Mária Kolonits

Molecular structure and ring distortions of p-xylene as determined by electron diffraction

Abstract The molecular structure of p -xylene in the gas phase has been determined by electron diffraction analysis. Least-squares refinement of a model with D 2h symmetry for the benzene ring, and with allowance for possible shrinkage effects in the longest C⋯C distances, has led to the accurate measurement of the small changes in the ring geometry caused by the methyl substituents. The most appreciable distortion from the D 6h symmetry of unsubstituted benzene is a reduction from 120° of the internal angle at the ipso carbon. The value of 117.1 ± 0.3° obtained in the present analysis is in agreement with solid state results for toluene and a variety of its para -substituted derivatives. Also altered are the carboncarbon bond lengths of the ring; the bonds that originate from the ipso carbon are about 0.01 A longer than the central bonds ( r g = 1.405 ± 0.003 vs 1.392 ± 0.003 A). Other geometrical parameters for the D 2h model are: r g (CCH 3 ) = 1.512 ± 0.003 A, r g (CH) phenyl = 1.103 ± 0.010 A, r g (CH) methyl = 1.113 ± 0.005 A, ∠CCH phenyl = 120.8 ± 1.3°, ∠CCH methyl = 110.3 ± 0.8°. The results of the present analysis indicate that gas-phase electron diffraction is a suitable tool for measuring accurately the ring distortions occurring in symmetrically para -disubstituted benzene derivatives, a class of molecules not otherwise accessible to gas-phase structural studies.

Molecular structure of first-row transition metal dihalides from combined electron diffraction and vibrational spectroscopic analysis

The average and equilibrium molecular geometries and vibrational characteristics of MnCl2, FeCl2, CoCl2, NiCl2, MnBr2, FeBr2, CoBr2, and NiBr2 have been determined by a combined analysis of gas‐phase electron diffraction and vibrational spectroscopic data. The nozzle temperatures ranged between 960 and 1100 K in the electron diffraction experiments, paralleled with mass spectrometric control of the vapor composition. Four approximations have been examined for the molecular Hamiltonian in the joint analysis. The overall utility of combined analysis has been demonstrated. The dynamic behavior of the first‐row transition metal dihalides is best described by the semirigid model. The equilibrium bond distance is best approximated by the cubic anharmonic potential. The cubic force constants and the bond Morse anharmonicity parameter can be determined even from electron diffraction data alone provided that experimental information is collected to large enough scattering angles. The present analysis has confirmed...

The gas-phase molecular structure of 1-fluorosilatrane from electron diffraction

The molecular geometry of 1-fluorosilatrane has been determined by gas-phase electron diffraction. The distance between the nitrogen and silicon atoms is much longer in the gas phase, viz., 2.324±0.014 Å, than in the crystal, 2.042 (1) Å [5]. This indicates a weakened donor-acceptor interaction possibly as a consequence of the absence of intermolecular interactions in the gas phase. The five-membered rings take envelope conformations with the carbon atoms adjacent to nitrogen at the envelope tips. The following bond distances (гg, Å) and bond angles (°) were obtained with their estimated total errors: N-C, 1.481±0.008; C-C, 1.514±0.011; O-C, 1.392±0.004; Si-O, 1.652±0.003; Si-F, 1.568±0.006; C-H, 1.118±0.005; N-C-C, 104.5±0.6; C-C-O, 117.0±0.7;C-O-Si, 123.7±0.6; O-Si-F, 98.7±0.3; O-Si-O, 117.8±0.1; C-N-C, 115.0±0.3.

Tetrathiafulvalene: gas-phase molecular structure from electron diffraction

Abstract The gas-phase molecular structure of 2,2-bi-1,3-dithiole (tetrathiafulvalene) was determined by electron diffraction and compared with the crystal molecular structure. A nonplanar “boat” structure was found to give the best agreement with experimental electron diffraction results. The electron diffraction results were found to be consistent overall with those from X-ray crystallographic studies.

The molecular geometry of iron trifluoride from electron diffraction and a reinvestigation of aluminum trifluoride

The molecular geometry of iron trifluoride has been determined at 1260 K by gas-phase electron diffraction. Use of a platinum envelope during the experiment prevented the iron trifluoride sample from partial reduction otherwise observed at high temperatures. The molecular geometry of aluminum trifluoride has been reinvestigated at 1300 K. The electron diffraction results for both AlF3 and FeF3 are compatible with planar bond configuration (D3hsymmetry) with bond lengths (rg): Al-F 1.630±0.003 Å and Fe-F 1.763±0.004 Å. Experimental vibrational frequencies support the notion of planarity for aluminum trifluoride. There is no such additional spectroscopic evidence available for iron trifluoride.

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.

On the effect of 4f electrons on the structural characteristics of lanthanide trihalides: computational and electron diffraction study of dysprosium trichloride.

The molecular and electronic structure of dysprosium trichloride, DyCl(3), was calculated by high-level quantum chemical methods in order to learn about the effect of the partially filled 4f subshell and of the possible spin-orbit coupling on them. High-temperature electron diffraction studies of DyCl(3) were also carried out so that we could compare the computed geometry with the experimental one, after thermal corrections on the latter. Dysprosium monochloride, DyCl, and the dimer of dysprosium trichloride, Dy(2)Cl(6), were also investigated by computation. We found that the electron configuration of the 4f subshell does not influence the geometry of the trichloride monomer molecule as the ground state and first excited state molecules have the same geometry. Nonetheless, taking the 4f electrons into account in the calculation, together with the 5s and 5p electrons, is important in order to get geometrical parameters consistent with the results from experiment. Based on electron diffraction and different levels of computation, the suggested equilibrium bond length (r(e)) of DyCl(3) is 2.443(14) A, while the thermal average distance (r(g)) from electron diffraction is 2.459(11) A. The molecule is trigonal planar in equilibrium. Although the ground electronic state splits due to spin-orbit coupling, the lowering of the total electronic energy is very small (about 0.025 hartree) and the geometrical parameters are not affected. In contrast with the monomeric trichloride molecule, the bond angles of the dimer seem to be different for different electronic states, indicating the influence of the 4f electronic configuration on their structure. We carried out an anharmonic analysis of the out-of-plane vibration of the trichloride monomer and found that the vibration is considerably anharmonic at 39.5 cm(-1), compared with the 30.5 cm(-1) harmonic value.

Electron diffraction and infrared spectroscopic study of the molecular structure of furan-2-aldehyde and 2-furanmethanethiol

Abstract Electron diffraction and IR spectroscopic studies of furan-2-aldehyde C 4 H 4 OCHO and 2-furanmethanethiol C 4 H 3 OCH 2 SH, have been carried out to obtain information on the conformational properties and geometry of these molecules. The electron diffraction data for both molecules were consistent with the assumption that the geometry of the ring skeleton and the hydrogen atoms attached to it were based on the furan structure as determined by microwave spectroscopy. The electron diffraction study ot furan-2-aldehyde confirmed the existence of two conformers with trans and cis oxygen atoms in the ring and aldehyde group. In the gas phase the trans conformer was found to be more stable, having less energy by an amount 0.5(4) kcal mol -1 . It was shown that the 2-furanmethanethiol molecule takes a non-planar conformation with dihedral angle O 1 C 2 C 6 S 7 = 39(4)° (0° value of the dihedral angle corresponds to the trans position of the O 1 -C 2 and C 6 -S 7 bonds, c.f. Fig. 4). The following values were obtained for bond lengths and bond angles characterizing the -CHO and -CH 2 SH groups in furan-2-aldehyde and 2-furanmethanethiol molecules, respectively (cf. Figs. 3 and 4) C 4 H 3 OCHO C 4 H 3 OCH 2 SH r a (C 2 -C 6 ) = 1.453(7) A r a (C 2 -C 6 ) = 1.481(15) A r a (C 6 O 7 ) = 1.212(4)A r a (C 2 -S 7 ) = 1.834(5) A ∠O 1 C 2 C 5 =117.6(4)° r a (C 6 -H) = 1.106(17) A ∠C 2 C 6 O 7 = 122.7(8)° ∠O 1 C 2 C 6 = 114.4(11)° ∠C 2 C 6 S 4 = 112.5(8)° The IR spectra of fuyan-2-aldehyde were measured in liquid and solid phases as well as in non-polar solvents at different temperatures between 308 and 210 K. Eight band pairs were identified which are dependent on the conformational equilibrium ; the band components are assigned to the trans or cis form. The energy difference determined from the 1470 cm −1 band pair is 0.51(4) kcal mol −1 . Evidence is given concerning the role of Fermi resonance in the splitting of the carbonyl stretching band. The IR spectra of 2-furanmethanethiol in the vapour and liquid phases measured between 233 and 323 K show the existence of only one conformer of the free molecule. The frequency of the symmetric methylene stretching vibration indicates a smaller hyperconjugative effect in 2-furanmethanethiol than in benzenemethanethiol. CNDO/2 and INDO calculations were performed for both rotamers of furan-2-aldehyde. The geometry was optimized in the CNDO/2 calculation starting with both microwave and electron diffraction data. The calculated energy difference of the conformers and their dipole moments are in good agreement with the experimental data.

MOLECULAR STRUCTURE OF CEI3 FROM GAS-PHASE ELECTRON DIFFRACTION AND VIBRATIONAL SPECTROSCOPY

Abstract CeI3 was investigated by electron diffraction and infrared spectroscopy, both in the gas phase, at high temperature (around 1270 K). The geometrical parameters are: r g (Ce-I) 2.948 ± 0.009 A and r g (I … I) 4.943 ± 0.032 A . One absorption band was detected in the stretching region of the spectrum and we have assigned it to the asymmetric stretch, v3 = 191 ± 10 cm−1. The unusually large uncertainty is due to the large population of excited vibrational and rotational levels. From the very low intensity band at the lower detection limit of the spectrometer, we can conclude that neither v2 nor v4 is above 35 cm−1. From the analysis of electron diffraction data and vibrational spectra we conclude that the molecule is very likely planar or at most slightly pyramidal (quasiplanar).

The molecular structure of selenium oxychloride as studied by electron diffraction

Abstract The molecular geometry of selenium oxychloride has been studied by electron diffraction. The internuclear distances (in terms of ra) are: r(Se-O) 1.612 ± 0.005 A, r(Se-Cl) 2.204 ± 0.005 A, r(Cl β O) 3.064 ± 0.012 A, r(Cl β Cl) 3.295 ± 0.016 A. The bond angles are ∠Cl-Se-O 105.8 ± 0.7° and ∠ Cl-Se-Cl 96.8 ± 0.7°. The structural parameters of three simple selenium-oxygen compounds are compared with those of their sulphur analogs in terms of the valence shell electron pair repulsion model.