Investigating sigma bonds in an electric field from the Pauling's perspective: the behavior of Cl-X and H-X (X = C, Si) bonds

In the general framework concerning the description and the rationalization of the behavior (weakening/strengthening) of a bond of a polyatomic molecular system under the experience of an electric field, we have considered the sigma bonds Cl-X and H-X (where X = C, Si). The Pauling's perspective for chemical bonding is adopted, and the corresponding ionic structures, responsible for the behavior of a considered bond inside the electric field, are examined by means of a recently developed poly electron population analysis. The CH3Cl, SiH3Cl, CH4 and SiH4 molecules are considered as model sigma systems, and the changes of their bonds induced by the electric field are investigated by means of full geometry optimizations in both HF and MP2 levels. The changes in the weights of the ionic structures are calculated in the basis of interatomically nonorthogonal natural hybrid orbitals, which are necessary to describe the ionic structures of sigma bonds. The correlation of these changes with the bond length variations is investigated, and the observed computational tendencies can be summarized in the following rule, concerning the behavior of a sigma heteronuclear Charge-Shift or covalent/Charge-Shift bond, or in general a sigma bond with non-negligible polarity: 'if the electric field decreases (or increases) the difference of the weights of the two principal ionic structures, then the bond length decreases (or increases, respectively)'.