Vibrational Dynamics in crystalline 4-(dimethylamino) benzaldehyde: Inelastic Neutron Scattering and Periodic DFT Study

Abstract

The structure and dynamics of crystalline 4-(dimethylamino) benzaldehyde, 4DMAB, are assessed through INS spectroscopy combined with periodic DFT calculations. The excellent agreement between experimental and calculated spectra is the basis for a reliable assignment of INS bands. 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. Crystal field splitting is predicted and observed for the modes assigned to the dimethylamino group. Concerning the torsional motion of methyl groups, four individual bands are identified and assigned to specific methyl groups in the asymmetric unit. The torsional frequencies of the four methyl groups in the asymmetric unit fall in a region of ca. 190 ± 20 cm^-1, close to the range of values observed for methyl groups bonding to unsaturated carbon atoms. The hybridization state of the X atom in X-CH₃ seems to play a key role in determining the methyl torsional frequency.

Keywords: density functional theory; inelastic neutron scattering; methyl torsion; molecular crystal; vibrational assignment.

Figure 1

Representation of 4-(dimethylamino) benzaldehyde with the atom labelling used throughout the text (left) and a schematic representation of the crystal structure, evidencing the relevant intermolecular contacts present in the crystal (right). Molecules in the asymmetric unit are labelled A and B, and methyl groups are labelled I-IV.

Figure 2

The INS spectra of 4-(dimethylamino) benzaldehyde in the 25–1800 cm⁻¹ range: experimental (top), simulated from periodic calculations (middle) and from single-molecule discrete calculations (bottom).

Figure 3

Experimental spectrum of 4-(dimethylamino) benzaldehyde in the range below 300 cm⁻¹ (top) compared with the corresponding INS spectra calculated at the Γ-point (Γ, bottom) and averaged by the dispersion throughout the Brillouin zone (BZ, middle).

Figure 4

Atomic displacements of heavy atoms in one of the phonon modes contributing to the band at ca. 719 cm⁻¹. The large displacements of hydrogen atoms were omitted for clarity.

Figure 5

Pictorial description of the calculated band maxima in the low-wavenumber region (from periodic-DFT calculations). External modes (translations and librations) in grey, internal modes in blue, except for the CH₃ torsional modes, with a differentiated color code. The CH₃ torsional modes are labelled I–IV, according to Figure 1.