The structures of tellurium(IV) halides in the gas phase and as solvated molecules.

The structures of molecular tellurium tetrafluoride and tellurium tetrachloride were determined by a combination of gas-phase electron diffraction, mass spectrometry and quantum chemical calculations. The combined GED/MS experiments showed no evidence of decomposition of TeF(4) and TeCl(4). No ions of oligomeric (dimeric, trimeric, etc.) or any other composition were found in the mass spectra. The monomeric molecules possess a pseudo trigonal bipyramidal structure (C(2v) symmetry) with the equatorial Te-X distances being shorter than the axial ones. The fluorine atoms are bent away from the lone pair resulting in X(eq)-Te-X(eq) and X(eq)-Te-X(ax) bond angles smaller than 120 and 90 degrees, respectively. The structure of solvates TeF(4) (THF)(2), TeF(4) (dioxane) TeF(4) (DME)(2), TeF(4)(Et(2)O) TeF(4)(toluene), TeCl(4)(CH(3)CN)(2), TeCl(4)(DME)(2) and TeCl(4)(dioxane) were determined by X-ray diffraction. The structures of tellurium tetrafluoride solvates are strongly influenced by the choice of the solvent molecules. Monomeric TeF(4) units were obtained with THF, DME and dioxane whereas fluoride bridged coordination polymers were formed using diethyl ether or toluene. All tellurium tetrachloride solvates studied contain monomeric TeCl(4) units with coordinated solvent molecules. Coordination numbers range from four in the gas phase to eight in the TeF(4) dimethoxyethane solvate. Geometric parameters of the TeX(4) molecules in the crystal, solvates and gas phase were compared. DFT, MP2, CCSD, CCSD(T) methods were applied for calculation of geometric and vibrational characteristics of free TeX(4) molecules (X = F, Cl). The pseudorotation barriers were estimated and an NBO analysis was performed. It was shown that both, GED and theoretical, quantitative results are in agreement with the qualitative results of the VSEPR model.

The molecular structure of selenium dibromide as determined by combined gas-phase electron diffraction-mass spectrometric experiments and quantum chemical calculations.

The vapour over solid SeBr(4) at 10 degrees C was investigated with a combined gas-phase electron diffraction/mass spectrometric (GED/MS) method. The composition of the vapour derived from the mass spectra (43% SeBr(2), 56.7% Br(2) and 0.3% Se(2)Br(2)) was in agreement with the composition obtained from the analysis of the simultaneously recorded GED intensities (41(3)% SeBr(2), 59(3)% Br(2)). The GED study results in the following geometric parameters (r(g), angle(g) values with total uncertainties): Se-Br = 2.306(5) A and Br-Se-Br = 101.6(6) degrees . Most quantum chemical approximations (B3LYP, MP2, CCSD and CCSD(T) with relativistic effective core potentials and cc-pVTZ as well as aug-cc-pVTZ basis sets for the outer shells) overestimate the Se-Br bond length by 0.01 to 0.03 A. All methods reproduce the bond angle correctly, except for the B3LYP method. Gas phase vibrational frequencies estimated from experimental vibrational amplitudes agree well with those measured by Raman spectroscopy in acetonitrile solutions. All computational methods overestimate vibrational frequencies, especially that for the symmetric stretch vibration, by about or 8 to 13%.