New Insights on the Vibrational Dynamics of 2-Methoxy-, 4-Methoxy- and 4-Ethoxy-Benzaldehyde from INS Spectra and Periodic DFT Calculations

• Main Authors: Paulo J. A. Ribeiro-Claro, Pedro D. Vaz, Mariela M. Nolasco, Francisco P. S. C. Gil, Luís A. E. Batista de Carvalho, Maria Paula M. Marques, Ana M. Amado
• Format: Article
• Language: English
• Published: MDPI AG 2021-08-01
• Series: Materials
• Subjects: density functional theory; inelastic neutron scattering; vibrational assignment; molecular crystal; torsional potential; C–H…O hydrogen bonds
• Online Access: https://www.mdpi.com/1996-1944/14/16/4561

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.