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The geometries and spectroscopic constants of the dihalosilanones, F 2 SiO, Cl 2 SiO, Br 2 SiO, and I 2 SiO, and their cis- and trans-XSiOX isomers were calculated by high-level computations. The potential energy surfaces of the isomerization reactions were investigated and the transition-state structures determined, together with the isomerization and activation energies and enthalpies. It was shown that, in contrast to silanone and the hydroxysilylenes, the dihalosilanones are much more stable than their other structural isomers. Thus, even at several thousand kelvin temperatures, a common condition inside a metal halide lamp, the presence of appreciable amounts of the cis- and trans-BrSiOBr and ISiOI isomers is unlikely.

The molecular structure of tin dibromide was investigated by high-level computational methods and gas-phase electron diffraction. The structural and vibrational characteristics of both SnBr 2 and Sn 2 Br 4 were determined by computations. To reach an agreement between computed and experimental bond lengths for SnBr 2 , very large bases and correlated methods are needed. For the dimer Sn 2 Br 4 , two low-energy geometries were found, one with C, and the other with C 2 v symmetry, the former with somewhat lower energy. Thermodynamic functions for gaseous SnBr 2 and Sn 2 Br 4 and their dimerization reaction have been calculated on the basis of the computed structures.

The molecular geometry of yttrium trichloride has been determined by high-temperature gas-phase electron diffraction. The vapor phase consisted of about 87% monomeric and 13% dimeric species. High-...

The molecular geometry of dimeric gold trichloride has been determined by gas-phase electron diffraction and high-level quantum chemical calculations. The molecule has a planar, D2h-symmetry halogen-bridged geometry, with the gold atom in an almost square-planar coordination. The geometrical parameters from electron diffraction (rg and ∠α) are: Au−Clt, 2.236 ± 0.013 A; Au−Clb, 2.355 ± 0.013 A; ∠Clt−Au−Clt, 92.7 ± 2.5°; and ∠Clb−Au−Clb, 86.8 ± 1.8° (t, terminal; b, bridging chlorine). Quantum chemical calculations have also been carried out on the ground-state and transition-state structures of monomeric AuCl3; both have C2v-symmetry structures due to Jahn−Teller distortion. CASSCF calculations show that the triplet D3h-symmetry structure lies ∼29 kcal/mol above the 1A1 symmetry ground state. The Mexican-hat-type potential energy surface of the monomer has three equal minimum-energy positions around the brim of the hat, separated by three transition-state structures, ∼6 kcal/mol higher in energy, at the C...

The molecular geometry of monomeric and dimeric gold trifluoride, AuF3 and Au2F6, has been determined by gas-phase electron diffraction and high-level quantum chemical calculations. Both experiment and computation indicate that the ground-state structure of AuF3 has C2v symmetry, rather than 3-fold symmetry, with one shorter and two longer Au−F bonds and an almost T-shaped form, due to a first-order Jahn−Teller effect. CASSCF calculations show the triplet D3h symmetry structure, 3A‘, to lie about 42 kcal/mol above the 1A1 symmetry ground state and the D3h symmetry singlet, 1A‘, even higher than the triplet state, by about a further 13 kcal/mol. The molecule has a typical “Mexican-hat”-type potential energy surface with three equal minimum-energy structures around the brim of the hat, separated by equal-height transition structures, about 3.6 kcal/mol above the minimum energy. The geometry of the transition structure has also been calculated. The dimer has a D2h symmetry planar, halogen-bridged geometry, w...

The molecular geometry of gaseous gallium tribromide has been determined by electron diffraction augmented with quadrupole mass spectrometry and quantum chemical calculations. Of the two experiments, at the higher temperature (638 K) only the monomeric form was present whereas at the lower temperature (357 K) the monomers and dimers were present in comparable amounts. The monomer is planar with bond lengths ( r g ) 2.239 ± 0.007 A at 357 K and 2.249 ± 0.005 A at 638 K. The dimer has a halogen-bridged structure with bond lengths ( r g ) of 2.250 ± 0.006 A and 2.453 ± 0.005 A and bond angles ∠ a Br b GaBr b 92.7 ± 0.3° and ∠ a Br t GaBr t 123.1 ± 1.4°.

A new high-temperature gas-phase electron diffraction analysis of manganese trifluoride, combined with high-level quantum chemical calculations, provides direct geometrical evidence of a Jahn−Teller distortion in a free molecule. The potential energy surface of the molecule was scanned extensively by computation at the SCF level. CASSCF and CASPT2 calculations established that of the many C2v symmetry stationary points on the potential energy surface the lowest energy ones are quintets. The global minimum is a quintet state of 5A1 symmetry. In this planar C2v symmetry structure there are two longer and one somewhat shorter Mn−F bonds, with two bond angles close to 106° and one bond angle of about 148°. The second lowest energy state was of 5B2 symmetry, which turned out to be a transition state with an imaginary b2 (in plane) bending frequency. A constrained planar structure of D3h molecular symmetry has appreciably higher energy than the 5B2 symmetry structure. The experimental data are in complete agree...

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.

The molecular geometry of gaseous cuprous chloride oligomers was determined by gas-phase electron diffraction at two different temperatures. Quantum chemical calculations were also performed for Cu(n)Cl(n) (n=1-4) molecules. A complex vapor composition was found in both experiments. Molecules of Cu(3)Cl(3) and Cu(4)Cl(4) were present at the lower temperature (689 K), while dimeric molecules (Cu(2)Cl(2)) were found in addition to the trimers and tetramers at the higher temperature (1333 K). All Cu(n)Cl(n) species were found to have planar rings by both experiment and computation. The bond lengths from electron diffraction (r(g)) at 689 K are 2.166+/-0.008 A and 2.141+/-0.008 A and the Cu-Cl-Cu bond angles are 73.9+/-0.6 degrees and 88.0+/-0.6 degrees for the trimer and the tetramer, respectively. At 1333 K the bond lengths are 2.254+/-0.011 A, 2.180+/-0.011 A, and 2.155+/-0.011 A, and the Cu-Cl-Cu bond angles 67.3+/-1.1 degrees, 74.4+/-1.1 degrees, and 83.6+/-1.1 degrees for the dimer, trimer, and tetramer, respectively.