Search

The simplest and most common of all symmetries is bilateral symmetry, yet at first sight, it does not appear as overwhelmingly important in chemistry as in our every-day life. The human body has bilateral symmetry, except for the asymmetric location of some internal organs.

The average and equilibrium molecular geometries and vibrational characteristics of MnCl2, FeCl2, CoCl2, NiCl2, MnBr2, FeBr2, CoBr2, and NiBr2 have been determined by a combined analysis of gas‐phase electron diffraction and vibrational spectroscopic data. The nozzle temperatures ranged between 960 and 1100 K in the electron diffraction experiments, paralleled with mass spectrometric control of the vapor composition. Four approximations have been examined for the molecular Hamiltonian in the joint analysis. The overall utility of combined analysis has been demonstrated. The dynamic behavior of the first‐row transition metal dihalides is best described by the semirigid model. The equilibrium bond distance is best approximated by the cubic anharmonic potential. The cubic force constants and the bond Morse anharmonicity parameter can be determined even from electron diffraction data alone provided that experimental information is collected to large enough scattering angles. The present analysis has confirmed...

A vibrational analysis of the electron diffraction intensity data of iron trifluoride provides firm evidence for the planarity of this molecule. The optimized deformation frequencies of iron trifluoride, ν 2 110(17) and ν 4 158(17) cm −1 , appear to be much lower than the available estimates. The optimized symmetric streching frequency for aluminium trifluoride is 683(54) cm −1 from the joint analysis of electron diffraction intensities and vibrational frequencies. Force constants are also obtained for both molecules.

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.

Scarcely any term connects art and science more closely than symmetry. This was already a theme in ancient times, as is found in Vitruvius, for example. On the beach of an unknown island, a shipwrecked traveler finds neatly arranged geometric figures. Can he conclude from this that there is an intelligent creator (perhaps even a god)?

The molecular and electronic structure of dysprosium trichloride, DyCl(3), was calculated by high-level quantum chemical methods in order to learn about the effect of the partially filled 4f subshell and of the possible spin-orbit coupling on them. High-temperature electron diffraction studies of DyCl(3) were also carried out so that we could compare the computed geometry with the experimental one, after thermal corrections on the latter. Dysprosium monochloride, DyCl, and the dimer of dysprosium trichloride, Dy(2)Cl(6), were also investigated by computation. We found that the electron configuration of the 4f subshell does not influence the geometry of the trichloride monomer molecule as the ground state and first excited state molecules have the same geometry. Nonetheless, taking the 4f electrons into account in the calculation, together with the 5s and 5p electrons, is important in order to get geometrical parameters consistent with the results from experiment. Based on electron diffraction and different levels of computation, the suggested equilibrium bond length (r(e)) of DyCl(3) is 2.443(14) A, while the thermal average distance (r(g)) from electron diffraction is 2.459(11) A. The molecule is trigonal planar in equilibrium. Although the ground electronic state splits due to spin-orbit coupling, the lowering of the total electronic energy is very small (about 0.025 hartree) and the geometrical parameters are not affected. In contrast with the monomeric trichloride molecule, the bond angles of the dimer seem to be different for different electronic states, indicating the influence of the 4f electronic configuration on their structure. We carried out an anharmonic analysis of the out-of-plane vibration of the trichloride monomer and found that the vibration is considerably anharmonic at 39.5 cm(-1), compared with the 30.5 cm(-1) harmonic value.

A joint analysis of electron diffraction and vibrational spectroscopic data has been performed for silicon dichloride and silicon dibromide. Various theoretical expressions for the model potentials have been tested. Both molecules appear to be rigid systems with small nuclear displacements and negligible vibrational anharmonicity. The equilibrium geometry and the force constants for both molecules have been determined and reliable deformation frequencies were estimated, viz., 195 ± 13 cm −1 and 130 ± 8 cm −1 for SiCl 2 and SiBr 2 , respectively.

The molecular geometry of iron trifluoride has been determined at 1260 K by gas-phase electron diffraction. Use of a platinum envelope during the experiment prevented the iron trifluoride sample from partial reduction otherwise observed at high temperatures. The molecular geometry of aluminum trifluoride has been reinvestigated at 1300 K. The electron diffraction results for both AlF3 and FeF3 are compatible with planar bond configuration (D 3h symmetry) with bond lengths (r g ): Al-F 1.630±0.003 A and Fe-F 1.763±0.004 A. Experimental vibrational frequencies support the notion of planarity for aluminum trifluoride. There is no such additional spectroscopic evidence available for iron trifluoride.

Quantum chemical calculations have become an everyday tool in chemistry. There are commercial program packages and downloadable basis sets for most needs. However, many chemists rarely go beyond the routine use of these programs, rarely, if ever, checking the original references for basis sets. In this letter, we point out some of the pitfalls of such an approach. Structural parameters of the aluminum-monohalides, AlF, AlCl, AlBr, and AlI, have been calculated using the Gaussian 03 program package and different basis set combinations.

The molecular structure of magnesium dibromide was investigated by high-level computational techniques and gas-phase electron diffraction. The vapor consisted of about 88% monomeric and 12% dimeric species at the electron diffraction experimental conditions at 1065 K. The geometrical parameters and vibrational characteristics of monomeric, dimeric, and trimeric magnesium dibromide species were determined by computations. Very high level computations with extended basis sets and relativistic pseudopotentials on bromine were needed to reach an agreement between computed and estimated experimental equilibrium geometries for the monomer. For both the dimer and the trimer, different geometrical arrangements were tested. Their ground-state structures have halogen bridges with four-membered ring geometries and D2h and D2d symmetry, respectively. Thermodynamic parameters have also been calculated.

The molecular structure of aluminum triiodide was investigated in the gas phase by high-temperature gas-phase electron diffraction and high-level computations. The geometries of monomeric, AlI3, and dimeric, Al2I6, molecules were determined from two separate experiments carried out under carefully controlled conditions to prevent decomposition. This is the first experimental determination of the dimer structure by modern techniques. The computed geometrical parameters strongly depend on the applied methods and basis sets as well as on core-valence correlation effects. The electron diffraction thermal average bond length, r(g), of AlI3 at 700 K is 2.448(6) A; while those of Al2I6 at 430 K are 2.456(6) A (terminal) and 2.670(8) A (bridging). The equilibrium geometry of the monomer molecule is planar with D(3h) symmetry. The dimer molecule is extremely floppy, and it is difficult to determine the symmetry of its equilibrium geometry by computation, as it is sensitive to the applied methods. MP2 and CCSD calculations find the Al2I6 molecule puckered with C(2v) symmetry (although with a very small barrier at planarity), while density functional methods give a structure with a planar central ring of D(2h) symmetry. Comparison of the computed vibrational frequencies with the gas-phase experimental ones favors the D(2h) symmetry structure.

Symmetry considerations are all-important in studying structures. This compilation provides examples in diverse areas of structural science.1 The examples include molecular symmetry, buckminsterfullerene, the Jahn-Teller effect, chirality and dissymmetry, the double helix and other biological macromolecules, molecular packing in crystals, and quasicrystals

The molecular structure of all silver halide monomers, Ag(2)X, AgX, AgX(2), and AgX(3), (X = F, Cl, Br, I), have been calculated at the B3LYP, MP2, and CCSD(T) levels of theory by using quasirelativistic pseudopotentials for all atoms except fluorine and chlorine. All silver monohalides are stable molecules, while the relative stabilities of the subhalides, dihalides, and trihalides considerably decrease toward the larger halogens. The ground-state structure of all Ag(2)X silver subhalides has C(2)(v)() symmetry, and the molecules can be best described as [Ag(2)](+)X(-). Silver dihalides are linear molecules; AgF(2) has a (2)Sigma(g) ground state, while all of the other silver dihalides have a ground state of (2)Pi(g) symmetry. The potential energy surface (PES) of all silver trihalides has been investigated. Neither of these molecules has a D(3)(h)() symmetric trigonal planar geometry, due to their Jahn-Teller distortion. The minimum energy structure of AgF(3) is a T-shaped structure with C(2)(v)() symmetry. For AgCl(3), AgBr(3), and AgI(3), the global minimum is an L-shaped structure, which lies outside the Jahn-Teller PES. This structure can be considered as a donor-acceptor system, with X(2) acting as donor and AgX as acceptor. Thus, except for AgF(3), in the other three silver trihalides, silver is not present in the formal oxidation state 3.