The HgF₂ Ionic Switch: A Triumph of Electrostatics against Relativistic Odds

Abstract

A remarkable transition in the chemical bonding in (HgF2)n clusters as a function of n is identified and characterized. HgF2 is a fascinating material. Certain significant consequences of relativistic effects on the structure of the HgF2 molecule, dimer, and trimer disappear in the extended solid. Relativistic effects in Hg ensure that HgX2 molecules (X≡F, Cl, Br, and I) are linear, rigid, and form weakly bound dimers and trimers held together by weak electrostatic and van der Waals-type forces (unlike ZnX2 and CdX2 systems in which the intermonomer contacts are strong polar covalent bonds). For HgF2, the location and nature of an apparent transition from weak interactions in the smallest (HgF2)n clusters to ionic bonding in the (fluorite) HgF2 extended solid has remained a mystery. Computational evidence obtained at the M06-2X, B97D3, and MP2 levels of theory and reported herein indicate that polar covalent bonding in (HgF2)n begins as early as n=5. For n=2 through to n=13, the transition or switch from weak (primarily dipole-dipole-type) intermonomer interactions to a preference for polar covalent bonding occurs within the range 5<n≤9. Thermodynamic evidence for this transition is provided. Our results demonstrate a significant risk associated with crystal structure prediction from the ground up (i.e., based on bonding patterns in small clusters). The path from monomers through to extended solids may be punctuated at one or several points (as n increases) with transitions in structure and bonding that are not anticipated or betrayed by the bonding in small clusters.

Twenty years of exceptional success: The molecular education and research consortium in undergraduate computational chemistry (MERCURY)

“…Padgett left the consortium, Gao ( Central State ) rejoined, and Kelly Anderson ( Roanoke College , physical), Aimée Tomlinson ( North Georgia College , physical), and Sudeep Bhattacharyay and Jim Phillips ( University of Wisconsin‐Eau Claire , biophysical and physical) joined, bringing the total to 17 MERCURY investigators. We published 79 peer‐reviewed papers, [ 159–237 ] or 1.5 papers/faculty/year, which is three times the rate for physical science faculty at undergraduate institutions. [ 46 ] The Shields group's collaboration with Brooks Pate resulted in a Science paper on the structures and energetics of the gas‐phase water hexamer, [ 160 ] which has been cited over 250 times.…”

“…Kelling Donald's research: In the Donald research group, work is focused on (a) halogen bonding and other sigma hole interactions in organic and inorganic systems, [ 210,278 ] (b) fluxionality in planar inorganic clusters and the nature and potential control of so‐called molecular machines, [ 208,263 ] (c) the evolution of structural preferences and thermal stability in metal halide and hydride molecules, clusters, and corresponding extended solids, [ 322 ] (d) the stability and catalytic applications of structurally interesting or unusual main‐group and transition‐metal organometallic compounds, [ 151,231,351,385 ] (e) the stabilization of unusual bonding arrangements, including planar tetra‐coordinate and other unusual coordination environments, [ 172 ] (f) quantifying currently qualitative but transferrable concepts in chemical bonding, [ 233 ] and (g) aromaticity and analogous phenomena in non‐hydrocarbon systems. [ 338,386 ] The MRI resources are essential for calculations on large and heavy elemental systems.…”

From Molecules to Solids: A vdW-DF-C09 Case Study of the Mercury Dihalides

“…The mercury dihalides are an intriguing series of compounds in which relativistic effects and competing weak electrostatic, covalent, and ionic contributions in chemical bonding collide. − It is now well accepted that in the gas phase , all of the simple mercury dihalide molecules (Hg X 2 for X = F, Cl, Br, and I) are linear. ,− It can be difficult to establish linearity unambiguously for simple triatomic molecules in matrix isolation studies, especially if the potential energy surface is flat (i.e., where the barrier to bending is very low), ,− but state-of-the-art computational investigations (and reliable electron diffraction data, where available) affirm in fact that all of the binary and ternary mercury dihalides (Hg XY ; for X = Y, and X ≠ Y ) are linear. ,− …”

“…The most stable form of HgI 2 is a red layered structure with vertex-linked tetrahedra, but the metastable yellow form, for example, is a molecular solid. , Yet, the fluoride adopts unequivocally the ionic fluorite structure , a fact that may be attributed to the polarity or strong ionic character of the Hg–F bonds . So, even though the minimum energy dimer and trimer of HgF 2 are found computationally to be weak assemblies of linear monomers (just like all of the other Hg dihalides), ,− ,, the accumulating crystal packing forces and the increasing polarization of the Hg centers stabilize an ionic (fluorite) solid. , For the Hg X 2 solids where X is less electronegative (Cl, Br, and I), no such ionic alternative is observed in theory or experiment; they rely substantially on weak interactions …”

“…23 So, even though the minimum energy dimer and trimer of HgF 2 are found computationally to be weak assemblies of linear monomers (just like all of the other Hg dihalides), 12,[21][22][23]36,37 the accumulating crystal packing forces and the increasing polarization of the Hg centers stabilize an ionic (fluorite) solid. 11,37 For the HgX 2 solids where X is less electronegative (Cl, Br, and I), no such ionic alternative is observed in theory or experiment; they rely substantially on weak interactions. 23 Given this diversity in the bonding in the HgX 2 solids, therefore, and the significant role of dispersion forces in some cases, we have selected this series of structures for a detailed analysis of the performance of the vdW-DF-C09 method and its ability to accurately model bonding preferences and relative stabilities.…”