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Gas-phase electron diffraction experiments have been carried out on NO 2 at three temperatures, viz., 294, 480, and 691 K. Applying the necessary vibrational corrections, the equilibrium structure ( r e ) of NO 2 was determined from the 480 K data where it was the sole component of the vapor, 1.195(3) A and 133.7(2)∘. This information was then utilized in the analysis of the mixture of NO 2 (82%) and N 2 O 4 (18%) at 294 K. The NO bond of NO 2 is shorter than that of N 2 O 4 by 0.008 A, and the ONO angle opens slightly in the dimer as compared with NO 2 . The barrier height to internal rotation in N 2 O 4 is estimated to be about 40 kJ mol −1 from the electron diffraction data. There is a slight increase of the average NO bond length ( r g ) at higher temperatures.

The molecular structure of hexamethyldisiloxane has been reinvestigated by electron diffraction. The two kinds of large amplitude motion in the molecule, the Si–O–Si bending and the torsion about the Si–O bonds, were treated independently in terms of their potential functions. In the equilibrium structure the molecule has probably C 2v symmetry with staggered C 3 Si–O–Si fragments and a nonlinear Si–O–Si linkage with an angle of 155°. The barrier to linearization was obtained to be at least 10 kJ mol −1 . In addition, there is free or nearly free internal rotation of the trimethylsilyl groups in hexamethyldisiloxane. In terms of r g bond lengths the electron diffraction analysis yielded Si–O 1.639±0.003 A, Si–C 1.871±0.004 A, and the r a angle Si–O–Si 152±2°.

The molecular geometries of 1,3-C 6 H 4 (SiMe 3 ) 2 ( 2 ) and 1,3,5,-C 6 H 3 (SiMe 3 ) 3 ( 3 ) have been determined by gas-phase electron diffraction. The mean CC bond lengths are slightly larger than those in benzene, and the endocyclic ipso bond angles C6C1C2 are considerably less than 120°, in accord with the electron-releasing inductive effect of the SiMe 3 substituent and the predictions of the VSEPR model. The SiC (methyl) bonds are longer than the SiC(aryl) bonds by about 0.02 A (see below). Only limited information concerning conformational preference could be obtained from the electron-diffraction data but this did not hinder the determination of the other structural characteristics. The following main bond length ( r g ) and angle parameters (with estimated total errors) have been obtained at nozzle temperatures of 86 and 105°C, respectively. For 2 : (CC) mean 1.405±0.003, (SiC) mean 1.87 9± 0.004, Δ(SiC)  SiC(methyl)  SiC(aryl) = 0.018 ± 0.008, SiC(methyl) 1.884 ± 0.004, SiC(aryl) 1.866 ± 0.007 A C6C1C2 116.2±0.6°, C4C5C6 119.5° (assumed), CSiC 109.5° (assumed); for 3 : CC 1.410±0.003, (SiC) mean 1.881 ± 0.004, Δ(SiC) 0.016±0.006, SiC(methyl) 1.885±0.004, SiC(aryl) 1.869±0.006 A, C6C1C2 117.0±0.6°, CSiC 109.5° (assumed). On the basis of available structural information on three trimethylsilyl-substituted benzene derivatives, and assuming additivity, the angular ring deformation impact of the trimethylsilyl substituent has been estimated. The predicted endocyclic angles for trimethylsilylbenzene are (starting from the ipso angle) 116.8, 122.2, 119.9, and 119.0°.

For CH 3 OCHCHCN, two conformers with C s symmetry were found in the E -isomer, and two C 1 symmetry forms in the Z -isomer by gas-phase electron diffraction. Parameters ( r g with estimated total errors) CC 1 , CC 1 , OC mean , CN, OCC, COC, CCC in the E -isomer: 1.428(5), 1.348(8), 1.393(6), 1.155(4) A, 122.5(6), 120.5(7), 119.1(6)°, in the Z -isomer: 1.418(6), 1.374(15), 1.392(7), 1.161(3) A 120.8(9), 117.9(9), 126.1(7)°. Through-conjugation is larger and competes with steric effects in the Z -isomer.

The structures of free Ph 4 M molecules (MSi, Ge, Sn) have been analysed by electron diffraction. Only a limited amount of reliable structural information could be determined since several models ( D 2d , S 4 , D 2 ) fit the experimental data equally well. The phenyl rings are slightly elongated. Assuming that b = c and γ = 120°, the r g parameters (with estimated total errors) have been obtained: 1 MC (CC) mean (CH) mean b  a α (A) (A) (A) (A) (deg.) Ph 4 Si 1.871(4) 1.403(3) 1.087(4) 0.005(5) 118.2(4) Ph 4 Ge 1.960(4) 1.401(3) 1.098(4) −0.012(6) 119.5(3) Ph 4 Sn 2.137(5) 1.401(3) 1.109(5) −0.037(8) 119.5(6) where a = r (C1-C2), b = r (C2-C3), c = r (C3-C4) α = ∠ C6-C1-C2, and γ = ∠ C2-C3-C4. The bond configuration of the central atom is tetrahedral, but the individual CMC bond angles as well as b - a and α are poorly determined because of their correlation with the conformations assumed in the analysis. Molecular parameters are consistent with those in the crystal phase.

The molecular structure of Ge(CH 3 ) 4 has been determined from gas-phase electron diffraction augmented by a normal coordinate analysis. Assuming tetrahedral symmetry for the germanium bond configuration, the following structural parameters are found: r g (GeC) = 1.958 ± 0.004 A, r g (CH) = 1.111 ± 0.003 A and ∠(GeCH) = 110.7 ± 0.2° ( R =4.0%). The methyl torsional barrier V 0 is estimated to be 1.3 kJ mol −1 on the basis of an effective angle of torsion 23.0 ± 1.5°, from the staggered form, yielded directly by the analysis. The GeC bond length of Ge(CH 3 ) 4 is the same, within experimental error, as that of Ge(C 6 H 5 ) 4 and is in agreement with the prediction of a modified Schomaker-Stevenson relationship.

1 Introduction 2 Cyclopropane 3 Substituted Cyclopropanes 4 Cyclopropylidenes 5 Cyclopropenes 6 Cyclopropenylidenes and Related Systems 7 Cyclopropane Rings in Polycyclic Systems 8 Acknowledgements Keywords: cyclopropane derivatives and structural chemistry; structure and electron-density distribution; vinyl and aryl cyclopropanes; carbonyl derivatives; nitrogen and phosphorus derivatives; cyclopropylidenes

The molecular geometries of 1,3-C 6 H 4 (SiMe 3 ) 2 ( 2 ) and 1,3,5,-C 6 H 3 (SiMe 3 ) 3 ( 3 ) have been determined by gas-phase electron diffraction. The mean CC bond lengths are slightly larger than those in benzene, and the endocyclic ipso bond angles C6C1C2 are considerably less than 120°, in accord with the electron-releasing inductive effect of the SiMe 3 substituent and the predictions of the VSEPR model. The SiC (methyl) bonds are longer than the SiC(aryl) bonds by about 0.02 A (see below). Only limited information concerning conformational preference could be obtained from the electron-diffraction data but this did not hinder the determination of the other structural characteristics. The following main bond length ( r g ) and angle parameters (with estimated total errors) have been obtained at nozzle temperatures of 86 and 105°C, respectively. For 2 : (CC) mean 1.405±0.003, (SiC) mean 1.87 9± 0.004, Δ(SiC)  SiC(methyl)  SiC(aryl) = 0.018 ± 0.008, SiC(methyl) 1.884 ± 0.004, SiC(aryl) 1.866 ± 0.007 A C6C1C2 116.2±0.6°, C4C5C6 119.5° (assumed), CSiC 109.5° (assumed); for 3 : CC 1.410±0.003, (SiC) mean 1.881 ± 0.004, Δ(SiC) 0.016±0.006, SiC(methyl) 1.885±0.004, SiC(aryl) 1.869±0.006 A, C6C1C2 117.0±0.6°, CSiC 109.5° (assumed). On the basis of available structural information on three trimethylsilyl-substituted benzene derivatives, and assuming additivity, the angular ring deformation impact of the trimethylsilyl substituent has been estimated. The predicted endocyclic angles for trimethylsilylbenzene are (starting from the ipso angle) 116.8, 122.2, 119.9, and 119.0°.

The molecular structure of Ge(CH 3 ) 4 has been determined from gas-phase electron diffraction augmented by a normal coordinate analysis. Assuming tetrahedral symmetry for the germanium bond configuration, the following structural parameters are found: r g (GeC) = 1.958 ± 0.004 A, r g (CH) = 1.111 ± 0.003 A and ∠(GeCH) = 110.7 ± 0.2° ( R =4.0%). The methyl torsional barrier V 0 is estimated to be 1.3 kJ mol −1 on the basis of an effective angle of torsion 23.0 ± 1.5°, from the staggered form, yielded directly by the analysis. The GeC bond length of Ge(CH 3 ) 4 is the same, within experimental error, as that of Ge(C 6 H 5 ) 4 and is in agreement with the prediction of a modified Schomaker-Stevenson relationship.

The structural parameters of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), (CH2NNO2)3, 1,3-dinitro-1,3-diazacyclopentane (DDCP), CH2(CH2NNO2)2, andN-nitropyrrolidine (NP), (CH2)4NNO2, have been determined by electron diffraction.

The structure of (E)- and (Z)-methylcyanovinyl sulphone has been determined by gas electron diffraction and by X-ray crystallography. Considerably widened bond angles in the Z-isomer indicate steric repulsions across the CC bond. The conformer occurring in the crystal and prevailing in the gas phase has nearly eclipsed CC and SO bonds and is stabilized by an intramolecular hydrogen bond in the E-isomer, and by an attractive interaction, O···C(N) 2.78–2.81 A, in the Z-isomer. Parameters (rg with estimated total errors) SO, SCmean, CC, CN and OSCvinyl, OSCmethyl, CSC, SCC, CCC, OSO in the free E-isomer: 1.437(3), 1.781(4), 1.344(5). 1.165(3) A and 105.2(5), 110.2(2), 102.3(9), 114.9(6), 121.5(7), 121.4(7)°; in the free Z-isomer: 1.437(3), 1.780(4), 1.344(5), 1.168(3) A and 109.6(3), 104.3(3), 108.4(5), 124.7(4), 127.3(7), 119.8(0)°. Crystal data: E-isomer: P1, a=5.798(1), b=6.185(1), c=8.959(2) A, α=102.83(2), β=108.74(2), γ=92.18(2)°, Z=2,ϱm=1.476(7),ϱc=1.480 g cm−3; Z-isomer; P21/c, a=10.613(4), b=16.066(4), c=8.630(2) A, α=90, β=122.14(2), γ=90°, Z=8, ϱm=1.395(4), ϱc=1.398 g cm−3. There is an O⋯H contact of 2.26 A between the two independent molecules in the asymmetric unit of the Z-isomer.

The structures of free Ph 4 M molecules (MSi, Ge, Sn) have been analysed by electron diffraction. Only a limited amount of reliable structural information could be determined since several models ( D 2d , S 4 , D 2 ) fit the experimental data equally well. The phenyl rings are slightly elongated. Assuming that b = c and γ = 120°, the r g parameters (with estimated total errors) have been obtained: 1 MC (CC) mean (CH) mean b  a α (A) (A) (A) (A) (deg.) Ph 4 Si 1.871(4) 1.403(3) 1.087(4) 0.005(5) 118.2(4) Ph 4 Ge 1.960(4) 1.401(3) 1.098(4) −0.012(6) 119.5(3) Ph 4 Sn 2.137(5) 1.401(3) 1.109(5) −0.037(8) 119.5(6) where a = r (C1-C2), b = r (C2-C3), c = r (C3-C4) α = ∠ C6-C1-C2, and γ = ∠ C2-C3-C4. The bond configuration of the central atom is tetrahedral, but the individual CMC bond angles as well as b - a and α are poorly determined because of their correlation with the conformations assumed in the analysis. Molecular parameters are consistent with those in the crystal phase.

An electron diffraction determination of the molecular geometry of hexamethyldisiloxane has removed much of the uncertainty concerning this structure. The length of the SiO bond and the SiOSi bond angle were determined to be 1.631 ± 0.003 A and 148 ± 3°, respectively. The experimental data are consistent with a staggered conformation ( C 2 v symmetry) while a model with twist angles around the SiO bonds of about 30° cannot be excluded. The molecule is probably performing large amplitude intramolecular motion.

The analysis of gas electron diffraction data yielded the bond lengths ( r g ) of diphenyl sulphone, (C 6 H 5 ) 2 SO 2 : CC 1.400(3) A, CH 1.100(11) A, SC 1.772(5) A and SO 1.440(5) A, and of diphenyl sulphoxide, (C 6 H 5 ) 2 SO: CC 1.398(3) A, CH 1.105(11) A, SC 1.804(6) A and SO 1.489(5) A. Limited information on the bond angles and conformation was obtained. The data on the stretching frequencies and lengths of SO bonds for sulphoxides are consistent with the relationship established for sulphones between [0.5 (ν 2 s + ν 2 as )] 0.5 and r (SO).

The molecular structure of ethylbenzene has been studied by gas electron diffraction. Experi-mental intensities and radial distribution are well reproduced by an average structure with r = 70(3)°, where r is the angle between the C—C—C(H3) plane and the ring plane, mean (rg) bond lengths (C-C)phenyi = 139.9(3) pm, Cphenyi-C(H2) = 152.4(9) pm, C(H2)-C(H3) = 153.5(12) pm, C—H = 109.4(4) pm, and bond angle C-C-C(HS) = 111.8(15)°. (Estimated total errors are parenthesized.) An also acceptable perpendicular model (r = 90°) is accompanied by very large vibrational amplitudes, while the coplanar conformation (r = 0°) is excluded. Mixtures of conformers fit the experimental data as well. The barrier to rotation of the phenyl group is estimated from the average structure to be about 7 kJ mol -1 . According to CNDO/2 calculations, only the perpendicular form is stable. The results of a geometry optimization are shown.

An electron diffraction analysis of the molecular structures of 1,1,1,3,3,3-hexachloro-1,3-disilapropane and octachloro-1,3-disilapropane has been carried out. Deviations from the staggered conformation are indicated. The data may be approximated by models with C 2 symmetry and a small tilt of the SiCl 3 groups. The main bond lengths ( r g ) and bond angles obtained for (SiCl 3 ) 2 CH 2 are: SiCl, 202.7(4); SiC, 186.6(6); CH, 109.8(24) pm, ClSiCl, 107.9(1); SiCSi, 118.3(7)°; and for (SiCl 3 ) 2 CCl 2 : SiCl, 202.0(4); SiC, 190.2(9); CCl, 179.6(9) pm; ClSiCl, 109.5(1); SiCSi, 120.6(9); ClCCl, 110.9(16); SiCCl, 106.3(3)°.

The molecular structure of free uracil has been determined by electron diffraction. Planar or nearly planar models fit the experimental data. Differences between the four CN bond lengths as well as those between the two CO bond lengths were assumed from ab initio (4–21) calculations in the electron diffraction analysis. The following bond lengths (rg) and bond angles (rα) were determined; the estimated total errors are parenthesized as expressed in units of the last digit: r(CN)mean 1.399(6), r(CC) 1.462(8), r(CC) 1.343(24), r(CO)mean 1.212(3) A, C2N1C6 123.2(12), N3C4C5 115.5(18), C4C5C6 119.7(21), C5C6N1 122.1(22), N1C2O8 123.8(14), C5C4O10 124.3(20)°. The structural parameters by electron diffraction are in agreement with those by quantum chemical optimizations (ab initio 4–21, MINDO/3). The bond lengths indicate partial delocalization in the ring.

A gas electron diffraction study yielded the following geometrical parameters for hexamethylcyclotrisilazane: r (Si-N) = 1.728 ± 0.004 A, r (Si-C) = 1.871 ± 0.004 A, r (C-H) = 1.124 ± 0.007 A, ∠N-Si-N = 108.4 ± 1.0°, ∠Si-N-Si = 126.8 ± 0.8°, ∠C-Si-C = 108.9 ± 2.3°, ∠H-C-H = 111.6 ± 0.9°. The (SiN) 3 ring was found to be puckered but the deviation from planarity is relatively small. Details of the ring shape could not be determined. The degree of ring puckering in six-membered rings with alternating atoms can be roughly predicted from the bond angles in analogous non-cyclic molecules.

The molecular geometry of gaseous triphenylsilane has been determined by electron diffraction. The silicon bond angles are ideal tetrahedral within experimental error. The benzene rings are slightly elongated in the direction of the SiC bond. The experimental data are consistent with a C3 model and a mean torsional angle of 37°, in agreement with published molecular mechanics calculations. The electron diffraction bond lengths (with estimated total errors), among them rg(SiC) 1.872(4) A and rg(CC mean) 1.403(3) A, refer to a well-defined nuclear configuration and differ from the analogous parameters from an X-ray crystallographic study of the same molecule.

Geometric and other relations of decorated leather bookbindings are analysed. Symmetry properties of the ornamentation, the symmetries of motifs and of the layout, and some correlations of symbols and ideas, furthermore the occurrence of one- and two-dimensional space groups and interlace designs are demonstrated by Romanesque and Gothic bindings, Hungarian, Italian, French and German Renaissance, as well as by Baroque and rococo bindings, and finally, by some pieces of modern bookbinding art.

Nearly regular tetrahedral silicon bond configuration and a considerably distorted ring characterize the p -bis(trimethylsilyl)benzene molecular geometry according to an electron diffraction study. The SiC methyl bond is longer than the SiC phenyl bond, in agreement with expectation but contrary to an X-ray diffraction determination. The extent of ring deformation is consistent with the electropositive character of the trimethylsilyl substituent and with the structural variations in other para -disubstituted benzene derivatives. The electron diffraction data are consistent with either free rotation around the SiC phenyl bonds or with a rotamer deviating by about 15° from the eclipsed form. The following bond lengths ( r g , pm) and bond angles (°) have been determined with parenthesized estimated total errors: (CC) mean 140.8(3), (C ipso )(C ortho C meta ) 1.6(7), (SiC) mean 188.0(4), (SiC methyl )(SiC phenyl ) 3.3(7), (CH) methyl 111.3(3), CC ipso C 115.7(6), and C phenyl SiC methyl 109.2(4).

The following parameters were obtained for a C 2V model of the bis(trimethylgermyl) ketene molecule in the vapour phase by electron diffraction: r (Ge-C) av. = 1.946±0.005 A, l (Ge-C) av. = 0.059 ±0.004 A, r (C-H) = 1.131 ±0.011 A, l (C-H) = 0.077±0.011 A, r (C-C)= 1.274±0.015 A, ∠Ge-C-Ge = 127.6±1.3°, ∠(H)C-Ge-C(H) = 108.1± 1.2°, l (Ge · Ge) = 0.090 ±0.010 A, l (Ge · H) = 0.162±0.017 A.

The gas-phase electron diffraction study of di-2-pyridyl sulphide has yielded rg(S-C) 1.786 ± 0.004 Å and ∢ C-S-C 104.4 ± 1.7°. The dimensions of the pyridyl groups were found to be consistent with those of free pyridine molecules. Models with various conformational properties could be used to approximate the experimental data.

An electron diffraction analysis of the molecular structures of 1,1,1,3,3,3-hexachloro-1,3-disilapropane and octachloro-1,3-disilapropane has been carried out. Deviations from the staggered conformation are indicated. The data may be approximated by models with C 2 symmetry and a small tilt of the SiCl 3 groups. The main bond lengths ( r g ) and bond angles obtained for (SiCl 3 ) 2 CH 2 are: SiCl, 202.7(4); SiC, 186.6(6); CH, 109.8(24) pm, ClSiCl, 107.9(1); SiCSi, 118.3(7)°; and for (SiCl 3 ) 2 CCl 2 : SiCl, 202.0(4); SiC, 190.2(9); CCl, 179.6(9) pm; ClSiCl, 109.5(1); SiCSi, 120.6(9); ClCCl, 110.9(16); SiCCl, 106.3(3)°.

Nearly regular tetrahedral silicon bond configuration and a considerably distorted ring characterize the p -bis(trimethylsilyl)benzene molecular geometry according to an electron diffraction study. The SiC methyl bond is longer than the SiC phenyl bond, in agreement with expectation but contrary to an X-ray diffraction determination. The extent of ring deformation is consistent with the electropositive character of the trimethylsilyl substituent and with the structural variations in other para -disubstituted benzene derivatives. The electron diffraction data are consistent with either free rotation around the SiC phenyl bonds or with a rotamer deviating by about 15° from the eclipsed form. The following bond lengths ( r g , pm) and bond angles (°) have been determined with parenthesized estimated total errors: (CC) mean 140.8(3), (C ipso )(C ortho C meta ) 1.6(7), (SiC) mean 188.0(4), (SiC methyl )(SiC phenyl ) 3.3(7), (CH) methyl 111.3(3), CC ipso C 115.7(6), and C phenyl SiC methyl 109.2(4).

An electron-diffraction structure analysis has yielded the following bond lengths (ra) for the title compound: SO 1.438(3), S–C 1.769(4), CC 1.332(4), and C–H 1.118(6)A. Of the bond angles, the following two are reliablydetermined: OSO 119.5(12) and S–CC121.5(3)°. CNDO/2 molecular-orbital calculations indicatethe coexistence of three conformers. At least two, possibly more, conformers are present in the vapour according to the electron-diffraction data, which also indicate that the CC bonds tend to eclipse the other bonds. Comparison of a number of bands in the i.r. and Raman spectra for different states of the compound shows the coexistence of at least two, possibly more, conformers in the liquid and in the supercooled liquid, while there is only one form in the crystalline phase. The SO bond lengths and stretching frequencies are consistent with empirical relations found for a relatively large series of sulphones.