Structure, bonding, relativistic effects, and dispersion in the group 12 dihalide (MX2)3 clusters, with lessons from the extended solids

Abstract

The prediction of crystal structures for inorganic solids remains a challenge at the frontier of materials science. We examine the relationship between the group 12 dihalide monomers, small oligomers (in particular the trimers), and the extended solids. The clusters are investigated at density functional B3PW91 and MP2 computational levels and at the M06-2X, B97D3, and CCSD(T) levels for the first time for special cases where dispersion is important. Significant patterns in the bonding preferences in the dimers and extended solids of these systems are known to reflect the rigidity of the linear monomers. The trimers evolve typically as one-dimensional extensions of the dimers, but the HgX 2 systems are different: They form loosely bound clusters with a few competitive geometries emerging that imply several potential directions for continued and more strongly bond higher-order clusters. The very weak intermolecular interactions in the Hg systems appear only when relativistic effects are turned on. Otherwise, they behave precisely like their Zn and Cd analogues. The HgF 2 trimers, which have not been considered before, are examined in detail as well. These trimers-especially the fluorides-are somewhat more polar than the isolated monomers and dimers, and we find evidence for a gradual switch to the ionic bonding regime in the HgF 2 clusters on the way to the fluorite extended solid structure. Differences between geometrical preferences obtained at the MP2 versus the B3PW91 levels for the (HgX 2 ) 3 clusters demonstrate the importance of dispersion interactions in the bonding. Successes are reported for other density functional methods that directly address dispersion.

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.…”

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

“…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.…”