Béla Rozsondai

The molecular structure of uracil: an electron diffraction study

Abstract 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.

An electron diffraction study of the molecular structure of hexamethyldisiloxane

Abstract 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 molecular geometries of some cyclic nitramines in the gas phase

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 six-membered ring of RDX has a chair form with axial positions of the nitro groups and close to planar bond geometry of the amine nitrogen atoms. The overallC3 symmetry of the molecule is in agreement with the experimental data.The conformation of the five-membered ring in DDCP is a half-chair ofC2 symmetry, while that in NP is an envelope ofCS symmetry. The nitro groups are in equatorial positions in both molecules. The conformations of pyrrolidine and imidazolidine cycles show interesting features.The pyramidal geometry of the amine nitrogen atom bonds flattens in going from pyrrolidine andN-chloropyrrolidine to NP and DDCP and then to RDX and to dimethylnitramine (DMNA), (CH3)2NNO2.

Gas electron diffraction study of the molecular structure of diphenyl sulphone and diphenyl sulphoxide

Abstract 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 p-bis(trimethylsilyl)-benzene from gas phase electron diffraction

Abstract 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).

Molecular structure and intramolecular motion of hexamethyldisiloxane from gas-phase electron diffraction

Abstract 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°.

Molecular structure of (E)- and (Z)-methylcyanovinyl sulphone in the gas phase and in crystal☆

Abstract 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.

Electron diffraction study of the molecular structure of bis(trimethylgermyl)ketene

Abstract 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 molecular structure of triphenylsilane from gas-phase electron diffraction

Abstract 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.

Electron diffraction study on the molecular structure of hexamethylcyclotrisilazane, [(CH3)2SiNH]3

Abstract 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.