An intricate molecule: aluminum triiodide. Molecular structure of AlI3 and Al2I6 from electron diffraction and computation

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 integration of structure determination by computation,electron diffraction and microwave spectroscopy

The history of the interaction between experimental structure determinations by microwave spectroscopy and by gas phase electron diffraction is briefly reviewed in terms of three eras: (1) competition and antagonism, (2) comparison and correction, and (3) integration of analysis. A similar progression is noted for the relation between experimental and theoretical methods for studying molecular structure, with the present time straddling ages (2) and (3). Examples are given from a variety of studies involving various degrees of methodological interaction. The true integration of experimental and computational structural studies is still in its infancy with the primary illustrations involving the evaluation of theoretical structural offset values from experimental evidence, the transfer of theoretically determined parameters into the fitting of experimental data, and the current development of methods for utilizing vibrational information obtained from the combined analysis of computed theoretical and experimental infrared data in the further analysis of experimental diffraction and microwave information.     

Iron dihalides: structures and thermodynamic properties from computation and an electron diffraction study of iron diiodide

The molecular structures, vibrational frequencies, and thermodynamic properties of all four monomeric and dimeric iron dihalide molecules, FeF₂, FeCl₂, FeBr₂, and FeI₂, were determined by quantum chemical calculations and the structure of iron diiodide also by gas-phase electron diffraction (ED). The earlier ED study of iron dibromide was also repeated. All iron dihalides are stable molecules in contrast to the iron trihalides, for which FeBr₃ and FeI₃ are unstable and easily decompose to the corresponding dihalides. The structures of the trimers and tetramers of FeCl₂ were also calculated and compared to the crystal structure.     

Molecular structure of bismuth trichloride from combined electron diffraction and vibrational spectroscopic study

The results of an electron diffraction reanalysis, augmented with a combined electron diffraction and vibrational spectroscopic elucidation, of the molecular structure of BiCl₃ are reported. The principal parameters arer _g (Bi-Cl)=2.424±0.005 å (r =2.417±0.005 å) and <Cl-Bi-Cl=97.5±0.2. They are in excellent agreement with previous electron diffraction analysis [1], utilizing a more limited data range from the same experiment. They are also fully consistent with the expected trends of geometrical variation in the Group V trihalide series. The force fields of BiCl₃, determined by normal coordinate analysis and by combined analysis, agree within experimental error.     

An electron diffraction study of the molecular structure of hexamethyldisiloxane

An electron diffraction determination of the molecular geometry of hexamethyldisiloxane has removed much of the uncertainty concerning this structure. The length of the SiO bond and the SiOSi bond angle were determined to be 1.631 ± 0.003 Å and 148 ± 3°, respectively. The experimental data are consistent with a staggered conformation (C_2v symmetry) while a model with twist angles around the SiO bonds of about 30° cannot be excluded. The molecule is probably performing large amplitude intramolecular motion.     

An electron diffraction study of the molecular structure of meta-difluorobenzene

The molecular structure of meta-difluorobenzene in the gas phase has been determined by electron diffraction at room temperature. The carbon ring deviates slightly from D_6h symmetry. The four C-C distances adjacent to the F atoms are 1.384 Å, the two other C-C distances are slightly longer, i.e. 1.405 Å. The C-F and C-H distances are 1.324 and 1.107 Å. The C-C-F valency angle is 119.5°.     

Gas electron diffraction study of the geometric structure of triallylborane molecule

The main structural parameters of the triallylborane molecule having the C ₃ symmetry were determined by gas electron diffraction and quantum-chemical calculations at the MP2/6-31G(d,p) and B3LYP/6-31G(d,p) levels. The parameters calculated by the MP2/6-31G(d,p) method are in better agreement with the experimental data than those calculated by the B3LYP/6-31G(d,p) method.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 98–101, January, 2005.     

A gas electron diffraction study of the molecular structure of cis-stilbene

The cis-Stilbene molecule is found to possess C₂ symmetry and may be described as having a propeller-like conformation with the phenyl groups rotated ca. 43° about the C-φ bonds. The deviation from planarity is found to be more extensive than predicted by most theoretical calculations. The steric strain in the molecule is also revealed by large valence angles at the central carbon-carbon double bond (∠CC-C: 129.5°).     

A gas electron diffraction study of the molecular structure of trans-stilbene

The molecular structure of trans-stilbene has been studied by the gas electron diffraction method. Unlike the approximately planar structure with C_i symmetry found for the solid state, the molecule in the gas phase was found to be non-planar and to possess C₂ symmetry. The phenyl groups were found to be rotated ca. 30° about the C-φ bonds. The non-planarity of the molecule is, however, not so large as to seriously influence the resonance energy.     

An electron diffraction study of the molecular structure of gaseous cyclobutylgermane

The molecular structure and conformation of cyclobutylgermane have been determined by gas-phase electron diffraction. Like its counterpart cyclobutylsilane (CBS) it possesses quasi-equatorial and quasi-axial conformers. The most interesting aspect of the structure of CBG is the influence of the germyl group on the ratio of equatorial to axial conformers. The predominance of the quasi-equatorial conformer (ΔG = 3.1(1) kJ mol⁻¹), the near equality of the skeleton C---C bond lengths (C---C = 1.557(3)A) (r_a value) and the values of the puckering angles for the equatorial angles form and the axial one of 25.3(3.1)° and −20.4(3.6)° respectively, all support the predictions made by Jonvik and Boggs concerning the correlation between electronegativity and structural parameters in four-membered rings. From a consideration of these predictions, a comparison of the most prominent structural factors in CBG and cyclobutylsilane indicates that the germanium atom is more electronegative than silicon. This result could be considered as the first structural evidence for the previously postulated inversion of the electronegativity order within group IV.     

A gas-phase electron diffraction study of the molecular structure of tetravinylsilane

The molecular structure of tetravinylsilane has been studied by gas-phase electron diffraction. The radial distribution curve suggests the absence of conformers having vinyl double bonds staggered with respect to the SiC₄ skeleton. Of the eclipsed or approximately-eclipsed conformers, the one with S₄ symmetry gives the best fit with experiment, although a small admixture of a C₁ conformation cannot be ruled out. Least-squares refinement gave the following values for the independent structural parameters (lengths, r_a basis; angles, r_α basis): C-H = 1.118 ± 0.003 Å, CC = 1.355 ± 0.002 Å, Si-C = 1.855 ±0.002 Å, ∠SiCC = 124.0 ± 0.3°, ∠SiCH = 118.4 ± 1.0°, torsion angles CSiCC are 17.5 ± 0.6° from the eclipsed conformation. During the refinement the vibrational amplitudes u and perpendicular amplitude corrections K were held constant at calculated values. The CC bond length provides evidence of interaction between the vinyl π-bonds and the vacant d-orbitals of silicon.     

Gas electron diffraction study of the molecular structure of NbCl4

The structure of the NbCl₄ molecule is studied experimentally by the synchronous electron diffraction and mass spectrometry methods. The model molecular geometries of C_2v, C_3v, D_2d, and T_d symmetries are verified. The advantages of the tetrahedral model over the other models are established. The thermally averaged parameters of the effective configuration of the NbCl₄ molecule are as follows: r_g(Nb?Cl)=2.279(5) Å, l(Nb?Cl)=0.073(2) Å, r_g(Cl?Cl)=3.692(17) Å, l(Cl?Cl)=0.275(11) Å, ∠_g(Cl-Nb-Cl)=108.2(5)°, and δ(Cl?Cl)=0.030(19) Å.     

On the effect of 4f electrons on the structural characteristics of lanthanide trihalides: computational and electron diffraction study of dysprosium trichloride

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.     

An electron diffraction study of the molecular structure of gaseous allyl silane

The molecular structure of allyl silane has been studied by gas-phase electron diffraction. The experimental radial distribution curve has only four prominent peaks, resulting in serious resolution problems in the structure determination. A single conformer whose dimensions resemble those of related molecules fits the diffraction data. The torsion angle φ_siccc is102 ± 1°, measured from the conformation having Si-C and CC eclipsed.