UV-induced conformational isomerization and photochemistry of 3-chloro-4-methoxybenzaldehyde in cryogenic inert matrices

Abstract

The interest in investigating the structure and reactivity of halogen- and methoxy-substituted benzaldehydes has been motivated by the practical relevance of these substances in various fields, from the fragrance industry to agrochemicals and drug manufacturing. In this study, 3-chloro-4-methoxybenzaldehyde (3CMBA) was investigated by matrix isolation infrared spectroscopy and quantum chemistry calculations. Molecules of the compound were isolated in cryogenic argon and xenon matrices and the conformational composition of the as-deposited materials characterized by infrared spectroscopy. Two nearly degenerated conformers, differing by the orientation of the aldehyde group relatively to the chlorine ring substituent, were successfully trapped into the matrices from the room temperature vapor of the compound, the cis conformer being slightly higher in energy than the trans form. Both conformers belong to the Cs point group and have the methoxy group pointing to the opposite direction of the chlorine atom. The as-deposited matrices of 3CMBA were then irradiated by tunable monochromatic UV-light. Irradiation within the 302–299 nm range induced conversion of the cis into the trans conformer, while irradiation in the 298–296 nm range induced the inverse photoisomerization. Irradiation at higher energy (250 nm) was found to lead to decarbonylation of both conformers. Very interestingly, the observed UV-induced processes were found to take place in an essentially selective way as a function of the irradiation wavelength. In particular, the conformational isomerization reactions of 3CMBA occur with only a residual number of molecules undergoing simultaneous decarbonylation.

Structure, Vibrational Spectra, and Cryogenic MatrixPhotochemistry of 6-Bromopyridine-2-carbaldehyde: From the Single Molecule of the Compound to the Neat Crystalline Material

“…The performed irradiations led to excitation to the S 2 to S 5 states and can be expected to allow for facile intersystem crossing to the triplet manifold, considering the existence of nine triplet states with energy below S 5 , where the bond cleavages leading to formation of the intermediate radical species involved in the decarbonylation of BPCA take place [ 39 ]. On the other hand, taking into account previous results for other aromatic aldehydes [ 4 , 5 , 6 , 7 , 8 , 38 ], the observed photo-induced trans → cis BPCA conformational isomerization occurs, much probably, in the singlet manifold (either in S 1 or S 0 , after internal conversion).…”

“…The decarbonylation of aromatic aldehydes has been observed frequently as a result of UV irradiation of these type of compounds under matrix isolation conditions [ 6 , 7 , 8 , 38 ]. The fact that the reaction was found to be more efficient in the Xe matrix indicates stabilization of the radical species initially formed upon breaking of the C–C(HO) and C–H (aldehyde) bonds, in this medium.…”

“…From the structural point of view, BPCA appears as an interesting compound due to the following characteristics: (i) the molecule of the compound possesses an internal rotational degree of freedom (the torsion around the exocyclic C–C bond), so that different conformers can be expected to exist; (ii) the location of the aldehyde substituent at the ortho position to the pyridine nitrogen atom can be anticipated to lead to relevant conformation-dependent intramolecular interactions between these two moieties; (iii) the presence of the voluminous, strongly polarizable bromine substituent in the molecule opens expectations regarding its importance in determining fundamental characteristics of the crystal structure of the compound; (iv) the unimolecular photochemistry of the isolated molecules of the compound appears as an interesting topic to be investigated, considering the a priori possibility of occurrence of both UV-induced conformational isomerization [ 4 , 5 , 6 ] and photofragmentation reactions, specifically decarbonylation [ 6 , 7 , 8 ], pyridine ring valence isomerization [ 9 , 10 , 11 ], and Br atom extrusion [ 12 , 13 ]. The investigation of all these characteristics appears as a fundamental step for the detailed understanding of the physicochemical behavior of BPCA, bridging the properties of the isolated molecule to those of the neat crystalline compound and potentiating its use in supramolecular chemistry, in synthesis, and other practical applications, as those mentioned above.…”

Structural Aspects of the Ortho Chloro- and Fluoro- Substituted Benzoic Acids: Implications on Chemical Properties

“…Potential energy profiles were obtained by scaning the conformationally relevant torsional coordinates and optimizing all remaining structural parameters. The calculated vibrational wavenumbers were scaled by 0.978 (our standard scaling factor for this combination of method and basis set [39][40][41]), mainly to account for the effects of basis set limitations, neglected part of electron correlation and anharmonicity. In the spectra simulation, the bands were represented by convoluting each peak (calculated scaled wavenumber and infrared intensity) with a Lorentzian function with a full-width-at-half-maximum (FWHM) of 2 cm −1 .…”