György Schultz

Molecular structure of nitrobenzene in the planar and orthogonal conformations - A concerted study by electron diffraction, X-ray crystallography, and molecular orbital calculations

The molecular structure and ring distortions of nitrobenzene have been determined by gas-phase electron diffraction and ab initio molecular orbital (MO) calculations as well as from the structures of six derivatives studied by X-ray crystallography. The experimental value of the ring angle at the ipso position isα = 123.4 ± 0.3° in the free molecule; this is about 1.5° less than the hitherto reported values. Regression analysis of the ring angles in the six derivatives studied by X-ray crystallography yieldsα = 122.7(1)° for nitrobenzene in a crystalline environment. The small difference in the two values of a is interpreted as an effect of intermolecular interactions in the crystal. The value produced by the MO calculations,α = 122.3° at the 6–31G* (5D) level, is smaller than either of the experimental results. As regards the ring angles at the meta and para positions, the three techniques of structure determination consistently indicate that these are larger than 120° by a few tenths of a degree. Other important geometrical parameters from the electron diffraction study are 〈rg(C-C)〉 = 1.399 ± 0.003 Å,rg(C-N) = 1.486 ± 0.004 Å,rg(N-O) = 1.223 ± 0.003 Å, and A sO-N-O = 125.3 ± 0.2°. X-ray diffraction experiments on 3,5-dimethyl-4-nitrobenzoic acid and 3,5-dimethylbenzoic acid and ab initio MO calculations provide solid evidence that the geometry of nitrobenzene is little affected when the nitrogroup is twisted by 90° out of the planar equilibrium conformation. This indicates that the extent of π-electron transfer from the benzene ring to the nitro group is small. The barrier to rotation is estimated to be 17 ± 4 kJ mol−1 from the electron diffraction data.

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 and ring distortions of fluorobenzene: an electron diffraction study, and a comparison with other experimental and ab initio MO results

Abstract The molecular structure of fluorobenzene has been investigated by electron diffraction. Least-squares refinement of a model with C2v symmetry has led to an accurate determination of the angular distortions of the benzene ring caused by the fluorine atom. The values of the ring angles are in excellent agreement with those obtained by microwave spectroscopy. X-ray crystallography, and NMR spectroscopy in liquid-crystal solutions. Particularly well determined is the angle at the ipso atom, α = 123.4 ± 0.2°. Also the length of the CF bond rg(CF) = 1.356 ± 0.004 A, agrees closely with previous results. Comparison of the experimental geometries of fluorobenzene, 1,3-difluorobenzene and 1,3,5-trifluorobenzene with those obtained by ab initio MO calculations (Pulays gradient method) shows that the calculations underestimate the angular distortion of the ipso region of the benzene ring.

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

Abstract The molecular structure of p -dichlorobenzene in the vapour phase has been studied by electron diffraction. Least-squares refinement of a model with D 2h symmetry has led to the accurate determination of the small deviations of the benzene ring from D 6h symmetry caused by the chlorine substituents. The most appreciable effect is an increase from 120° to 121.6 ± 0.2° of the internal angle at the ipso carbon, associated with a shortening of the distance between the two ipso atoms. A less pronounced effect is a shortening of the C-C bonds that originate from the ipso carbons as compared to the central C-C bonds ( r g = 139.0 ± 0.3 pm vs. 139.5 ± 0.4 pm). Other bond distances are r g (C-Cl) - 173.0 ± 0.4 pm and r g (C-H) = 109.4 ± 1.0 pm. The observed ring distortions are in agreement with those obtained by low-temperature X-ray crystallography on three different crystal phases of p -dichlorobenzene. They are also consistent with those obtained for chloro-benzene by gas-phase electron diffraction and by NMR spectroscopy in a nematic solvent. The r s structure of chlorobenzene obtained in a recent study by micro-wave spectroscopy is shown to need revision, as far as the ipso region of the ring is concerned.

Molecular structure and ring distortions of cyanobenzene: an electron diffraction study

Abstract The molecular structure of cyanobenzene has been investigated by gas-phase electron diffraction. Least-squares refinement of a model with C 2v symmetry leads to an accurate determination of the angular distortion of the ring at the place of substitution. The value of the internal angle at the ipso carbon, α = 121.9 ± 0.3°, is in excellent agreement with that obtained by microwave spectroscopy. The smaller value obtained by X-ray crystallography from several para -substituted cyanobenzenes, α = 121.1(2)°, probably reflects a change in the electronic properties of the cyano group, caused by strong intermolecular interactions in the solid state. Comparison of the experimental geometry of the ring in the free molecule with that obtained by ab initio MO calculations at the 6-31G and 6-31G** levels indicates that the calculations do not reproduce the angular distortion of the ipso region. Other important parameters from the present study are: 〈 r g (CC)〉 = 1.400 ± 0.003 A; r g (CCN) = 1.438 ± 0.005 A; r g (CN) = 1.168 ± 0.003 A.

Molecular structure and ring distortion of phenol. An electron diffraction study

Abstract The molecular structure of gaseous phenol has been determined by electron diffraction. This has led to an accurate measurement of the internal ring angle at the place of substitution, α = 121.6° ± 0.2°, an important geometrical parameter that is hard to determine accurately by microwave spectroscopy or ab initio MO calculations. Comparison with solid state results suggests that the angle α decreases by about 1° in going from the free molecule to the crystal. This may reflect a change in the electronic properties of the OH substituent, caused by intermolecular hydrogen bonding in the solid state.

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 ethynylbenzene from electron diffraction and ab initio molecular orbital calculations

The molecular structure and benzene ring distortions of ethynylbenzene have been investigated by gas-phase electron diffraction and ab initio MO calculations at the HF/6-31G* and 6-3G** levels. Least-squares refinement of a model withC2v, symmetry, with constraints from the MO calculations, yielded the following important bond distances and angles:rg(Ci-Co)=1.407±0.003 Å,rg(Co-Cm)=1.397±0.003 Å,rg(Cm-Cp)=1.400±0.003 Å,rg(Cri-CCH)=1.436 ±0.004 Å,rg(C=C)=1.205±0.005 Å, ∠Co-Ci-Co=119.8±0.4°. The deformation of the benzene ring of ethynylbenzene given by the MO calculations, including <Co-Ci-Co=119.4°, is insensitive to the basis set used and agrees with that obtained by low-temperature X-ray crystallography for the phenylethynyl fragment, C6H5-C≡C-, in two different crystal environments. The partial substitution structure of ethynylbenzene from microwave spectroscopy is shown to be inaccurate in the ipso region of the benzene ring.

An electron diffraction study of the 1,4-thioxane molecular geometry

Abstract The following parameters have been obtained for the 1,4-thioxane ring geometry by means of a gas electron diffraction study: S-C 1.826 ± 0.004 A, C-C 1.521 ± 0.006 A, O-C 1.418 ± 0.004 A, C-S-C 97.1 ± 2.0°, C-C-S 111.4 ± 1.0°, C-C-0 113.2 ± 1.7° and C-O-C 115.1 ± 2.2°. The molecule takes a chair conformation. The ring geometry is consistent with an average puckering angle of 58.3°.

The Structure of Gaseous Carbon Tetraiodide from Electron Diffraction and All Carbon Iodides, CIn (n = 1–4), and Their Dimers, C2I2n (n = 1–3) from High-Level Computation. Any Other Carbon-Iodide Species in the Vapor?

The geometry of a series of carbon iodides have been determined, CI4 by gas-phase electron diffraction and CIn (n = 1–4) and C2I2n (n = 1–3) by high-level quantum chemical calulations. The bond length of the tetrahedral CI4 molecule from electron diffraction is (rg):2.157(10) Å. The indication of about 20% I2 in the vapor suggests partial decomposition and it has been thoroughly investigated what other carbon iodide species might be present beside CI4. There is no appreciable amount of either of the dimeric species in the vapor phase, in spite of the suggestion from thermodynamics. On the other hand, the electron diffraction data are compatible with the presence of about 18% of either of the monomeric free radicals, CI3 or CI2, beside CI4 and I2. Possible reasons for these observations are discussed. Our correlated level computations, in agreement with other high level computations, found the singlet 1A1 state to be the ground state for CI2. This is in contrast with a recent photoelectron spectroscopic study according to which the triplet state is the ground state though with a large margin of error (1 ± 3 kcal/mol energy difference). The computed singlet-triplet separation strongly depends on the level of the computation, but it is at least 9 kcal/mol. Geometrical parameters, singlet-triplet separations, and dipole moments have been calculated for the CX2 series (X = F, Cl, Br, I, H) and their variations are discussed. The thermodynamic stability of different carbon iodide species has also been investigated.