Richard L. Hilderbrandt

The structure of methyl silatrane (1-methyl-2,8,9-trioxa-5-aza-1-silabicyclo(3.3.3)undecane) as determined by gas phase electron diffraction

Abstract The structure of methyl silatrane is investigated by gas-phase electron diffraction at 185° C. The molecule possesses C 3v symmetry. The result obtained for the Si—N distance (2.45(5) A) indicates essentially no dative bonding between Si and N in the gas phase. This result is quite different from the solid-state result which indicates a Si←N dative bond length of 2.175(4) A. Other structural parameters compare favorably with both the solid state results and with values obtained in the gas phase for similar molecules.

Molecular structures of silacyclohexane and silacyclopentane as determined by gas phase electron diffraction

Abstract The molecular structures of silacyclopentane and silacyclohexane are determined by gas phase electron diffraction. Silacyclohexane is found to exist in a modified chair conformation which is somewhat flattened (relative to cyclohexane) at the silicon atom and somewhat puckered at the C 4 position. The SiC ( r g = 1.885(3) A) and CC ( r g = 1.550(3) A) bond lengths were found to be comparable with those observed in more highly strained polycyclic molecules such as 1-methyl-1-silaadamantane and 1-methyl-1-silabicyclo(2.2.1)heptane. Valence angles for the ring were found to be: ∠C 5 SiC 1 = 104.2(1.4)°, ∠SiC 1 C 2 = 110.6(0.6)°, ∠C 1 C 2 C 3 = 113.7(1.1)° while the observed dihedral angles were: τ(SiC) = 44.0(4.2)°, τ(C 1 C 2 ) = 57.3(2.0)° and τ(C 2 C 3 ) = 67.5(2.0)°. Combined analysis of electron diffraction and microwave spectroscopic data for silacyclopentane shows that the molecule exists in the C 2 or twist conformation with SiC and average CC bond lengths of 1.892(2) A and 1.549(3) A respectively. The valence angles obtained for the ring are: ∠C 4 SiC 1 = 96.3(0.3)°, ∠SiC 1 C 2 = 103.6(0.3)° and ∠C 1 C 2 C 3 = 108.4(0.7)°, while the ring dihedral angles are: τ(SiC) = 13.3(0.4)°, τ(C 1 C 2 ) = 36.1(1.0)° and τ(C 2 C 3 ) = 49.7(1.4)°. Molecular mechanics calculations are found to be helpful in interpreting the structures and conformations of these two molecules in terms of simple Bayer and Pitzer strain energy concepts.

Application of Newton-Raphson optimization techniques in molecular mechanics calculations

Abstract Applications of the Newton-Raphson optimization technique to molecular mechanics calculations are discussed. The theory for applying this method is developed in terms of the spectroscopic Wilson S vectors for stretching, bending and torsional internal coordinates. An exact method for calculating the geometries of transition states for interconversion processes is shown to follow directly from the use of Newton-Raphson optimization. Also, a precise method is developed for calculating the detailed minimum energy pathway for interconversions. A computer program EMIN is described, and applications of this program to geometry optimization, normal coordinate analysis, thermodynamic calculations and synthesis of electron diffraction radial distribution curves are illustrated using the molecule cyclohexane.

The structure of bis(1,1,1,5.5,5-hexafluoro-2,4. pentadionato) copper(II) as determined by gas phase electron diffraction

Abstract The rg structure of bis(1,1,1,5,5,5-hexafluoroacetylacetonato) copper(II) has been determined by gas phase electron diffraction. The experimental data were found to be consistent with a D2h model in which the oxygens from the two ligands are arranged in an essentially square planar configuration about the copper atom (∠OCuO = 90.6° ± 1.2°). It was possible to obtain a precise value for the copper oxygen bond length, rg = 1.919 ± 0.008 A, since this distance appeared as an isolated peak in the radial distribution curve. Structural parameters for the ligand (rg(C-O) = 1.276 ± 0.009 A, rg(C-Cring) = 1.392 ± 0.015 A, rg(C-CF3)= 1.558 ± 0.009 A and rg(C-F) = 1.339 ± 0.003 A), while less precisely determined are, nevertheless, consistent with reported values for related molecules. A model for the rotational isomerism of the four CF3 groups was invoked in order to explain various features in the radial distribution curve in a region from 2.5 to 5.5 A.

The structure and conformation of dichloroacetyl chloride as determined by gas-phase electron diffraction

Abstract The structure and conformation of dichloroacetyl chloride have been determined by gas-phase electron diffraction at nozzle temperatures of 20 and 119°C. The molecules exist as a mixture of two conformers with the hydrogen and oxygen atoms syn and gauche to each other. The composition (mole fraction of syn form) of the vapor was found to be 0.72 ± 0.06 and 0.73 ± 0.12 at 20 and 119°C, respectively, corresponding to almost equal energy for the two forms. The results for the distance ( r g ), angle ∠α and r.m.s. amplitude ( l ) parameters obtained at the two temperatures are entirely consistent. At 20°C the more important parameters, with estimated uncertainties of 3σ are: r (C-H) = 1.062(0.049)A, r (C0) = 1.189(0.003)A, r (C-C) = 1.535(0.008)A, r (CO-Cl) = 1.752 (0.009)A, r (CHCl-Cl) = 1.771(0.004)A, ∠C-CO = 123.3(1.3)°, ∠C-CO-Cl = 113.9 (5.9)°, ∠C-CHCl—Cl = 109.5(1.5)°, ∠C1-C-Cl = 111.7(0.5)°, ∠Cl-C-H = 108.0(1.5), φ 1 (HCCO torsion angle in the syn conformer) = 0.0° (assumed), φ 2 (HCCO torsion angle in the gauche conformer) = 138.2(5.1)°.

The structure and pseudorotation of cyanocyclopentane as determined by gas phase electron diffraction

Abstract The structure of cyanocyclopentane has been determined by gas phase electron diffraction. The molecule was found to have a low barrier to pseudorotation with two minima corresponding to C S envelope conformations with the CN in the quatorial and axial positions. The best least squares value obtained for the barrier to pseudorotation was 240(330) cal mol −1 , and the energy difference between the equatorial and axial conformers was found to be 180(330) cal mol −1 with the equatorial form being preferred. The puckering amplitude for the five-membered ring was found to be 0.42(28) A, and an average CC(ring) distance of 1.544(1) A was obtained. Other parameters obtained from least squares analysis of the experimental data include: r g (CCN) = 1.477(7) A, r g (CN) = 1.160(2) A, r g (CH) = 1.102(6) A, avg = 107.2(3.2)°. The results obtained are in excellent agreement with related cyclopentyl compounds, and with the microwave spectroscopic results obtained for cyanocyclopentane.

The structure of 1-chloro-1-silabicyclo(2.2.2)octane as determined by gas-phase electron diffraction

Abstract The structure of 1 -chloro-1 -si labicyclo( 2.2.2 )octane is determined by gas-phase electron diffraction. The molecule is found to have a large amplitude twisting motion with a double minimum quartic potential function of the form V (φ) = V o [1 + (φ/φ o ) 4 - 2(φ/φ o ) 2 ]. Least-squares analysis of the experimental data gives values of 1.4(0.8) kcal mole − for V o and 17.5(2.5)° for φ o . Other structural parameters for the “quasi- C 3v ” cage-like molecule include: r g (Si-Cl) = 2.061(3) A, r g (Si-C) = 1.863(3) A, r g (C-C av ) = 1.559(2) A, and r g (C-H av ) = 1.098(7) A. Several valence angles exhibit large deviations from tetrahedral values, e.g. ∠Cl-Si-C 2 = 114.6(0.2)°, ∠Si-C 2 -C 3 = 105.8(0.4)°, ∠C 2 -C 3 -C 4 = 114.2(1.2)°, ∠C- 3 -C 4 -C 5 = 111.4(0.8)° and ∠C 2 -Si-C 6 = 103.9(0.2)°. Many of the structural features in this strained polycyclic compound. Including the nature of the quartic potential function, can be rationalized in terms of a simple molecular mechanics model. A new method for the calculation of an analytical Jacobian of the intensity function with respect to parameters of the potential function is also discussed.

The structure of tris(1,1,1,5,5,5-hexafluoro-2,4-pentanedionato)aluminum(III) determined by gas phase electron diffraction

Abstract The r g structure of tris(1,1,1,5,5,5-hexafluoroacetylacetonato)aluminum(III) has been determined by gas phase electron diffraction. The experimental data were found to be consistent with a D 3 model in which the oxygen atoms of the three ligands are arranged in a slightly distorted octahedron about the aluminum atom [∠ OAlO for the ligand is 87.2° (1.4)]. The aluminumoxygen bond length obtained [AlO = 1.893(14) A] is in excellent agreement with the value previously obtained by X-ray diffraction for the related acetyl-acetonato complex of aluminum [AlO = 1.893(14) A]. Structural parameters for the ligand [CO = 1.277(11) A, CC ring = 1.412(14) A, CCF 3 = 1.561(11) A, and CF = 1.338(3) A] are in excellent agreement with the values obtained in an earlier electron diffraction study of the Cr(III) complex with hexafluoroacetylacetonate. The resulting structure was found to agree quite well with the predictions based on a force law model for tris bidentate complexes developed by Kepert.

The structure of 1-methyl-1-silaadamantane as determined by gas phase electron diffraction

Abstract The structure of 1-methyl-1-silaadamantane (MSA) has been determined by gas phase electron diffraction. There appears to be somewhat less ring strain at the silicon bridgehead of MSA than in the previously studied 1-methyl-1-silabicyclo[2.2.1]heptane (MSBH). The average SiC bond length [1.879(3) A is comparable to those found in acyclic organosilicon systems. Also, the average CC bond length (1.547(2) A) is only slightly longer than that observed for adamantane (1.540(2) A). Valence angles at the silicon bridgehead experience only a moderate perturbation away from their unstrained tetrahedral values. On this basis it is expected that MSA should be somewhat less reactive than MSBH under S N 2 conditions according to the reaction mechanism suggested by L.H. Sommer.

2-Chloropropene and 2-bromopropene: gas-phase molecular structures as determined by combined analysis of electron diffraction and microwave spectroscopic data

Abstract The molecular structures of 2-chloropropene and 2-bromopropene have been determined at 20°C by combined analysis of electron diffraction and microwave spectroscopic data. The following results, expressed in terms of r g distances and r α angles, were obtained for 2-chloropropene: r (CC) = 1.338(3) A, r (CC) = 1.495(4) A, r (CCl) = 1.744(2) A, r (=CH) avg = 1.089(8) A, Δ r (CH) = 0.008(10) A, ∠CCC = 126.3(2)°, ∠CCCl = 119.0(3)°, ∠CCH( trans to Cl) = 118.7(2.0)°, ∠CCH( cis to Cl) = 125.0(2.0)°, ∠CCH(in plane) = 112.1(2.3)°, ∠CCH(out of plane) = 110.6(1.7)°. The results obtained for 2-bromopropene were: r (CC) = 1.343(4) A, r (CC) = 1.496(5) A, r (CBr) = 1.909(3) A, r (CH) avg = 1.113(9) A, ∠CCC = 126.9(3)°, ∠CCBr = 118.3(5)°, ∠CCH avg = 103.8(2.1)°. For 2-bromopropene a second least-squares minimum was found for a model having ∠CCBr = 123.5(7)°. This model could not be rejected on a statistical basis. Quoted uncertainties in geometrical parameters are 3σ.