The duodecet rule: Part 2. C-H hydrogen bonding by sulfonyl compounds

By combining linear relationships between log k and log r, bond order, n, and force constants, k, where k is the CS stretching force constant of a bond of length r, a correlation is established between CS bond lengths and simple valence-bond bond orders. For a variety of S (VI) species with bonds to electronegative ligands, including methyl groups, this is used to demonstrate that the total bond order at sulfur in these species is six, consistent with a duodecet rule. Apparently, S-CH3 groups in compounds such as CH3SO2F, CH3SO2OH, (CH3)2SO2, and related species, compete with other ligands for the delocalization of electron pairs into the valence shell of the central sulfur atom, in this case through hyperconjugation. This accounts for the enhanced acidities of hydrogen atoms bonded to α-carbon atoms in such compounds, and their ability to form hydrogen bonds. Evidence for hydrogen bonding in CH3SO2F, CH3SO2Cl, and (CH3)2SO2, and in CH3SO2OH (via both S-OH and S-CH3), is discussed. The use of C-H bond lengths and stretching force constants as a possible diagnostic tool to select molecules capable of CH hydrogen bonding is also discussed.