Theoretical analysis of bonding properties in 1,1-dicyanocyclopentane and 1,1-dichlorocyclopentane by applying the AIM and NBO approaches

Abstract

Using Bader’s quantum-topological theory of atoms in molecules (AIM) and Weinhold’s Natural Bond Orbital (NBO) analysis we could rationalize the impact of the geminal substitution by C≡N and Cl on the geometry and electronic structure of the cyclopentane ring in 1,1-dicyanocyclopentane (DCCP) and 1,1-dichlorocyclopentane (DClCP). Among the crucial results we obtained are: 1. The topological quantities, particularly the bond ellipticity of 0.035 for the C–CN bond, indicate that this bond possesses higher bond order than a single bond. This conclusion is clearly supported by the NBO results. 2. The AIM theory as well as the NBO analysis confirm uniformly the non-linearity of the C–C≡N moiety. 3. Regardless the quantum mechanical method that has been employed for a variety of nitriles the sign of the Laplacian of the charge density, ∇2ρ(r), of the C≡N group changes its sign from negative to positive upon moving from the triple zeta to the double zeta basis set. A possible explanation for this striking behavior has been provided. By invoking the AIM and NBO approaches the different endocyclic C–C bond lengths in DCCP and DClCP as a consequence of the geminal substitution could be explained. Also the variations of these bond lengths upon moving from the more stable Cs to the energetically less favorable C2 conformer of both compounds could be rationalized. For the purpose of comparison and verification of some findings of this work, we also carried out AIM and NBO calculations on various related cyclic and non-cyclic compounds.