Calculations of the molecular interactions in 1,3-dibromo-2,4,6-trimethyl-benzene: which methyl groups are quasi-free rotors in the crystal?

Dibromomesitylene (DBMH) is one of the few molecules in which a methyl group is a quasi-free rotor in the crystal state. Density functional theory calculations - using the Born-Oppenheimer approximation (BOa) - indicate that in isolated DBMH, Me4 and Me6 are highly hindered in a 3-fold potential V3 > 55 meV while Me2 symmetrically located between two Br atoms has a small 6-fold rotation hindering potential: V6 ∼ 8 meV. Inelastic neutron scattering studies have shown that this is also true in the crystal, the Me2 tunneling gap being 390 µeV at 4.2 K and V6 ∼ 18 meV. In the monoclinic DBMH crystal, molecules are packed in an anti-ferroelectric manner along the oblique a axis, favoring strong van der Waals interactions, while in the corrugated bc planes each molecule has a quasi hexagonal environment and weaker interactions. This results in the nearby environment of Me2 only being composed of hydrogen atoms. This explains why the Me2 rotation barrier remains small in the crystal and mainly 6-fold. Using the same potentials in the Schrödinger equation for a -CD3 rotor has allowed predicting a tunneling gap of 69 µeV for deuterated Me2 in very good agreement with inelastic neutron scattering measurements. Therefore, because of a rare and unexpected local symmetry in the crystal, the Me2 rotation barrier remains small and 6-fold and hydrogen nuclei are highly delocalized and not relevant to the Born-Oppenheimer approximation. This and the neglect of spin states explain the failure of density functional theory calculations for finding the rotation energy levels of Me2.

Crystal structure and Hirshfeld surface analysis of 2,6-di-iodo-4-nitro-toluene and 2,4,6-tri-bromo-toluene.

The title compounds, 2,6-di-iodo-4-nitro-toluene (DINT, C7H5I2NO2) and 2,4,6-tri-bromo-toluene (TBT, C7H5Br3,), are tris-ubstituted toluene mol-ecules. Both mol-ecules are planar, only the H atoms of the methyl group, and the nitro group in DINT, deviate significantly from the plane of the benzene ring. In the crystals of both compounds, mol-ecules stack in columns up the shortest crystallographic axis, viz. the a axis in DINT and the b axis in TBT. In the crystal of DINT, mol-ecules are linked via short N-O⋯I contacts, forming chains along [100]. In TBT, mol-ecules are linked by C-H⋯Br hydrogen bonds, forming chains along [010]. Hirshfeld surface analysis was used to explore the inter-molecular contacts in the crystals of both DINT and TBT.

Anharmonic Computations Meet Experiments (IR, Raman, Neutron Diffraction) for Explaining the Behavior of 1,3,5-Tribromo-2,4,6-trimethylbenzene.

In the present paper we first show the experimental Raman, infrared, and neutron INS spectra of tribromomesitylene (TBM) measured in the range 50-3200 cm(-1) using crystalline powders at 6 or 4 K. Then, the bond lengths and angles determined by neutron diffraction using a TBM single crystal at 14 K are compared to the computed ones at different levels of theory. Anharmonic computations were then performed on the relaxed structure using the VPT2 approach, and for the lowest normal modes, the HRAO model has led to a remarkable agreement for the assignment of the experimental signatures. A particularity appears for frequencies below 150 cm(-1), and in particular for those concerning the energy levels of "hindered rotation" of the three methyl groups, they must be calculated for one-dimensional symmetrical tops independent of the frame vibrations. This fact is consistent with the structure established by neutron diffraction: the protons of the methyl groups undergoing huge "libration" motions are widely spread in space. The values of the transitions between the librational levels determined by inelastic neutron scattering indicate that the hindering potentials are mainly due to intermolecular interactions different for each methyl group in the triclinic cell.

Crystal structure of 1,3,5-trimethyl-2,4-di-nitro-benzene.

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 mol-ecule possesses approximate mirror symmetry about a plane normal to the mol-ecule. In the crystal, mol-ecules form stacks in the [100] direction with adjacent mol-ecules related by translation, although the centroid-centroid separation of 4.136 (5) Šis probably too long to regard as a significant aromatic π-π stacking inter-action. An extremely weak C-H⋯O inter-action is also present.

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 0.009 Å. The largest deviation from the mean plane is 0.016 (4) Šfor 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 motif.

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

In 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 network.

Iodo-durene.

THE TITLE COMPOUND (SYSTEMATIC NAME: 1-iodo-2,3,5,6-tetra-methyl-benzene), C10H13I, crystallizes in the chiral space group P212121. The I atom is displaced by 0.1003 (5) Šfrom the mean plane of the ten C atoms [maximum deviation = 0.018 (6) Å]. In the crystal, there are no significant inter-molecular inter-actions present.

Bis(4-acetyl-phen-yl) selenide.

In the title compound, C(16)H(14)O(2)Se, the dihedral angle between the benzene rings is 87.08 (11)°. In the crystal, mol-ecules are linked into layers parallel to the bc plane by inter-molecular C-H⋯O hydrogen bonds.

Rotationally disordered phase of 1,3-dibromo-5-iodo-2,4,6-trimethylbenzene at 293 K.

In the crystal state at room temperature, the molecule of dibromoiodomesitylene (1,3-dibromo-5-iodo-2,4,6-trimethylbenzene), C9H9Br2I, is prone to strong disorder, apparently involving only the three halogen sites (occupied identically by 66.7% Br and 33.3% I). This disorder, of the rotational type according to previously published NMR measurements, corresponds to fast 2pi/3 stochastic in-plane reorientations of the whole molecule between three discernable locations. This kind of rotational disorder can be revealed for the first time by diffractometry thanks to the C2v idealized molecular symmetry of the title compound, although it has been indirectly suspected at room temperature in other trihalogenomesitylenes of similar crystal packing but of D3h molecular symmetry. The average endocyclic angles facing the Br/I sites and the methyl groups are 124.14 (6) and 115.85 (2) degrees, respectively. The angle between the normal to the aromatic ring and the normal to the (100) plane is 4.1 degrees. TLS analysis indicates that only the aromatic ring and the methyl groups behave as a rigid body with respect to the thermal librations.

Vibrational spectra of triiodomesitylene: combination of DFT calculations and experimental studies. Effects of the environment.

A 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 D(3h) 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 C(3h) 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 C(s) 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 C(s) 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.

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.

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.

The low-temperature phase of 1,3-dibromo-2,4,6-trimethylbenzene: a single-crystal neutron diffraction study at 120 and 14 K.

In the low-temperature phase of dibromomesitylene (1,3-dibromo-2,4,6-trimethylbenzene), C(9)H(10)Br(2), the molecule deviates significantly from the C(3h) molecular symmetry encountered in tribromomesitylene (1,3,5-tribromo-2,4,6-trimethylbenzene), even for the endocyclic bond angles. An apparent C(2v) molecular symmetry is observed. The angle between the normal to the molecular plane and the normal to the (100) plane is approximately 20 degrees. The overall displacement was analysed at 120 K with rigid-body-motion tensor analysis. The methyl group located intermediate between the two Br atoms is rotationally disordered at both temperatures. This disorder was treated using two different approaches at 14 K, viz. the conventional split-atom model and a model using the special annular shapes of the atomic displacement parameters that are available in CRYSTALS [Watkin, Prout, Carruthers & Betteridge (1999). Issue 11. Chemical Crystallography Laboratory, Oxford, England], but only through the latter approach at 120 K. The disorder locally breaks the C(2v) molecular symmetry at 14 K only. Intra- and intermolecular contacts are described and discussed in relation to this methyl-group disorder. The bidimensional pseudo-hexagonal structural topology of trihalogenomesitylenes is altered in dibromomesitylene insofar as the (100) molecular layers are undulated and are not coplanar as a result of an alternating tilt angle of approximately 34 degrees propagating along the [011] and [0-11] directions between successive antiferroelectric molecular columns oriented roughly along the a axis.