Vibrational spectroscopy and DFT calculations of 1,​3-​dibromo-​2,​4,​6-​trimethylbenzene: Anharmonicity, coupling and methyl group tunneling

International audienceThe Raman, IR and INS spectra of 1,3-dibromo-2,4,6-trimethylbenzene (DBMH) were recorded in the 80-3200 cm-1 range. The molecular conformation and vibrational spectra of DBMH were computed at the MPW1PW91/LANL2DZ level. Except for the methyl 2 environment, the agreement between the DFT calculations and the neutron diffraction structure is almost perfect (deviations < 0.01 Å for bond lengths, < 0.2° for angles). The frequencies of the internal modes of vibration were calculated with the harmonic and anharmonic approximations; the later method yields results that are in remarkable agreement with the spectroscopic data, resulting in a confident assignment of the vibrational bands. Thus, no scaling is necessary. The coupling, in phase or anti-phase, of the motions of symmetrical C-Br and C-Me bonds is highlighted. Our DFT calculations suggest that the torsion of methyl groups 4 and 6 is hindered in deep wells, whereas methyl group 2 is a quasi-free rotor. The failure of the calculations to determine the frequencies of the methyl torsional modes is explained as follows: DFT does not consider the methyl spins and assumes localization of the protons, whereas the methyl groups must be treated as quantum rotors

Bis(4-acetyl­phen­yl) selenide

In the title compound, C16H14O2Se, the dihedral angle between the benzene rings is 87.08 (11)°. In the crystal, mol­ecules are linked into layers parallel to the bc plane by inter­molecular C—H⋯O hydrogen bonds

Vibrational Spectra of Triiodomesitylen: Combination of DFT Calculations and Experimental Studies. Effects of the Environment

International audienceA study of the internal vibrations of triiodomesitylene (TIM) is presented. It is known from X-rays diffraction at 293 K that the molecule has nearly D3h symmetry because of the large delocalization of the methyl protons. By using Raman and infrared spectra recorded at room temperature, a first assignment is done by comparing TIM vibrations with those of 1,3,5-triiodo- and 1,3,5-trimethyl-benzene. This assignment is supported by DFT calculations by using the MPW1PW91 functional with the LanL2DZ(d,p) basis set and assuming C3h symmetry. The agreement between the calculated and experimental frequencies is very good: always better than 97% for the observed skeletal vibrations. The calculations overestimate the methyl frequencies by 7%, and experiment shows only broad features for these excitations. Because a neutron diffraction study had established that the TIM conformation at 14 K is not exactly trigonal, new theoretical calculations were done with Cs symmetry. This shows that all previous E′ and E″ modes of vibration are split by 2−12 cm−1. This is confirmed by infrared, Raman, and inelastic neutron scattering spectra recorded below 10 K. Apart from two frequencies, all the TIM skeleton vibrations have been detected and assigned by using Cs symmetry. For the methyl vibrations, experiment has confirmed the splitting of the previously degenerate modes; only some small discrepancies remain in the assignment. This is partly due to the difference of the model conformation used in the calculations and the crystallographic one. All these results confirm that each of the three methyl groups has not only its own tunnel splitting but also a different specific spectroscopic behavior for all the molecular modes

Crystal structure of 1,3,5-trimethyl-2,4-dinitrobenzene

In the title compound, C9H10N2O4, the planes of the nitro groups subtend dihedral angles of 55.04 (15) and 63.23 (15)° with that of the aromatic ring. These tilts are in opposite senses and the molecule possesses approximate mirror symmetry about a plane normal to the molecule. In the crystal, molecules form stacks in the [100] direction with adjacent molecules related by translation, although the centroid–centroid separation of 4.136 (5) Å is probably too long to regard as a significant aromatic π–π stacking interaction. An extremely weak C—H...O interaction is also present

Crystal structure and Hirshfeld surface analysis of N-[(2-hydroxynaphthalen-1-yl)(3-methylphenyl)methyl]acetamide

The title compound, C20H19NO2, is of interest as a precursor to biologically active substituted quinolines and related compounds. This compound crystallizes with two independent molecules (A and B) in the asymmetric unit. The dihedral angles between mean planes of the methylphenyl ring and the naphthalene ring system are 78.32 (6) and 84.70 (6)° in molecules A and B, respectively. In the crystal, the antiferroelectric packing of molecules A and B is of an ABBAABB type along the b-axis direction. The crystal structure features N—H...O, O—H...O and weak C—H...O hydrogen bonds, which link the molecules into infinite chains propagating along the b-axis direction

Iododurene

The title compound (systematic name: 1-iodo-2,3,5,6-tetramethylbenzene), C10H13I, crystallizes in the chiral space group P212121. The I atom is displaced by 0.1003&amp;#8197;(5)&amp;#8197;&amp;#197; from the mean plane of the ten C atoms [maximum deviation = 0.018&amp;#8197;(6)&amp;#8197;&amp;#197;]. In the crystal, there are no significant intermolecular interactions present

Crystal structure of 1,1'-[selanediyl-bis(4,1-phenyl-ene)]bis-(2-chloro-ethan-1-one)

International audienceIn the title mol-ecule, C16H12Cl2O2Se, the C-Se-C angle is 100.05 (14)°, with the dihedral angle between the planes of the benzene rings being 69.92 (17)°. The average endocyclic angles (Se-Car-Car; ar = aromatic) facing the Se atom are 120.0 (3) and 119.4 (3)°. The Se atom is essentially coplanar with the benzene rings, with Se-Car-Car-Car torsion angles of -179.2 (3) and -179.7 (3)°. In the crystal, mol-ecules are linked via C-H⋯O hydrogen bonds forming chains propagating along the a-axis direction. The chains are linked via C-H⋯π inter-actions, forming a three-dimensional networ

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

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

2,6-Dichloro-4-nitrotoluene

The title compound, C7H5Cl2NO2 [systematic name: 1,3-dichloro-2-methyl-5-nitrobenzene], crystallizes in the chiral space group P212121 with a Flack parameter of −0.03 (5). The methyl C atom, the Cl atoms and the N atom of the nitro substituent all lie extremely close to the plane of the benzene ring; the deviations are 0.028 (3) Å for the methyl C atom, −0.016 (1) and 0.007 (1) Å for the two Cl atoms, and −0.017 (3) Å for the nitro N atom. Hence, no significant steric hindrance of the methyl group by the ortho halogen atoms is observed. The nitro group is inclined to the benzene ring by 9.8 (3)°. In the crystal, molecules are linked by weak C—H...O and C—H...Cl hydrogen bonds, forming layers parallel to the ab plane

Dibromonitrotoluene

The title compound, C7H5Br2NO2 (common name: dibromonitrotoluene; systematic name: 1,3-dibromo-2-methyl-5-nitrobenzene) crystallizes with two independent molecules (A and B) in the asymmetric unit. In molecule A, the Br atoms lie almost in the plane of the benzene ring, with deviations of 0.012 (1) and 0.009 (1) Å, while for the methyl C atom the deviation is 0.038 (4) Å. In molecule B, the opposite is observed; for the methyl C atom the deviation is 0.003 (4) Å, while the two Br atoms deviate by 0.032 (1) and 0.025 (1) Å. In the crystal, the B molecules are linked via C—H...Br hydrogen bonds, forming chains along [101]. The A molecules are also aligned along the same direction, and there is a short Br...O contact of 3.101 (4) Å involving the A and B molecules. The molecules stack in layers parallel to (101) and are linked by weak π–π interactions [intercentroid distances = 3.564 (3) Å between A molecules and 3.662 (3) Å between B molecules]