Matrix-Isolation Infrared Spectroscopy of the Rotational Isomers of 1,2-, 1,3-, and 1,4-Benzenedicarboxaldehyde

Abstract

Rotational isomers (rotamers) of the three structural isomers of benzenedicarboxaldehydes (1,2-, 1,3-, and 1,4-derivatives) have been investigated in detail using matrix-isolation infrared spectroscopy in the 600-4000 cm-1 region, combined with UV photoexcitation and density-functional theory (DFT) calculations. Two rotamers were identified for 1,2- and 1,4-benzenedicarboxaldehyde (1,2- and 1,4-BDA, respectively), while three rotamers were identified for 1,3-benzenedicarboxaldehyde (1,3-BDA) in infrared spectra upon UV-irradiation. Most of the observed infrared bands of each rotamer have been assigned. The energetic relationships among the rotamers were revealed based on the infrared data and the DFT calculations. It is shown that the intramolecular C-H...H-C interaction in the H-syn rotamer or the C-H...O=C hydrogen bonding in the anti rotamer of 1,2-BDA results in the blue-shift of the aldehyde C-H stretching band and the shortening of the aldehyde C-H bond length. Both photoinduced rotational isomerization and rearrangement were observed upon UV irradiation for 1,2-BDA. The structure of the major enol isomer formed as the result of the photochemical rearrangement of 1,2-BDA is determined.

Thermal and Photoinduced Control of Relative Populations of 4-Methoxybenzaldehyde (p-Anisaldehyde) Conformers

“…Hence, these barriers can also be easily crossed during radiationless dissipation of the excitation energy. The profiles presented in Figure can explain the UV-induced O- trans ↔ O- cis isomerizations observed for p -anisaldehyde and (by analogy) the photochemical trans ↔ cis isomerizations previously observed for compounds with the aldehyde or with the methoxy groups attached to aromatic rings. − …”

“…The profiles presented in Figure 10 can explain the UV-induced O-trans T O-cis isomerizations observed for p-anisaldehyde and (by analogy) the photochemical trans T cis isomerizations previously observed for compounds with the aldehyde or with the methoxy groups attached to aromatic rings. [38][39][40][41] The presented observations indicate that the ratio of conformers at a photostationary state is specific to every wavelength of UV light used for irradiation. The observed net direction of the photoinduced conversions is dependent on the wavelength range of the applied UV light as well as on the initial population ratio of the conformers.…”

“…UV-induced mutual conversions of conformers differing in the orientation of their aldehyde group have been observed for several analogues of benzaldehyde. − Photochemical rotations were found also for methoxy groups attached to a benzene ring . It could then be expected that conformational isomerization would also occur for matrix-isolated p -anisaldehyde subjected to UV irradiation.…”

Infrared Spectra and Photochemistry of Matrix-Isolated Pyrrole-2-carbaldehyde

“…For these matrix-isolated molecules, the photoproduced minor conformers could be successfully trapped in the matrix and characterized. 10,11 One of the objectives of the present work was the experimental characterization of the P2C conformers, with emphasis on its minor form. The method of matrix isolation in solidified inert gases combined with infrared spectroscopy is one of the most appropriate experimental techniques for such studies.…”

“…Indeed, the experimental observation of the trans conformation of P2C has not been previously reported. However, in similar molecules, like 3-pyridinecarbaldehyde in the gas phase, and a series of aromatic aldehydes isolated in cryogenic argon matrices, , internal rotation of the aldehyde group has been shown to take place upon UV excitation. For these matrix-isolated molecules, the photoproduced minor conformers could be successfully trapped in the matrix and characterized. , …”