Brian Beagley

Electron-diffraction studies of tetramethylsilane and hexamethyldisilane, and discussion of the lengths of Si-C bonds

Abstract Studies of tetramethylsilane and hexamethyldisilane by standard electron-diffraction techniques give rise to the following parameters: in tetramethylsilane, Si-C = 1.875 ± 0.002A, ∠SiCH = 109.2 ± 0.8°, ∠HCH = 109.8 ± 0.8°; in hexamethyldisilane, Si-C = 1.877 ± 0.003 A, Si-Si = 2.340 ± 0.009 A, C-H = 1.127 ± 0.009 A, ∠SiCH = 108.7±0.8°, ∠CSiC = 110.5±0.4°, ∠SiSiC = 108.4 ± 0.4°, ∠HCH = 110.3 ± 0.8°. Vibrational amplitudes were also determined for all major distances. A 0.009 A shrinkage of the long C·C( trans ) distance in hexamethyldisilane leads to an apparent preferred configuration 10 ± 4° from full staggering about Si-Si. Three refinements of tetramethylsilane were carried out, providing checks for consistency between differing computational techniques. The mean Si-C bond length is 0.060 ± 0.005 A shorter than the sum of the tetrahedral covalent radii for silicon and carbon, and about 0.01 A shorter than that found in the solid phase. Upon correction to an r c value, the Si-C bond length agrees closely with microwave results for other methyl silanes. Arguments which have been used to explain C-C bond lengths cannot be carried over to the Si-C system, probably because d orbitals are involved.

Vibrational force fields and amplitudes, and zero-point average structures of (CH3)3Y molecules (Y = N, P, As, Sb, Bi): A Combination of electron-diffraction and spectroscopic data

Abstract Following the development of methods for placing electron-diffraction and spectroscopic geometrical parameters on a common basis, available data on (CH 3 ) 3 Y molecules (Y = N, P, As, Sb, Bi) have been used to derive force fields, r.m.s. amplitudes of vibration u along the internuclear vectors, and perpendicular amplitude correction coefficients K for these molecules. For trimethylamine, the amplitudes are similar whether or not off-diagonal elements are included in the force field; hence, only diagonal elements are considered for the other molecules. Among the interesting trends, as group V is descended, is that the C-Y r.m.s. amplitudes increase only from 0.049 to 0.058 A, whereas the C-Y stretching force constant decreases by over 60% from 5.3 to 1.8 mdyn A −1 . There is evidence for an increasing tendency for torsional motion of methyl groups, as group V is descended. For each of the molecules, the amplitude data were used to derive zero-point average (r o α ) structures and to make estimates of a partial equilibrium ( r e ) structure. For trimethylamine the results suggest a systematic error in the electron-beam wavelength of the literature study, and the structural parameters were appropriately revised. The r o α lengths of the C-Y bonds in the five molecules are 1.458 ± 0.002, 1.844 ± 0.003, 1.979 ± 0.010, 2.169 ± 0.010 and 2.263 ± 0.004 A, respectively. The estimated r e parameters for the bonds in trimethylamine agree well with the microwave r s structure.

An electron diffraction determination of the molecular structure of methyl silyl ether

Abstract The molecular structure of methyl silyl ether CH 3 OSiH 3 , has been deter- mined in the gas phase by the sector microphotometer method of electron, diffrac- tion. The Si-O bond length is 1.640±0.003 A, the C-O bond length is 1.418±0.009 A, and the Si-O-C angle is 120.6±0.9°.

An electron diffraction study of gaseous α-alanine, NH2CHCH3CO2H

Abstract The molecular structure of α-alanine has been studied using gas-phase electron diffraction data collected on the Balzers KDG2 instrument at UMIST. Only one conformer exists in the vapour of α-alanine and it is a neutral species. The principal structure,parameters of the molecule were observed to be: r g (C-N)=1.471(7) A, r g (C-C)=1.544(10) A, r g (C=O)=1.192(2) A, r g (C-O)=1.347(3) A, r g (C- m )=1.509(16) A, ∠NCC=110.1(8)°, ∠CC=O=125.6(7)°, ∠CC=0110.3(7)°, ∠CCC m 111.6(11)°, ∠NCC m =111.0(16)° and =l7(2)°,where is the dihedral angle between the NCC plane and the CC=O plane. The distortion is in the direction of reducing the repulsion between the methyl group and the carboxyl group.

An electron-diffraction study of the molecular structure of trimethylgallium

Abstract An electron-diffraction study of trimethylgallium in the vapour phase gave rise to the following geometrical parameters ( r g values): Ga-C = 1.967 +-0.002 A, C-H = 1.082±0.003 A, ∠Ga-C-H = 112.1 ±0.8°; the vibrational shrinkage of the C⋯C distance was found to be 0.021 ±0.010 A, which corresponds to an apparent ∠C-Ga-C of 118.6°. The least-squares refinements of the structure employed a technique for ill-conditioned problems, which assisted the study of the torsional motion of the methyl groups. There is little doubt that at room temperature the methyl groups are rotating, but whether the rotation is entirely “free” is not certain.

The crystal and molecular structure of cis-diammine-1,1-cyclobutanedicarboxoplatinum(II) [cis-Pt(NH3)2CBDCA]. Dynamic puckering of the cyclobutane ring

Abstract C6H12N2O4Pt, Mr = 371.27, orthorhombic, Pnma, a = 7.724(2), b = 10.464(3), c = 11.176(2) A, V = 903.29 A3, Z = 4, DX = 2.73 Mg m−3, μ (Mo Kα) = 16.32 mm−1, λ = 0.71069 A. Final R = 0.029 for 856 reflexions. A mirror plane containing the platinum and average cyclobutane ring bisects the molecule. One of the cyclobutane carbon atoms shows excessive thermal motion, which is interpreted in terms of a dynamically puckering ring.

On the molecular structure of 1,1,1-trifluoroethane. A modern gas-phase electron diffraction study

Abstract The molecular structure of 1,1,1-trifluoroethane has been re-examined by gas-phase electron diffraction to resolve the confusion arising from the contradictory results of earlier structural studies (including spectroscopic ones) and to provide reference data for the parameterization of molecular mechanics calculations. Electron-diffraction data, collected on the Balzers KDG2 instrument, were processed to yield molecular intensity curves which were employed in refinements of the molecular structure. The preferred refinement yielded the following structural parameters for the staggered conformation (ra values with e.s.d. in parentheses): CC = 1.494(3) A, CF = 1.340(2) A, CH = 1.081(7) A, ∠ CCF = 111.9(2)° and ∠ CCH = 112(1)°. These results are in good agreement with the microwave r0 values of Thomas, Heeks and Sheridan, and confirm the occurrence of a very short CC bond.

A combined electron-diffraction and vibrational study of the molecular structure and compliance field of n-methylsulfinylamine

Abstract A gas-phase electron-diffraction study of N -methylsulfinylamine, CH 3 NSO, has yielded the following geometric parameters ( r g basis): C-H = 1.134±0.009 A, C-N =1.421±0.005 A, N-S = 1.525±0.004 A, S-O = 1.466±0.004 A, ∠C-N-S = 126±2°, ∠N-S-O = 117±2°. The angle H-C-N has not been established with certainty. The C-N-S-O skeleton is planar and has the cis configuration. A hydrogen atom is in or close to the principal molecular plane, eclipsed with the N-S bond. The geometric parameters are highly correlated, and mean-square amplitudes of vibration were restrained to their spectroscopic values, as derived from a compliance field calculation carried out in parallel with the ED refinements. As a geometric structure is required to calculate the G matrix elements needed for the compliance constant refinement, a double iteration procedure was adopted. The overall calculation was cycled to simultaneous convergence of both geometric and compliance-field refinements.

The C-C bond in neo-pentane, the bonding radius of the tertiary-butyl group, and their bearing on bonding hypotheses

Abstract The molecular structure of neo-pentane, (CH 3 ) 4 C, has been studied in the gas phase by electron diffraction. The C-C bond length, 1.539 ± 0.002 A, is intermediate between the lengths in ethane and diamond. The observed length is readily understood in terms of separate bonding radii, r p = 0.767 and r t = 0.772 A, for primary and tertiary carbon atoms, respectively. The origin of the significant difference between these radii is discussed. Two semi-empirical theoretical approaches are tested, (i) the hypothesis that all variations in C-C bond lengths are due to changes in the degree of hybridisation of the two carbon atoms, (ii) the hypothesis that hybridisation, conjugation and hyperconjugation each have a role in determining C-C bond lengths. Hybridisation differences are considered to be the origin of the difference between r p and r t . Quantitatively, however, the observed difference suggests that the second of the approaches is the more realistic, i.e. changes in hybridisation have only a limited effect on C-C bond lengths; the predominant effects are conjugation and hyperconjugation. The anomalous effect of adjacent hetero-atoms on C-C bond lengths is pointed out.

The gas-phase rotamers of 1,1,2,2-tetrafluoroethane - force field, vibrational amplitudes and geometry - a joint electron-diffraction and spectroscopic study

Abstract The gas-phase conformational mixture of the anti and gauche rotamers of 1,1,2,2-tetrafluoroethane has been subjected to an electron-diffraction study at 253 K. Effective least-squares refinement of the geometry and relative proportions of the conformers was achieved with vibrational amplitudes for both conformers fixed at values calculated from spectroscopic data. In order to calculate the amplitudes, a force field was deduced which reproduced the observed wave numbers for both conformers; the assignment of the modes proposed in the literature was modified slightly. At 253 K, the rotamer composition was found to be 84% anti : 16% gauche , which corresponds to an energy difference of 1170 cal mol −1 ; the geometrical parameters ( r a values) and e.s.d. are C-C = 1.518 ± 0.005 A, C-H = 1.098 ± 0.006 A, C-F = 1.350 ± 0.002 A. ∠CCF = 108.2 ± 0.3°, ∠FCF = 107.3 ± 0-3°, ∠ CCH = 110.3 ± 1.0δ, and the torsion angleτ hcch in the gauche form is 78 ± 2°.