Abstract
The dynamics of 2-methoxybenzaldehyde, 4-methoxybenzaldehyde, and 4-ethoxybenzaldehyde in the solid state are assessed through INS spectroscopy combined with periodic DFT calculations. In the absence of experimental data for 4-ethoxybenzaldehyde, a tentative crystal structure, based on its similarity with 4-methoxybenzaldehyde, is considered and evaluated. The excellent agreement between calculated and experimental spectra allows a confident assignment of the vibrational modes. Several spectral features in the INS spectra are unambiguously assigned and torsional potential barriers for the methyl groups are derived from experimental frequencies. The intramolecular nature of the potential energy barrier for methyl rotation about O–CH3 bonds compares with the one reported for torsion about saturated C–CH3 bonds. On the other hand, the intermolecular contribution to the potential energy barrier may represent 1/3 of the barrier height in these systems.
Highlights
IntroductionIn inelastic neutron scattering (INS), there are no selection rules and band intensities are proportional to the nuclei scattering cross-section and atomic displacements in the vibrational mode, both of which are large for hydrogen atoms
There has been an increasing number of reports assessing the structure and dynamics of molecular crystals based on the synergistic combination of inelastic neutron scattering (INS) spectra with periodic density functional (DFT) calculations (e.g., [1,2,3,4,5,6,7,8,9])
In INS, there are no selection rules and band intensities are proportional to the nuclei scattering cross-section and atomic displacements in the vibrational mode, both of which are large for hydrogen atoms
Summary
In INS, there are no selection rules and band intensities are proportional to the nuclei scattering cross-section and atomic displacements in the vibrational mode, both of which are large for hydrogen atoms. In this way, INS spectroscopy provides information not accessible using optical vibrational techniques (IR and Raman) and presents high sensitivity for low wavenumber/large amplitude vibrations, such as torsional vibrations and molecular librational and translational modes. DFT calculations—either periodic or discrete—are highly efficient in predicting the eigenvectors (atomic displacements) for the vibrational normal modes, all that is required to simulate the corresponding
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