V. S. Mastryukov

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.

Electron diffraction determination of the vapour phase molecular structure of azetidine, (CH2)3NH

Abstract The electron diffraction study of azetidine yielded the following main geometrical parameters ( r a structure): dihedral angle (the angle between the C-C-C and C-N-C planes) φ = 33.1 ± 2.4°, r (C-N) = 1.482 ± 0.006A, r (C-C) = 1.553 ± 0.009A, r (C-H) = 1.107 ± 0.003A, ∠C-N-C = 92.2 ± 0.4°, ∠C-C-C = 86.9 ± 0.4° and ∠C-C-N = 85.8 ± 0.4°.

An electron diffraction study of 3-methyldiaziridine and 1,2-dimethyldiaziridine

Abstract The structures of the title compounds, diaziridines, (the first to be studied in the gas phase) have been determined by electron diffraction. The following principal structural parameters were obtained with the estimated standard deviations parenthesized: 3-methyldiaziridine, N-C = 1.489(9) A, N-N = 1.444(13) A, C-C = 1.505(16) A, C-H = 1.107(5) A, α =∠ (C-C, NCN) = 61.3° (0.9); 1,2-dimethyldiaziridine, (parameters of the cycle CN 2 were assumed from the previous molecule), N-C (methyl) = 1.445(3) A, C-H = 1.108(9) A, ∠ C-N-Me = 112.0° (0.5), the two methyl groups are in the trans position. Vibrational amplitudes were also determined for all important distances.

Molecular structure of cycloheptene, C7H12, as determined by electron diffraction and molecular mechanics calculations☆

Abstract The structure of cycloheptene has been investigated by gas-phase electron diffraction at 20°C and by molecular mechanics (MM2). The energetically less favorable conformer, the boat, is shown to be incompatible with experiment. The geometrical parameters of the chair conformer were found to be (ra structure): CC 1.343(8) A, C2–C3 1.516(16) A, C3–C4 (C4–C5) 1.536(8) A, CH 1.108(7) A; ∠ CCC 123.8(8)°, ∠C4C5C6 113.9(24)°, ∠HCH 107.9(36)°; flap angles φ1 128.0(33)° and φ2 120.2(44)°. The differences between the parameters calculated by molecular mechanics are all within the reported uncertainties.

An electron diffraction study of the molecular structure of gaseous bicyclo[3.3.1]nonane

Abstract A gas-phase electron diffraction study of the title compound, carried out at 65°C, has found no statistically significant evidence for the presence of any conformer other than the double chair. The geometrical parameters were found to be: r g (CC) av = 1.538(1) A, r g (CH) av = 1.116(3) A, ∠C1C9C5 = 111.5(1.1)°, ∠C2C3C4 = 111.2(1.4)°, ∠HCH = 107.0(2.6)°, ∠θ = 121.5(0.8)°, ∠ϕ = 42.6(2.5)°. In an attempt to refine electron diffraction data for models with three non-equivalent CC distances, two sets of molecular parameters were obtained. Amplitudes of vibration were calculated from an approximate force field and the uncertainties in the fixed amplitudes were analyzed. The resulting structural parameters, chosen on the basis of molecular mechanical calculations, were: ( r g structure for bond lengths, r α structure for angles) C1C2 = 1.559(10) A, C2C3 = 1.541(10) A, C1C9 = 1.480(9) A, CH = 1.114(3) A, ∠C1C9C5 = 110.1(2.3)°, ∠C2C3C4 = 113.2(2.7)°, ∠HCH = 111.4(3.5)°, ∠θ = 124.1(0.9), ∠ϕ = 40.0(2.0)°. Comparison with structures of other bicyclo[ n.m. 1]alkanes has been made.

Empirical relation between bond angles in C-CH2-C fragments

Abstract Collected experimental data (17 from electron diffraction and 13 from microwave spectroscopy) show that the HCH bond angle is a linear function of the CCC bond angle in the C-CH 2 -C fragments: ∠HCH = 126.1–0.175 (∠ CCC) (°). The CCC angle changes from 50.8° in cyclopropene to 116-56° in cyclooctane. This empirical relation can be used in the structural analysis of molecules which have non-equivalent methylene groups.

Relations between mean amplitudes of vibration and corresponding internuclear distances: I. bonded and nonbonded carbon-carbon distances

Abstract Mean amplitudes of vibration ( u ) for bonded and non-bonded CC distances from gas electron diffraction were treated statistically. The following empirical relation was established on the basis of 71 measurements u (CC) = 0.013837 + 0.023398 r -0.000147 r 2 where r is the internuclear distance in the range 1.217 ⩽ r ⩽ 5.618 ( r and u are both in A). The experimental data come mainly from electron diffraction laboratories in U.S.A., Norway, Japan and England.

Relations between mean amplitudes of vibration and corresponding internuclear distances: II. Bonded carbon-carbon distances

Abstract The empirical relation u (CC bond ) = −0.071856 + 0.124162 r − 0.028974 r 2 was established from a statistical treatment of 57 electron diffraction measurements (1.2086 ⩽ r ⩽ 1.549, where r is the bond distance, and u is the mean amplitude of vibration; both in A units). The agreement between observed and calculated u values is significantly improved when compared with the corresponding treatment of bonded and non-bonded CC distances taken together.

Molecular structure and conformations of bicyclohexyl, (C6H11)2, as studied by electron diffraction, vibrational spectroscopy and molecular mechanics

Abstract The molecular structure of bicyclohexyl, (C6H11)2, has been studied by gas-phase electron diffraction with the aid of vibrational spectroscopy and molecular mechanics calculations (MM3 force field). According to these calculations, the most stable conformer is ee anti (C2h symmetry), whereas the next stable conformer, ee gauche, has C2 symmetry and differs by only 0.96 kJ mol−1 in energy. The third conformer, ea anti, has Cs symmetry and its energy content is 7.36 kJ mol−1. Infrared spectra have been recorded in the vapour phase and in argon and nitrogen matrices. From variable temperature measurements by Raman spectroscopy, the enthalpy difference ΔH√O (ee gauche—ee anti) was found equal to 1.5 ±1 kJ mol−1. The electron diffraction results have been interpreted in terms of a mixture of C2h and C2 con- formers only, with a ratio of 52.6 ±9.0% and 47.4 9.0%, respectively, at 112°C. The average main structural parameters of these conformers are as follows (ra structure): CCring1.535 (2) A, CCpivot1.550 (14) A, CH 1.102 (3) A; bond angles CCC 110.7 (10)o,CCH 109.1 (6)o the torsional angle, τ, in the gauche conformer is 74.90° The estimated standard deviations, while allowing for the effects of correlations (3σ), have been increased for internuclear distances to take account of systematic errors in the electron wavelength.

Molecular structure of cyclododecane, C12H24, as determined by electron diffraction and molecular mechanics

Abstract The molecular structure of cyclododecane has been studied at 120°C using gas phase electron diffraction assisted by molecular mechanics calculations. Two models suggested earlier by X-ray data have been tested. Electron diffraction data are compatible with a D4 model while the D2d model can be rejected, in agreement with molecular mechanics calculations which favor the first model by 10.5 kcal mol−1 over the second. For a model with D4 symmetry the experimental results give average bond lengths (rg), independent bond and torsion angles (∠α) with estimated uncertainties of 3σ. CC 1.540(1) A, CH 1.114(3) A, ∠ HCH 104.2(18)°, ∠ C1C2C3 115.0(7)°, τ (C1C2C3C4) 159.5(12)°.