Molecular structure and force field of cerium tetrafluoride

Combined interpretation of electron diffraction and spectroscopic data for the CeF₄ molecule is reported. It is shown that the diffraction pattern corresponds to a tetrahedral structure of the molecule with the following effective configuration parameters (T=1180(50) K): r_g(Ce-F)=2.036(5) Å, r_g(F-F)=3.312(25) Å, l(Ce-F)=0.074(3) Å, l(F-F)=0.261(17) Å. The total force field of the CeF₄ molecule is found in a harmonic approximation. Possible participation of the Ce f-electrons in chemical bonding is discussed.     

Molecular structure and conformations of 2,2-di-tert-butyl-1,3-diaza-2-silacyclopentane: gas electron diffraction and quantum chemical calculations

The geometric structure and conformational properties of the saturated five-membered-ring compound 2,2-di-tert-butyl-1,3-diaza-2-silacyclopentane, (t-Bu)(2)Si(NH)(2)(CH(2))(2), was investigated by gas electron diffraction and quantum chemical methods (B3LYP and MP2 with 6-31G basis sets). The compound exists as a mixture of two conformers, both possessing a twist conformation and C(2) symmetry. In the prevailing form (76(6) % at 305 K) the N-H bonds stagger the adjacent CH(2) groups, and in the minor form the N-H bonds eclipse the CH(2) groups. This conformational mixture corresponds to a free energy difference of DeltaG degrees = 0.69(19) kcal/mol. The B3LYP method predicts a preference for the eclipsed conformer. The largest torsion occurs around the C-C bond with tau(NCCN) = 29.2(24) degrees. The degree of puckering in the title compound is considerably smaller than that in silacyclopentane with tau(CCCC) = 49.7(14) degrees. This has been rationalized by larger angle strain in the title compound.     

Structure and Energetics of β-Diketonates. XII. Structure of Lanthanide tris-Dipivaloylmethanates for Er(thd)3 Used as an Example

A simultaneous electron diffraction and mass spectroscopic study of saturated vapors of erbium tris-dipivalylmethanate has revealed that at 136(5)°C, the vapor consists solely of Er(thd)₃ molecules. Electron diffraction data may be described by two alternative models (of C_3 and D_3 symmetry), for which r_a, r_g, and r structural parameters have been determined. D₃ symmetry is recognized to be preferable for free Er(thd)₃ molecules. The main structural parameters of the model are r (Er – O) 2.218(5), r (O – C) 1.279(5), r (C – C_r) 1.404(6), r (C – C_t) 1.512(3), r(C_t – C_m) 1.542(5), r (rm C_m– H) 1.804(4) , The ErO 75.0(0.4)°. The ErO₆ coordination polyhedron has a structure close to an antiprism. A rotational angle of the O–O–O trigonal face relative to the position in a regular prism is 20.7(0.8)°. Possible reasons for the differences in the structure of Er(thd)₃ molecules in the gas phase and crystal are discussed.     

Structural,vibrational, and energy characteristics of halide molecules of group II–V elements

Correlations between the internuclear distances r(M-X), the bond energies E(M-X), and the valence force constants f_r for di- and trihalides of Group II–V elements are established. Some missing values are estimated. Ivanovo State Chemical Technological Academy. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 5, pp. 859–875, September–October, 1996. Translated by L. Smolina Ivanovo State Chemical Technological Academy. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 5, pp. 859–875, September–October, 1996.     

Structure and Energetics of β–Diketonates. XI. Molecular Structure of Sc(thd)3 from Gas–Phase Electron Diffraction Data

Electron–diffraction and mass–spectrometric studies of saturated vapor of scandium tris–dipivaloyl–methanate showed that at 135(5)°, the vapor contains only monomeric Sc(thd)₃, whose structural parameters r _a , r _g , and r were determined. The internuclear distances in the chelate ring were found to be rSc=O) = 2.066(5), r(O=C) = 1.272(3), and r(C=C_r) = 1.385(3). The ScO₆ coordination polyhedron has a D ₃ symmetry configuration close to a regular antiprism. The angle of rotation of the O=O=O trigonal faces relative to their position in a regular prism is 25.7(1.5)°.     

Structure and energy studies of Β-diketonates. 4. vibrational characteristics of Y(DPM)3 and Y(DPM)2

The vibrational problem for the Y(DPM)₃ and Y(DPM)₂ molecular forms that are present in the overheated vapor of yttrium tris-dipivaloylmethanate is solved. For this purpose, the experimental Raman spectra of Y(DPM)₃ solutions are interpreted. The force field of the Y(DPM)₂ radical is estimated on the basis of the force constants determined for Y(DPM)₃. Rms vibrational amplitudes and corrections for perpendicular displacements to the internuclear distances of the two molecular forms needed for interpreting electron diffraction data are calculated. Translated fromZhurnal Struktumoi Khimii, Vol. 38, No. 3, pp. 470–479, May–June, 1997.     

Unexpected conformational properties of 1-trifluoromethyl-1-silacyclohexane, C5H10SiHCF3: gas electron diffraction, low-temperature NMR spectropic studies, and quantum chemical calculations

The molecular structure of axial and equatorial conformers of 1-trifluoromethyl-1-silacyclohexane, (C5H10SiHCF3), as well as the thermodynamic equilibrium between these species was investigated by means of gas electron diffraction (GED), dynamic nuclear magnetic resonance (DNMR) spectroscopy, and quantum chemical calculations (B3LYP, MP2, and CBS-QB3). According to GED, the compound exists as a mixture of two Cs symmetry conformers possessing the chair conformation of the six-membered ring and differing in the axial or equatorial position of the CF3 group (axial=58(12) mol%/equatorial=42(12) mol%) at T=293 K. This result is in a good agreement with the theoretical prediction. This is, however, in sharp contrast to the conformational properties of the cyclohexane analogue. The main structural feature for both conformers is the unusually long exocyclic bond length Si--C 1.934(10) A. A low-temperature 19F NMR experiment results in an axial/equatorial ratio of 17(2) mol%:83(2) mol% at 113 K and a DeltaG (not equal) of 5.5(2) kcal mol-1. CBS-QB3 calculations in the gas-phase and solvation effect calculations using the PCM(B3LYP/6-311G*) and IPCM(B3LYP/6-311G*) models were applied to estimate the axial/equatorial ratio in the 100-300 K temperature range, which showed excellent agreement with the experimental results. The minimum energy pathways for the chair-to-chair inversion of trifluoromethylsilacyclohexane and methylsilacyclohexane were also calculated using the STQN(Path) method.