Abstract:Gas-phase electron diffraction (GED) studies at high temperatures have several special common features that justify their separate discussion. Due to the difficulties connected with the experiment this technique has developed only in a few laboratories. Most often inorganic systems are studied; lowervalence metal halides and metal oxides. Their low volatility requires high-temperature experimental conditions. Due to the complex vapor composition, other techniques, such as quadrupole mass spectrometry and, to a… Show more
“…[10][11][12]25] Unfortunately, there are no experimental geometries for the gas-phase dimers (M 2 X 4 ) of the Group 12 dihalides. Could this be because the dimers are not thermodynamically stable?…”
Section: Introductionmentioning
confidence: 99%
“…Dimers: The significance of the relativistic and shell structure effects on the structural chemistry of the dihalides has been identified in computational studies of the dihalide dimers as well. [10][11][12]25] Unfortunately, there are no experimental geometries for the gas-phase dimers (M 2 X 4 ) of the Group 12 dihalides. Could this be because the dimers are not thermodynamically stable?…”
Connections between the structures of Group 12 dihalides in their vapor and crystal phases are sought and discussed. The molecular structures of all monomers and dimers (MX(2): M=Zn, Cd, Hg and X=F, Cl, Br, I) were calculated at the density functional B3PW91 and MP2 computational levels. All the monomers are linear, with the mercury dihalide molecules having shorter bonds than their cadmium analogues; the ZnX(2) and CdX(2) structures are similar. The shorter Hg-X distances are traced back to relativistic effects. For the dimers, many possible geometrical arrangements were considered. The zinc and cadmium dihalide dimers have the usual D(2h)-symmetry geometry, whereas the mercury dihalide dimers are loosely-bound units with C(2h) symmetry. The origins of this C(2h) structure are discussed from different points of view, including frontier orbital interactions. The crystals of Group 12 dihalides span a wide range of structure types, from three-dimensional extended solids to molecular crystals. There is an obvious connection between the structures and characteristics of monomers, their dimers, and the crystals they form. The similarities as well as startling differences from the Group 2 dihalides are analyzed.
“…[10][11][12]25] Unfortunately, there are no experimental geometries for the gas-phase dimers (M 2 X 4 ) of the Group 12 dihalides. Could this be because the dimers are not thermodynamically stable?…”
Section: Introductionmentioning
confidence: 99%
“…Dimers: The significance of the relativistic and shell structure effects on the structural chemistry of the dihalides has been identified in computational studies of the dihalide dimers as well. [10][11][12]25] Unfortunately, there are no experimental geometries for the gas-phase dimers (M 2 X 4 ) of the Group 12 dihalides. Could this be because the dimers are not thermodynamically stable?…”
Connections between the structures of Group 12 dihalides in their vapor and crystal phases are sought and discussed. The molecular structures of all monomers and dimers (MX(2): M=Zn, Cd, Hg and X=F, Cl, Br, I) were calculated at the density functional B3PW91 and MP2 computational levels. All the monomers are linear, with the mercury dihalide molecules having shorter bonds than their cadmium analogues; the ZnX(2) and CdX(2) structures are similar. The shorter Hg-X distances are traced back to relativistic effects. For the dimers, many possible geometrical arrangements were considered. The zinc and cadmium dihalide dimers have the usual D(2h)-symmetry geometry, whereas the mercury dihalide dimers are loosely-bound units with C(2h) symmetry. The origins of this C(2h) structure are discussed from different points of view, including frontier orbital interactions. The crystals of Group 12 dihalides span a wide range of structure types, from three-dimensional extended solids to molecular crystals. There is an obvious connection between the structures and characteristics of monomers, their dimers, and the crystals they form. The similarities as well as startling differences from the Group 2 dihalides are analyzed.
“…[1][2][3] In this Account, our most important results of the past decade are summarized. [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] Metal halides usually evaporate at high temperatures; hence their experiments require special conditions. Their computational requirements are also different from the mainstream applications of quantum chemistry.…”
Metal halides are a relatively large class of inorganic compounds that participate in many industrial processes, from halogen metallurgy to the production of semiconductors. Because most metal halides are ionic crystals at ambient conditions, the term "molecular metal halides" usually refers to vapor-phase species. These gas-phase molecules have a special place in basic research because they exhibit the widest range of chemical bonding from the purely ionic to mostly covalent bonding through to weakly interacting systems. Although our focus is basic research, knowledge of the structural and thermodynamic properties of gas-phase metal halides is also important in industrial processes. In this Account, we review our most recent work on metal halide molecular structures. Our studies are based on electron diffraction and vibrational spectroscopy, and increasingly, we have augmented our experimental work with quantum chemical computations. Using both experimental and computational techniques has enabled us to determine intriguing structural effects with better accuracy than using either technique alone. We loosely group our discussion based on structural effects including "floppiness", relativistic effects, vibronic interactions, and finally, undiscovered molecules with computational thermodynamic stability. Floppiness, or serious "nonrigidity", is a typical characteristic of metal halides and makes their study challenging for both experimentalists and theoreticians. Relativistic effects are mostly responsible for the unique structure of gold and mercury halides. These molecules have shorter-than-expected bonds and often have unusual geometrical configurations. The gold monohalide and mercury dihalide dimers and the molecular-type crystal structure of HgCl(2) are examples. We also examined spin-orbit coupling and the possible effect of the 4f electrons on the structure of lanthanide trihalides. Unexpectedly, we found that the geometry of their dimers depends on the f electron configuration. Metal halides are unique in exhibiting strong vibronic interactions such as the Jahn-Teller effect and the related Renner-Teller effect. Some metal trihalide molecules have an almost T-shape due to static Jahn-Teller distortions. The nonlinear structure with a 150 degree bond angle of the chromium dichloride molecule demonstrates the Renner-Teller effect. Finally, we present a few examples of unknown structures that appear to be thermodynamically stable, including gold and silver triiodides and all silver subhalides. The combination of experimental and computational techniques has brought new insights to the structural chemistry of metal halides. We expect that the continuing progress in computational chemistry will shed further light on the intricate details of these and other molecular structures.
“…An interesting structural diversity has been observed experimentally and computationally in the series of dimers of the group 2 dihalides, (MX 2 ) 2 , as well. The Be 2 X 4 and Mg 2 X 4 dimers have a D 2 h minimum energy structure with two bridging halides, while Ca 2 F 4 , Sr 2 X 4 , and Ba 2 X 4 , X = F, Cl, feature a C 3 v minimum energy geometry with three bridging halides (Figure ). ,− 1 Minimum-energy structural isomers of the groups 2 dihalide dimers.…”
The link between structural preferences in the monomers, dimers, and extended solid-state structures of the group 2 dihalides (MX(2): M = Be, Mg, Ca, Sr, Ba and X = F, Cl, Br, I) is examined theoretically. The question posed is how well are geometric properties of the gas-phase MX(2) monomers and lower order oligomers "remembered" in the corresponding MX(2) solids. Significant links between the bending in the MX(2) monomers and the D(2)(h)()/C(3)(v)() M(2)X(4) dimer structures are identified. At the B3LYP computational level, the monomers that are bent prefer the C(3)(v)() triply bridged geometry, while the rigid linear molecules prefer a D(2)(h)() doubly bridged structure. Quasilinear or floppy monomers show, in general, only a weak preference for either the D(2)(h)() or the C(3)(v)() dimer structure. A frontier orbital perspective, looking at the interaction of monomer units as led by a donor-acceptor interaction, proves to be a useful way to think about the monomer-oligomer relationships. There is also a relationship between the structural trends in these two (MX(2) and M(2)X(4)) series of molecular structures and the prevalent structure types in the group 2 dihalide solids. The most bent monomers condense to form the high coordination number fluorite and PbCl(2) structure types. The rigidly linear monomers condense to form extended solids with low coordination numbers, 4 or 6. The reasons for these correlations are explored.
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