Charge saturation and neutral substitutions in halomethanes and their group 14 analogues

A computational analysis of the charge distribution in halomethanes and their heavy analogues (MH_4-nX_n: M = C, Si, Ge, Sn, Pb; X = F, Cl, Br, I) as a function of n uncovers a previously unidentified saturation limit for fluorides when M ≠ C. We examine the electron densities obtained at the CCSD, MP2(full), B3PW91, and HF levels of theory for 80 molecules for four different basis sets. A previously observed substituent independent charge at F in fluoromethanes is shown to be a move toward saturation that is restricted by the low polarizability of C. This limitation fades into irrelevance for the more polarizable M central atoms such that a genuine F saturation is realized in those cases. A conceptual model leads to a function of the form [q _M(n′) - q_M(n)] = a[Χ_A′ - Χ_A] + b that links the electronegativities (Χ) of incoming and leaving atoms (e.g., A′ = X and A = H for the halogenation of MH _4-nX_n) and the associated charge shift at M. We show that the phenomenon in which the charge at the central atom, q_M, is itself independent of n (e.g., at carbon in CH_4-nBr_n) is best described as an "M-neutral substitution" - not saturation. Implications of the observed X saturation and M-neutral substitutions for larger organic and inorganic halogenated molecules and polymeric materials are identified.