Frontside versus backside S_N2 substitution at group 14 atoms: Origin of reaction barriers and reasons for their absence

We have theoretically studied the gas-phase nucleophilic substitution at group-14 atoms (S_N2@A) in the model reactions of Cl ^-+AH₃Cl (A= C, Si, Ge, Sn, and Pb) using relativistic density functional theory (DFT) at ZORA-OLYP/TZ2P. Firstly, we wish to explore and understand how the reaction coordinate ζ, and potential energy surfaces (PES) along ζ, vary as the center of nucleophilic attack changes from carbon to the heavier group-14 atoms. Secondly, a comparison between the more common backside reaction (S_N2-b) and the frontside pathway (S _N2-f) is performed. The S_N2-b reaction is found to have a central barrier for A=C, but none for the other group-14 atoms, A=Si-Pb. Relativistic effects destabilize reactant complexes and transition species by up to 10 kcal mol^-1 (for S_N2-f@Pb), but they do not change relative heights of barrier s. We also address the nature of the transformation in the frontside S_N2-freactions in terms of turnstile rotation versus Berry-pseudorotation mechanism.