Interplay Between Microwave Spectroscopy and X‐ray Diffraction: The Molecular Structure and Large Amplitude Motions of 2,3‐Dimethylanisole

Abstract

To determine the structural properties of 2,3-dimethylanisole, a multidisciplinary approach was carried out where gas phase rotational spectroscopy recording a spectrum from 2 to 26.5 GHz using a pulsed molecular jet Fourier transform microwave spectrometer was combined with solid-state X-ray diffraction. Both methods revealed that only one conformer with a planar heavy-atom structure exists. In the solid state, the packing in the monoclinic space group is P2 /n with Z=4. In the gas phase spectrum, torsional splittings due to the internal rotations of two methyl groups attached to the phenyl ring were resolved and analyzed, providing an estimate of the barriers to methyl internal rotation of V =26.9047(5) and 518.7(12) cm for the methyl groups at the ortho- and meta-position, respectively. The coupling between the two internal rotations is modeled on a two-dimensional potential energy surface, which was obtained by quantum chemical calculations at the B3LYP/6-311++G(d,p) level of theory.

Structure of anisole derivatives by total neutron and X-ray scattering: Evidences of weak C H⋯O and C H⋯π interactions in the liquid state

“…In the common case of aliphatic or aromatic C-H groups as hydrogen donors, the energy of a C-H•••π hydrogen bond is around 2 kcal mol -1 . 10 In the case of 2,4 dimethylanisole, one of the methyl groups hydrogen is pointing towards the π- 8 A similar geometry has also been observed in crystalline anisole (see above), although involving C-H donors belonging to methoxy methyl group. 2 In the context of the present study, it makes sense to refer to previous structural studies on benzene that highlighted interesting features: benzene has been largely explored in the past, using synergy of X-ray and /or neutron scattering and computational tools (see e.g.…”

“…A joint microwave spectroscopy and X-ray Diffraction study on the gas and solid state, respectively, of 2,4 dimethylanisole was also reported. 8 In the solid state, the structural study suggests the establishment of C-H•••π interactions: such interactions are considered the weakest extreme of hydrogen bonds between a soft acid such as C-H and a soft base such as the π-system. [9][10][11][12][13][14] It has been reported that dispersive energy is the main contributor to C-H•••π interactions, as electrostatic or charge transfer contributions are often negligible; nevertheless the dual origin (both dispersive and electrostatic) of this interaction has been stressed as the reason for the importance of this non-covalent force in chemistry and, especially, biochemistry.…”

Two Equivalent Internal Rotations in the Microwave Spectrum of 2,6‐Dimethylfluorobenzene

“…The systematic investigations on dimethylanisoles have indicated some problems associating with the oscillation motions of the methoxy group which is suspect to influence the internal rotation of the two methyl groups. [16][17][18] The same effects could also happen in dimethylbenzaldehyde or other derivatives, whenever a group of more than two atoms are attached on the phenyl ring in addition to the two methyl groups. Therefore, we decided to study the dimethylfluorobenzene family, where the third substitution is a fluorine atom and the molecular planarity becomes evident.…”

“…For comparison, we also applied the Kohn-Sham density functional theory [31] using Becke's three parameter hybrid exchange functional [32] and the Lee-Yang-Parr correlation functional [33] (B3LYP), as our experiences and several studies in the literature have shown that this method is adequate for predictive guidance of phenyl ring containing molecules. [14,17,34] Grimme's distortion corrections [35] and Becke-Johnson damping [36] were used in addition, and the basis set 6-311 + + G(d,p) was chosen. We also carried out optimizations at the MP2/6-311 + + G(d,p) level of theory, a method quite often used in the spectroscopic community.…”

“…[7] Investigations on toluene derivatives with two methyl groups attached on the phenyl ring are scarce, such as those on the three isomers of dimethylbenzaldehyde [15] and the three isomers of dimethylanisole. [16][17][18] The reason is probably the complexity of the microwave spectrum due to the coupled LAMs of the two methyl groups, which makes it hard to assign and to model. As a consequence, effective Hamiltonians often have to be included to reproduce the experimental spectra with sufficient accuracy.…”