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 (MH4-nXn: 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 [qM(n) -- qM(n)] = a[χA\u27 -- χA] + b that links the electronegativities (χ) of incoming and leaving atoms (e.g., A\u27 = X and A = H for the halogenation of MH4-nXn) and the associated charge shift at M. We show that the phenomenon in which the charge at the central atom, qM, is itself independent of n (e.g., at carbon in CH4-nBrn) 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