4-Fluorobenzaldehyde, also known as p-fluorobenzaldehyde, has a molecular structure in which a fluorine atom and an aldehyde group are attached to the para positions of the benzene ring, respectively. It is chemically reactive; the aldehyde group can undergo oxidation, reduction, condensation, and addition reactions, while the electron-withdrawing effect of the fluorine atom modulates the reactivity of the benzene ring.

Vibrational dynamics of 4-fluorobenzaldehyde

At present, there is a limited number of reports addressing the vibrational spectra of molecular crystals using periodic density functional (DFT) calculations. In INS, the signal intensity is highly dependent on the motions of hydrogen atoms and, due to the absence of selection rules, all vibrational modes are permitted. In this way, low wavenumber/large amplitude vibrational motions are readily observed in INS spectra. This works aims to assess the vibrational dynamics of solid 4-fluorobenzaldehyde through the analysis of INS and IR spectra. To this end, a detailed description of the vibrational modes is obtained from periodic DFT calculations. In the absence of a reported crystalline structure of 4-fluorobenzaldehyde, the starting crystalline structure used in geometry optimization was derived from the one reported for 4-chlorobenzaldehyde, which is structurally similar to its parent compound 4-bromobenzaldehyde, lending credence to the hypothesis that 4-fluorobenzaldehyde is isomorphous with its chloro- and bromo- analogues. Additionally, the feasibility of using periodic DFT calculations to assist the interpretation of vibrational spectra of liquids and non-crystalline solids is discussed.[1]

Periodic calculations predict a factor group splitting for a few modes in 4-fluorobenzaldehyde crystal, and those splittings are actually observed in the vibrational spectra of the solid samples. Factor group splitting is inherently present in a crystal cell with eight molecules (where each molecular mode is expected to give rise to eight crystal modes), but its magnitude depends on the magnitude of intermolecular forces in the crystal. For most of the vibrational modes, the factor group components are not far apart, considering the spectral resolution, and have limited influence in the observed spectra.  The splitting of the νCO mode has been previously reported in the Raman spectra of solid state 4-fluorobenzaldehyde. The two components of the CHO torsion were identified in the INS spectrum. The splitting of the additional four modes has been identified in the infrared spectrum only, as resolution limitations do now allow its identification in the INS spectrum. Periodic DFT calculations, using a model crystal structure, are able to correctly describe the vibrational spectra and dynamics of 4-fluorobenzaldehyde in both the liquid and solid phases. This evidence supports the use of the periodic DFT calculations for amorphous solids and even for liquid phases, especially in systems presenting soft intermolecular interactions, for which the contacts with neighbouring molecules have negligible effects on molecular vibrations.

Electron and positron scattering from 4-fluorobenzaldehyde

A linear transmission time-of-flight method has been used to measure total cross sections (TCSs) for 0.4–1000 eV electron and 0.2–150 eV positron scattering from 4-fluorobenzaldehyde (C6H4(CHO)F) molecules. The disubstitution of the C6H6 parent molecule's two H atoms by the CHO radical and F atom in the 1- and 4-positions, on the benzene ring, has resulted in an electron TCS curve that shows that the original benzene ring symmetry has been partially preserved; a 1.6 eV peak corresponding to the 1.6 eV 2E2u C6H6 resonance peak, a 2.8 eV weak peak in contrast to a minimum for C6H6, a 4.6 eV shoulder corresponding to the 4.93 eV 2B2g resonance peak for C6H6, the main resonance peak centered at 8 eV corresponding to the 9.5 eV 2E1u main resonance peak for C6H6, and the characteristic change of the TCS slope producing a weak shoulder centered at 50 eV for both molecules. On the contrary the positron TCSs show low energy features that differ greatly from the parent C6H6 molecule; two relatively sharp peaks centered at 1 and 4 eV in contrast to the almost smooth and structureless 0.7–10 eV C6H6 peak. Nevertheless, these two positron TCSs resemble each other below 1 eV and above 6 eV. In all, TCS data for 0.4–1000 eV electron and 0.2–150 eV positron scattering from 4-fluorobenzaldehyde molecules have been studied in comparison with our previous data for 1,4-C6H4F2 and C6H6 molecules.[2]

References

[1]Ribeiro-Claro, P. J. A., Vaz, P. D., Nolasco, M. M., Araujo, C. F., Gil, F. P. S. C., & Amado, A. M. (2019). Vibrational dynamics of 4-fluorobenzaldehyde from periodic DFT calculations. Chemical Physics Letters, 737, 100006. https://doi.org/10.1016/j.cplett.2019.100006

[2]C. Makochekanwa,  M. K.,  O. Sueoka. (2004). Total cross sections for electron and positron scattering from 4-fluorobenzaldehyde (C6H4(CHO)F) molecules. Nuclear Instruments & Methods in Physics Research Section B-Beam Interactions With Materials and Atoms, 221, Pages 76-83. https://doi.org/10.1016/j.nimb.2004.03.034