A.V. Belyakov

An analysis of electron diffraction data on bis(trimethylsilyl)acetylene taking into account nonlinear relations between Cartesian and internal vibrational coordinates

Abstract The electron diffraction data on bis(trimethylsilyl)acetylene were analyzed in terms of the one-dimensional dynamic model of free Si(CH 3 ) 3 group rotations about the Si–C C–Si axis. The root-mean-square amplitudes and harmonic shrinkage corrections were calculated taking into account nonlinear relations between Cartesian and internal vibrational coordinates at the level of first-order perturbation theory ( h 1) and with the use of the traditional scheme ( h 0). The experimental r α distances were virtually independent of the approximation used to calculate vibrational effects. The r h 1 parameter values much better approximated the equilibrium geometry than the familiar r α r h 0 parameters. The r h 1 -structure of bis(trimethylsilyl)acetylene refined to Si–C(H 3 ) 1.877(4), Si–C 1.841(4), C C 1.239(3), C–H(av.) 1.108(3) A, (H 3 )C–Si–C 109.2(2)° and Si–C–H(av.) 111.3(2)°. Electron diffraction data on silylacetylenes were systematized in terms of r g parameters equal to r h 1 for bonded distances.

Gas-phase electron diffraction study of tris(trimethylstannyl)amine, N(SnMe3)3

Abstract The geometrical parameters of tris(trimethylstannyl)amine have been determined by gas-phase electron diffraction. The a structure has been refined using mean amplitude values calculated from the force fields of a number of tin derivatives. The experimental data are consistent with a planar bond configuration at the nitrogen in N(SnMe 3 ) 3 . The final set of geometrical parameters is as follows (average bond distances, r g , in A, angles in degrees): SnC 2.166(5), SnN 2.038(3), CH 1.117(17). NSnC 108.5(1.5), SnCH 112.1(1.6). Mean amplitude values have been varied for those distances which give considerable contributions to scattering. The results obtained fill a gap in the knowledge of structures of Group IV element μ-nitrido derivatives. They confirm the conclusion that lowering of ligand MR n electro-negativity weakens the tendency to deviation from planarity in the central fragment NM 3 . This tendency may be considered as a manifestation of the second-order Jahn-Teller effect.

Vibrational spectra of (CH3)3SnCCH, (Cd3)3SnCCH and (CD3)3SnCCD and force field of trimethylstannylacetylene

Abstract The IR (32–4000 cm −1 ) and polarized Raman (50–3500 cm −1 ) spectra of liquid (CH 3 ) 3 SnCCH, (Cd 3 ) 3 SnCCH and (CD 3 ) 3 SnCCD are measured at room temperature. The spectra are interpreted in terms of a model of point group C 3v . The CH (CD) methyl bonds lying in symmetry planes are assumed to be in an eclipsed arrangement with respect to the acetylene bond. The force constants of trimethylstannylacetylene are calculated by the least-squares method.

Molecular structures of acetylene derivatives of tin: Part IV. Gas-phase electron-diffraction study of tetraethynyltin, Sn(CCH)4, and triethynyltin iodide, ISn(CCH)3

Abstract The geometrical parameters of tetraethynyltin and triethynyltin iodide have been determined by gas-phase electron diffraction. Triethynyltin iodide was present as an admixture in both the tetraethynyltin samples studied. Because the samples differed significantly in percentage of the iodide (17.4 ± 4.0 and 47.1 ± 3.5 mol %, in samples A and B, respectively), it was possible to determine the structures of both molecules to a sufficient degree of accuracy. The r α , structures were solved by the least-squares treatment of the molecular intensities, using mean amplitudes and shrinkage corrections calculated from the force fields of a number of tin derivatives. The T d -symmetry model of Sn(CCH) 4 was refined to give the following parameters: Sn-C, 2.068(5); CC, 1.228(8); CH, 1.079(51). The structural parameters for ISn(CCH) 3 (on the basis of the C 3v model with linear Sn-CC-H fragments) are as follows: Sn-I, 2.646(4); Sn-C, 2.062(17); CC, 1.226(6); ∠ISnC 108.0(2.8). (The thermal average bond distances, r g , are given in A, and the valence angle, rα, in degrees; the values in paren- theses are three times the standard deviations, 3σ.) The Sn-C bonds in Sn(CCH) 4 , and ISn(CCH) 3 are shorter than the corresponding bonds in the monoethynyltin derivatives, Me 3 SnCCH and Me 3 SnCCSnMe 3 . The SnI bond in ISn(CCH) 3 is noticeably shorter than those in stannane iodide and trimethylstannane iodide.

Gas phase electron diffraction study of tetraphenyltin, Sn(C6H5)4

Abstract The geometrical parameters of the tetraphenyltin molecule have been determined by gas phase electron diffraction at about 310°. The S 4 and “open” D 2d molecular models with the tetrahedral bond configuration at tin were chosen for the structure analysis. The former gave the better fit. The thermal average bond lengths ( r g , in A) are as follows: The benzene ring geometry appears to be almost unaffected by bonding to tin. However, tin causes an increase in the endocyclic valence angle at the ipso -carbon atom to 121.0(0.9)° rather than a decrease of that angle as might be expected, tin being a σ-electron donor. The ring plane and the plane containing the bond and S 4 axis make an angle, ϕ, of 34.1(2.1)°. The bond length in tetraphenyltin is longer than not only the bond in tetravinyltin ( r g = 2.117(4) A) but also the bond in tetramethyltin ( r g = 2.144(7) A).

Gas phase electron diffraction study of basketene, C10H10

Abstract A gas phase electron diffraction study of the cage hydrocarbon, basketene, is reported. A least squares treatment of molecular intensities has been carried out in terms of a geometrically consistent rα structure. The mean amplitude values and shrinkage corrections have been calculated using the force field parameters estimated from the data on simpler molecules. Structure refinement of the C2v molecular model yields the following parameter values (bond lengths, ra, in nm; angles, rα in degrees): av. 0.1092(8); In addition to the geometric parameters listed, the mean amplitudes for all bonded and C· C nonbonded distances have been determined by the least squares method. All the other amplitudes (C· H and H· H) have been fixed at the values estimated from the spectral data. Comparison of the results obtained with the literature data on similar polycyclic molecules points to the stronger internal strain in the basketene molecule.

Molecular structures of acetylene derivatives of tin: 7. Bis(trimethylstannyl)acetylene: analysis of electron diffraction data taking into account nonlinear relations between Cartesian and internal vibrational coordinates

The electron diffraction data on bis(trimethylstannyl)acetylene, Me3SnC≡CSnMe3, were analyzed in the framework of the one-dimensional dynamic model of free internal rotation of the SnMe3 group about the axis of the Sn−C≡C−Sn linear fragment. The root-mean-square amplitudes and harmonic shrinkage corrections used in the analysis were calculated from the scaled quantum-chemical force field (i) taking into account nonlinear relations between Cartesian and internal vibrational coordinates at the first-order level of perturbation theory (h1) and (ii) using a conventional approach (h0). Therh1 parameters of internuclear distances describe the equilibrium geometry of the Me3SnC≡CSnMe3 molecule much better than the commonly accepted parametersrα≡rh0. Substituent effects on the geometry of the acetylene fragment are discussed.

Molecular structure of acetylene derivatives of tin: Part V. Application of vibrational spectroscopy results for refinement of electron diffraction data on trimethylstannylacetylene, Me3SnCCH

Abstract The force field of trimethylstannylacetylene previously obtained by us was used for calculation of the mean amplitudes and perpendicular amplitude corrections. Shrinkages for the linear fragment SnCCH(D) have also been evaluated. Using these data, we attempted to refine the geometrical parameters of the trimethylstannylacetylene molecule in terms of r a -structure. To account for the large amplitude motions due to the internal rotation of the methyl groups, a dynamic molecular model was used. The following values were obtained (the thermal average bond distances at 22°C, r g , are given in A and the valence angles, ∠ a , in degrees): SnC Me 2.141(10), SnC Ethyn 2.126(25), CC 1.233(18), CH Me 1.095(12), C Me SnC Ethyn 104.7(2.0), SnCH 113.4(1.2). The values in parentheses are three times the standard deviations, 3σ. The SnC Ethyn bond in trimethylstannylacetylene is longer than the bonds in the tetraethynyltin derivatives, Sn(CCH) 4 and Sn(CCF 3 ) 4 .

Electron diffraction study of the molecular structure of gaseous 1,1-dibromo-3,3,5,5-tetramethyl-1-stanna-3,5-disila-4-oxacyclohexane, Br2Sn(CH2SiMe2)2O

The molecular structure of gaseous Br2Sn(CH2SiMe2)2O was studied by electron diffraction. The six-membered ring has a chair conformation whereas the entire molecule possessesCssymmetry. The existence of a boat conformer cannot be completely excluded. The results of theoretical calculations for a twisted-boat conformation are at variance with the experimental data. Steric strain caused by mutual repulsion of the two axial methyl groups is reduced to the tilt of the Me2Si fragments in opposite directions. This results in an increase (up to 26°C) in the angle formed by the bisector of the CM-Si-CM angle with the CcSiO plane. The main geometrical parameters are as follows:rg (Å): Si-O 1.708(20); Si-CM 1.862(20); Si-Cc 1.882(9); Sn-C 2.108(26); Sn-Br 2.456(3); C-H 1.099(30); γα (degr.): C-Sn-C 105(2); Br-Sn-Br 107.9(1.2); Si-O-Si 129.6(3); CM-Si-CM 112; Si-C-H 113 (fixed value in accordance with experiment); Cc-Si-O 107(2); Sn-C-Si 109(2); torsion angles: ϕ(Si-C) 52(2); ϕ(Si-O) 62(1); ϕ(Cc-Sn) 54(1). The average amplitudes were fixed at the values calculated from the force field. Structural parameters of molecules with similar structures were analyzed and compared.