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

Abstract

Two almost isoenergetic conformers of 4-methoxybenzaldehyde (p-anisaldehyde), O-trans and O-cis, are nearly equally populated in gas phase at room temperature. The existence of these two conformers of similar energy makes p-anisaldehyde an attractive molecule for conformational investigations, in which the relative populations of the two forms might be subjected to optical control. In the present study, monomers of the compound were trapped from the room-temperature gas phase into cryogenic argon and xenon matrices. The initial relative amount of the two conformers present in the freshly deposited matrices is shifted slightly in favor of the O-trans conformer. The ratio of the two forms could be reversibly varied by irradiating the sample with UV light in different wavelength ranges or by using the temperature variation. Increasing the temperature of the xenon matrix up to ca. 57 K led to conversion of the less stable O-cis form into the O-trans conformer, shifting the O-cis/O-trans ratio to ca. 1:7. A series of UV irradiations with different long-pass cutoff filters was carried out. UV excitation induced transformation of O-cis and O-trans conformers into each other. These transformations were leading to the UV-wavelength-specific photostationary equilibria characterized by the O-cis/O-trans ratios of about 1:2.2, 1:1.4, 1:1.1, and 1:0.89 for λ > 328, 295, 288, and 234 nm cutoff filters, respectively. The isomerization processes were probed by infrared spectroscopy and supported by quantum chemical calculations. The absorption bands observed in the infrared spectra of p-anisaldehyde isolated in argon and xenon matrices were assigned to the theoretically predicted normal modes.

Conformers of diheteroaryl ketones and thioketones: a quantum chemical study of their properties and fundamental intramolecular energetic effects

“…The presence of a heteroaryl group may hinder the rotation of another heteroaryl group in diheteroaryl ketone or thioketone. The comparison of the difference in relative energy at the highest and lowest energy points on the PESs for 4a with the energy barrier calculated previously for 2-pyrrolyl aldehyde [56,57] shows that the former is approximately twice as large as the latter. Similarly, the energy barrier for 2a demonstrates a higher value when it is collated with the values of energy barriers calculated for 2-thiophenyl aldehyde [58] and thiophene-2-carbonyl fluoride [59].…”

Conformational changes in matrix-isolated 6-methoxyindole: Effects of the thermal and infrared light excitations

“…24,25,41 In all cases, no conformational relaxation was observed upon annealing the argon matrices up to the limit for the thermal stability of the host-material (∼35 K). Conformational conversions were only observed after depositing some of these molecules (i.e., those with accessible energy barriers for conformational isomerization) in xenon and heating the matrices up to temperatures ranging from 30 to 55 K (∼65 K is the limit for this host matrix).…”

“…21 On the other hand, in previous studies of molecules with conformers differing from each other by rotation of bulky fragments (such as OCH 3 or OCOH), no isomerizations have been observed after subjecting the matrices to broadband or narrowband IR radiation. [23][24][25] From a wider perspective, the results obtained in this work provide basic knowledge that can be used to understand local conformational features in many relevant methoxyindole-containing naturally occurring and synthetic biomolecules. [26][27][28][29][30] Also, the possibility of controlling the conformational distribution of 6MOI in matrices opens a window for exploiting its conformer-specific reactivity in condensed media (as opposite to the gas phase).…”

Light induced reactions in cryogenic matrices

“…[28][29][30] The conformational space of monomeric pyrrole-2-carbaldehyde (P2C) was investigated theoretically at the MP2 and DFT (B3LYP) levels, with the 6-311þþG(d,p) basis set. 28 The compound can assume two conformations, cis and trans, regarding the orientation of the N-C-C¼O dihedral angle.…”

“…28 Particularly interesting results were obtained for 4-methoxybenzaldehyde (p-anisaldehyde). 29 This compound possesses two almost isoenergetic conformers, which are nearly equally populated in gas phase at room temperature. Once deposited in argon or xenon matrix, the population ratio of the two forms could be reversibly varied by irradiating the sample with UV light in different wavelength ranges or by varying the temperature of the matrix.…”