Jean Meinnel

Reassessment of Methyl Rotation Barriers and Conformations by Correlated Quantum Chemistry Methods

Internal rotations of the methyl group in ortho‐substituted and 2,6‐disubstituted toluenes in their ground state have been investigated by means of various ab initio quantum chemistry methods. Computed barriers at the Hartree‐Fock (HF) level using medium sized basis sets agreed reasonably with experimental results in the case of the studied ortho‐substituted toluenes. However, this agreement worsens when using very large basis sets. Furthermore, the determination of the conformation and barriers of more weakly hindered methyl groups, that is, for 2,6‐dihalogenotoluenes or toluene itself, necessitates high level correlated computations, because of a possible failure of HF calculations in this case. Density functional theory (DFT) techniques required, in several cases, much more extended basis sets than the post‐HF Møller‐Plesset perturbation (MP2, MP4) ones, to insure the convergence of the computed barriers. Non‐negligible variations of the computed barriers when using different DFT functionals are observed for some systems.

Molecular conformation and methyl proton delocalization in triiodomesitylene: A combined density functional theory and single-crystal neutron diffraction study

First the conformations of various ortho and di-ortho substituted toluenes calculated by quantum chemistry methods are discussed as well as the hindering potentials deduced from the latter results and those established experimentally by microwaves and fluorescence techniques. It appears that methyl (Me) groups are much less hindered in di-ortho than in ortho substituted compounds. Then the study of the 1,3,5-triiodo-2,4,6-trimethylbenzene (triiodomesitylene or TIM) is reported. Density functional theory (DFT) calculations indicate that two conformations of the TIM molecule have the same formation energies. One has C3h symmetry, the other with the Cs symmetry is obtained from the C3h by a rotation of 60° for one Me. Experimentally, the TIM structure has been determined at 15 K using single-crystal neutron diffraction data. TIM crystallizes in the triclinic space group P−1. Molecules are stacked in an antiferroelectric manner along the oblique a axis. For two Me groups the experimental conformation is close...

1,3,5-Triiodo-2,4,6-trimethylbenzene at 293 K.

In the structure of triiodomesitylene (1,3,5-triiodo-2,4,6-trimethylbenzene), C(9)H(9)I(3), at 293 K, the benzene ring is found to be significantly distorted from ideal D(6h) symmetry; the average endocyclic angles facing the I atoms and the methyl groups are 123.8 (3) and 116.2 (3) degrees, respectively. The angle between the normal to the molecular plane and the normal to the (100) plane is 5.1 degrees. No disorder was detected at 293 K. The thermal motion was investigated by a rigid-body motion tensor analysis. Intra- and intermolecular contacts are described and topological differences compared with the isomorphous compounds trichloromesitylene and tribromomesitylene are discussed.

4-dimethylamino-beta-nitrostyrene and 4-dimethylamino-beta-ethyl-beta-nitrostyrene at 100 K.

The structures of 4-dimethylamino-beta-nitrostyrene (DANS), C10H12N2O2, and 4-dimethylamino-beta-ethyl-beta-nitrostyrene (DAENS), C12H16N2O2, have been solved at T=100 K. The structure solution for DANS was complicated by the presence of a static disorder, characterized by a misorientation of 17% of the molecules. The molecule of DANS is almost planar, indicating significant conjugation, with a push-pull effect through the styrene skeleton extending up to the terminal substituents and enhancing the dipole moment. As a consequence of this conjugation, the hexagonal ring displays a quinoidal character; the lengths of the C-N [1.3595 (15) A] and C-C [1.448 (2) A] bonds adjacent to the benzene ring are shorter than single bonds. The molecules are stacked in dimers with antiparallel dipoles. In contrast, the molecule of DAENS is not planar. The ethyl substituent pushes the nitropropene group out of the benzene plane, with a torsion angle of -21.9 (3). Nevertheless, the molecule remains conjugated, with a shortening of the same bonds as in DANS.

3,5-Di­bromo-4-methyl­pyridine

The title compound, C6H5Br2N, lies on a mirror plane. In the crystal, molxadecules are linked by Br⋯N and Br⋯Br interxadactions, forming zigzag chains along [010]. The chains are linked by offset π–π interxadactions [interxadcentroid distance = 3.5451u2005(3)u2005A], forming a three-dimensional framework.

Crystal structure of 4,6-di-chloro-5-methyl-pyrimidine.

The title compound, C5H4Cl2N2, is essentially planar with an r.m.s. deviation for all non-H atoms of 0.009u2005Å. The largest deviation from the mean plane is 0.016u2005(4)u2005Å for an N atom. In the crystal, molecules are linked by pairs of C—H⋯N hydrogen bonds, forming inversion dimers, enclosing an R 2 2(6) ring motif.

The Crystal Structure of Dibromonitrotoluen (DBNT) C7Br2H5 NO2

The crystal structure of Dibromonitrotoluen (DBNT) obtained at the ambient temperature 293k from the X-ray diffraction crystallizes in the space group P-1 with 2 molecules by mesh. The crystal growth is made along the a axis. In parallel with this study, a theoretical calculation of molecular conformation from the density functional theory (DFT) carried out using the Gaussian03 chain program in the case of isolated molecule, led to results of Optimization very close to the experiment. The molecular conformation calculations were made from two different functional the B3LYP and MPW1PW91. The values of bond lengths obtained from the functional MPW1PW91 and base set 6-311G are very close to the experiment with a gap of 1.25% (2.14% for the B3LYP) while for bond angles, calculation results are better for functional B3LYP (Lan2DZ) 0.95% and 1.02% for MPW1PW91.In this work a study of internal modes of Dibromonitrotoluen (DBNT) is presented, while comparing the infrared spectroscopic experimental results and the DFT studies of the isolated molecule. This material is a good probe to test the model precision and the calculated methods used to interpret dynamic properties by experimental spectroscopy.

4‐Dimethyl­amino‐β‐nitro­styrene and 4‐dimethyl­amino‐β‐ethyl‐β‐nitro­styrene at 100 K

The structures of 4-dimethylamino-beta-nitrostyrene (DANS), C10H12N2O2, and 4-dimethylamino-beta-ethyl-beta-nitrostyrene (DAENS), C12H16N2O2, have been solved at T=100 K. The structure solution for DANS was complicated by the presence of a static disorder, characterized by a misorientation of 17% of the molecules. The molecule of DANS is almost planar, indicating significant conjugation, with a push-pull effect through the styrene skeleton extending up to the terminal substituents and enhancing the dipole moment. As a consequence of this conjugation, the hexagonal ring displays a quinoidal character; the lengths of the C-N [1.3595 (15) A] and C-C [1.448 (2) A] bonds adjacent to the benzene ring are shorter than single bonds. The molecules are stacked in dimers with antiparallel dipoles. In contrast, the molecule of DAENS is not planar. The ethyl substituent pushes the nitropropene group out of the benzene plane, with a torsion angle of -21.9 (3). Nevertheless, the molecule remains conjugated, with a shortening of the same bonds as in DANS.