of the effects controlled by X. The major factors which control the X–Z bond length change are: (a) negative hyperconjugative donation of electron density from X-group to X–Z σ* antibonding molecular orbital (ABMO) in the parent X–Z, (b) induced negative hyperconjugation from the lone pair of electrons on Z to the antibonding orbitals of the X-group, and (c) charge transfer (CT) from the Y-group to the X–Z σ* orbital. The exchange repulsion from the Y-group that shifts partial electron density at the X–Z σ* ABMO back to X leads to blue-shifting and the CT from the Y-group to the σ* ABMO of X–Z leads to red-shifting. The balance between these two opposing forces decides red-, zero- or blue-shifting. A continuum of behaviour of X–Z bond length variation is inevitable in X–Z⋯Y complexes.

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Zero-Point Corrections for Isotropic Coupling Constants for Cyclohexadienyl Radical, C6H7 and C6H6Mu: Beyond the Bond Length Change Approximation

Molecules ◽

10.3390/molecules18054906 ◽

2013 ◽

Vol 18 (5) ◽

pp. 4906-4916 ◽

Cited By ~ 5

Author(s):

Bruce Hudson ◽

Suzanne Chafetz

Keyword(s):

Bond Length ◽

Zero Point ◽

Length Change ◽

Coupling Constants ◽

Cyclohexadienyl Radical

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The nature of the bond-length change upon molecule complexation

Collection of Czechoslovak Chemical Communications ◽

10.1135/cccc2009532 ◽

2010 ◽

Vol 75 (3) ◽

pp. 243-256 ◽

Cited By ~ 5

Author(s):

Weizhou Wang ◽

Yu Zhang ◽

Baoming Ji

Keyword(s):

Charge Transfer ◽

Complex Formation ◽

Bond Length ◽

Electrostatic Interaction ◽

Length Change ◽

Attractive Interaction ◽

Charge Transfer Interaction ◽

The Difference ◽

Hydrogen Bonded

The nature of the bond-length change upon molecule complexation has been investigated at the MP2/aug-cc-pVTZ level of theory. Our results have clearly shown that the X–Y bond-length change upon complex formation is determined mainly by the electrostatic attractive interaction and the charge-transfer interaction. In the case of strongly polar bond, the electrostatic interaction always causes bond elongation while in the case of weakly polar bond it causes bond contraction. The charge-transfer interaction generally results in the X–Y bond elongation; either it is a more polar bond or it is a less polar bond. Employing this simple “electrostatic interaction plus charge-transfer interaction” explanation, we explained and predicted many interesting phenomena related to the bond-length change upon molecule complexation. In addition, the difference between the origin of the bond-length change upon hydrogen-bonded complex formation and the origin of the bond-length change upon halogen-bonded complex formation was also discussed.

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