The torsional frequencies of the four methyl groups fall in a region of ca. Since the molecules are in orthogonal planes, all the atoms in both molecules move along the same “direction” (more rigorously, move in parallel planes). Figure 4 illustrates the situation for one of the phonon modes contributing to the 719 cm −1 band, which is nominally described as “ring out-of-plane deformation” but includes contributions from the “ring in-plane deformation” of the B-type molecules. In a few cases, the phonon mode includes an in-plane mode of one B-type molecule and an out-of-plane mode of the neighboring A-type molecule. However, since 4DMAB crystal results from the packing of molecules contained in nearly orthogonal planes (namely, molecules linked via a C-H …π interaction), mixing between in-plane and out-of-plane molecular modes is observed. In that sense, bands in Table 2 are described in terms of molecular normal modes of vibration. The two molecules in the asymmetric unit are schematically represented in Figure 1, right, evidencing the C-H …π and C-H …O contacts (along with the labels A and B for the two molecules and I-IV for the methyl groups).Īpart from the external or collective modes, phonon modes in 4DMAB crystal were found to be grouped in combinations of eight identical molecular normal modes-generally, combinations of four A-type molecules and four B-type molecules-giving rise to the observed INS band maxima. In addition, the structure is stabilized by π -stacking interactions involving the benzene rings.”. In the crystal structure, C-H …O hydrogen bonds link one type of independent molecules into a chain along the a axis. In both molecules, the aldehyde and dimethylamine groups are essentially coplanar with the attached benzene ring.
Gao and Zhu state that 4DMAB “ crystallizes with two independent but essentially identical molecules in the asymmetric unit, which are linked via a C-H …π interaction. The crystal structure of 4DMAB (monoclinic, space group P21/n, Z = 8) has been reported by Gao and Zhu and by Vicente et al. Furthermore, INS is of paramount importance since it makes it possible to study low-wavenumber (and high-amplitude) vibrational modes with high sensitivity, which provides critical structural information about molecular systems that cannot be accessed using IR or Raman.įigure 1, left, presents the molecular structure of 4-(dimethylamino) benzaldehyde (4DMAB), along with the numbering scheme adopted in this work. In INS, the largest scattering cross-section is found for hydrogen, thus making this technique ideal to study organic or other hydrogenous systems. While IR and Raman are optical spectroscopy techniques relying on optical modes that are symmetry-allowed, INS does not obey the same selection rules, with the intensity of the bands being proportional the incoherent neutron scattering cross-section and corresponding atomic displacements in a given vibrational mode. Such vibrational optical spectroscopy techniques can be complemented with the use of inelastic neutron scattering (INS). By using chemical bonds as probes, vibrational spectroscopy provides information about how a given molecule “communicates” with its surroundings. The latter offer extremely valuable information about the local structural environments of molecules. This can be done using a complete toolbox including crystallography (X-ray and neutron), nuclear magnetic resonance and vibrational spectroscopy (infrared and Raman). Assessing such molecular interactions unravels understanding of not only its structure but, more importantly, how a system’s dynamics contributes at the molecular level to the observable behavior at the macroscopic level. Molecular interactions lie in a field that is still far from being fully understood. The hybridization state of the X atom in X-CH 3 seems to play a key role in determining the methyl torsional frequency. 190 ± 20 cm −1, close to the range of values observed for methyl groups bonding to unsaturated carbon atoms. The torsional frequencies of the four methyl groups in the asymmetric unit fall in a region of ca. Concerning the torsional motion of methyl groups, four individual bands are identified and assigned to specific methyl groups in the asymmetric unit. Crystal field splitting is predicted and observed for the modes assigned to the dimethylamino group. The external phonon modes of crystalline 4DMAB are quite well described by the simulated spectrum, as well as the modes involving low-frequency molecular vibrations.
The excellent agreement between experimental and calculated spectra is the basis for a reliable assignment of INS bands. The structure and dynamics of crystalline 4-(dimethylamino) benzaldehyde, 4DMAB, are assessed through INS spectroscopy combined with periodic DFT calculations.
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