Halogen substituents electrostatic interactions

Figure 10 electrostatic interactions of OH or SH with C2 (upper part) and with halogen substituents (lower part). 

These calculations show that, if vinyl alcohol and vinyl thiol indeed mainly exist in planar syn conformations, as previously stated, the situation can be very different when halogen substituents are present. In some cases, the anti conformation is more or less out-of-plane staggered and can become more stable than the syn one. The general trends of substituent effects can be discussed by molecular orbitals analysis and electrostatic interactions. The results are subject to experimental confirmation, at least in the case of enols since their conformers can be prepared by the action of singlet oxygen atom on substituted ethylen, and trapped in low temperature matrices. 

It has been known for some time that certain organic halides can favorably interact through their C X bonds (X = Cl, Br, I) with heteroatom lone pairs and with aromatic n electrons [142-144], This phenomenon, which appears to contradict the normally negative nature of halogen substituents, has been termed halogen bonding [145], An explanation can be given in terms of molecular surface electrostatic poten-... 

The transmission of substituent effects in the cis- and trans-spiro[3,4]octan-6-one-2-carboxylic acids (31a and b) and related systems has been explored by comparing the observed ApK values with those calculated using spherical and ellipsoidal cavities in the Tanford-modified Kirkwood-Westheimer electrostatic field expression. The solution dipole moments of the trons-a-halogenodecalones (32a, b, and c) have shown that within experimental error the observed data are independent of halogen but dependent on whether it is located in an axial or an equatorial postion. Strong polar interactions between the halogen, carbonyl, and nitrile groups are indicated. 

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