Search

The bonding preferences in the mixed dihalides (MXY) of groups 2 and 12 metals, including the extent of any anomalous bending, are assessed and established. The deviation from linearity in group 2 metal binary dihalides is well-known, runs contrary to simple bonding models, and is believed to be decisive for structural preferences in the extended solids. Yet the bonding in the ternary, MXY, molecules has not been investigated systematically until now. The structure and bonding in these ternary systems (and, for completeness, the binary cases as well) are determined herein at high levels of theory. A softness criterion formulated by Szentpály and Schwerdtfeger, and tested initially on binary dihalides with predictions for mixed systems, is confirmed to apply broadly for binary and ternary species of the group 2 and 12 metals. For each M, a function of the form E(Θ) = Ae - kΘ is shown to predict the barriers to linearization for all of the bent molecules. The extended solids of some of the ternary dihalides are of interest for their optical properties. The bonding in the molecular (MXY) units may offer we think some new perspectives from which to rationalize the bonding preferences in those crystal structures.

A class of neutral 18-electron molecules with planar tetracoordinate carbon (ptC) centers is introduced. We show computationally that when n = 3 the neutral singlet molecule C(BeH)(n)(BH(2))(4-n) and other isoelectronic (18-valence electron) molecules of main group elements collapse from locally tetrahedral arrangements at the C-center to (near) planar tetracoordinate structures. For C(BeH)(3)BH(2) and C(CH(3))(BH(2))Li(2), for example, the tetrahedral type conformation is not even a minimum on the potential energy surface at the B3PW91, MP2(full), or CCSD levels of theory. The Mg analogue C(MgH)(3)BH(2) of the Be system also features a completely flat global minimum (with even higher energy planar minima in both cases as well). Other neutral compounds that may prefer planar geometries are apparent, and new openings for experimental investigations and theoretical analyses of planar tetracoordinate main group systems are identified. The planar conformation persists at one center in the C(BeH)(3)BH(2) dimer, and may be identifiable in higher order clusters of ptC molecules as well.

A computational analysis of the charge distribution in halomethanes and their heavy analogues (MH(4-n)X(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-n)X(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-n)Br(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.

How short can a C-C single bond get? The bonding in a set of molecules that are related structurally to previously synthesized or theoretically examined systems with short C-C bonds is investigated. According to calculations, a single C-C bond could be compressed to 1.313 Å! To the best of our knowledge, this is the shortest single C-C bond reported to date. This shortening is a consequence of a change in the C-C-C bond angle, θ, to minimize strain in the cages and an effort to offset the tension in the surrounding bridges.

Halogen bonding between halide sites (in substituted organic molecules or inorganic halides) and Lewis bases is a rapidly progressing area of exploration. Investigations of this phenomenon have improved our understanding of weak intermolecular interactions and suggested new possibilities in supramolecular chemistry and crystal engineering. The capacity for halogen bonding is investigated at the MP2(full) level of theory for 100 compounds, including all 80 MH(4-n)X(n) systems (M = C, Si, Ge, Sn, and Pb; X = F, Cl, Br, and I). The charge redistribution in these molecules and the (in)stability of the sigma-hole at X as a function of M and n are catalogued and examined. For the mixed MH(3-m)F(m)I compounds, we identify a complicated dependence of the relative halogen bond strengths on M and m. For m = 0, for example, the H(3)C-I----NH(3) halogen bond is 6.6 times stronger than the H(3)Pb-I----NH(3) bond. When m = 3, however, the F(3)Pb-I----NH(3) bond is shorter and approximately 1.6 times stronger than the F(3)C-I----NH(3) bond. This substituent-induced reversal in the relative strengths of halogen bond energies is explained.