Frontside versus Backside S(N)2 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(N)2@A) in the model reactions of Cl(-)+AH(3)Cl (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 zeta, and potential energy surfaces (PES) along zeta, 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(N)2-b) and the frontside pathway (S(N)2-f) is performed. The S(N)2-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(N)2-f@Pb), but they do not change relative heights of barriers. We also address the nature of the transformation in the frontside S(N)2-f reactions in terms of turnstile rotation versus Berry-pseudorotation mechanism.