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

Jesus, A. J. Lopes; Reva, Igor; Araujo-Andrade, Cuauhtémoc; Fausto, Rui

doi:10.1063/1.4944528

Cited by 12 publications

(12 citation statements)

References 41 publications

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“…5 In this respect, it appears interesting to us to report that for matrix-isolated 5MOI the experimental integrated intensities of νNH and 2νNH differ by a factor of 34 (see Figure 4). This is similar to the intensity drop from νNH to 2νNH in 6MOI (near 35), 21,22 as well to the intensity drop from νOH to 2νOH in furoic acid 63 (near 35) and in thiazole-carboxylic acid 64 (near 38), all these compounds studied in the same matrix isolation experimental setup. 65 On the basis of the experimental NIR spectrum of matrixisolated 5MOI, the OPO was tuned at 6853 cm −1 (maximum of the 2νNH absorption band), and monomers of the compound isolated in solid xenon at 16 K were exposed to this monochromatic NIR light for about 30 min (three successive irradiations, 10 min each).…”

Section: Resultssupporting

confidence: 78%

“…In order to undoubtedly confirm these assignments and to identify other nontrivial spectral signatures of both conformers, the deposited xenon matrix was annealed from 16 to 40 K, and in the course of this process, various infrared spectra were recorded at increments of 2 K. Owing to the moderate value of the energy barrier corresponding to the internal torsion of the OCH 3 group in the direction of the conformational relaxation, if both conformers are trapped in the deposited matrix, then, annealing of the sample should promote a syn → anti transformation . In the experiment, this would be manifested by an increase of bands due to the most stable form at the expense of bands due to the less stable form, as shown in Figure a.…”

Section: Results and Discussionmentioning

confidence: 99%

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Conformational Changes in 5-Methoxyindole: Effects of Thermal, Vibrational, and Electronic Excitations

2017

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The molecule of 5-methoxyindole (5MOI) may adopt two conformational states, syn and anti, with respect to the relative orientation of the NH and OCH groups. The structure of monomeric 5MOI was characterized spectroscopically, in mid- and near-infrared domains. The conformational composition of 5MOI could be controlled in three different ways. Thermally, two conformers of 5MOI could be trapped in xenon matrixes at 16 K. Upon annealing the xenon matrix to temperatures about 30-40 K, the higher-energy syn form converted to the ground-state anti conformer. Vibrational excitations in the near-infrared domain, at the frequency of the first NH stretching overtone, 6853 cm, afforded the inverse conformational transformation, and a part of the anti conformer was upconverted to the syn form. Electronic excitations in the UV domain, at 315-310 nm, resulted in a total consumption of the syn form again, in favor of anti. Upon further irradiations at 308 nm, a partial repopulation of the syn form, at the expense of anti, was observed. We propose a mechanistic explanation of the observed transformations, which is based on computations of the vibrational spectra of the two conformers and also on computations of the ground state S and the first excited state S potential energy surfaces along the coordinate for conformational isomerization. The highlights of the present work are the first experimental observation of the minor syn conformer of 5MOI, evidence of the long-range vibrational energy transfer resulting in conformational isomerization upon excitation of the NH stretching overtone, and the possibility of partial conformational control of 5MOI by using electronic excitations.

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Section: Resultssupporting

confidence: 78%

Section: Results and Discussionmentioning

confidence: 99%

Conformational Changes in 5-Methoxyindole: Effects of Thermal, Vibrational, and Electronic Excitations

2017

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“…The observed phototransformation is similar to that previously observed for 6-methoxyindole isolated in argon and xenon matrices [37], where rotamerization of the methoxy substituent was successfully induced upon UV irradiation in the 300-305 nm range. In 6-methoxyindole, the (0.50xVII+0.50xVIII-V) methoxy rotamerization barrier has been predicted to be 9-10 kJ mol -1 [38], which is similar to that found in the present study for the V → VII and V → VIII conversions (~ 7 kJ mol -1 ; see Fig. 2).…”

Section: Matrix Isolation Infrared Experimentssupporting

confidence: 89%

UV-induced –OCH3 rotamerization in a matrix-isolated methoxy-substituted ortho-hydroxyaryl Schiff base

Sıdır

Góbi

et al. 2022

Photochem Photobiol Sci

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A new methoxy-substituted ortho-hydroxyaryl Schiff base, 4-(3-methoxy-2-hydroxybenzylidene-amino) phenol was synthesized from 4-aminophenol and 2-hydroxy-3-methoxybenzaldehyde in methanol solution and characterized by 1 H-NMR, 13 C-NMR and infrared spectroscopies and elemental analysis. The compound was isolated in a cryogenic (10 K) argon matrix, and the analysis of the infrared spectrum of the matrix-isolated compound revealed that it corresponds to the E-enol-imine isomeric form, with 3 different conformers being present in the matrix. These conformers share as common structural features the conformation of the free hydroxyl group (trans relatively to the para-substituent of the ring) and the presence of an OH … N intramolecular H-bond involving the methoxy-substituted phenol ring and the azomethine bridge, while they differ in the orientation of the methoxy-substituent group. The structures and relative energies of the conformers of the molecule, and relevant barriers for their interconversion were obtained through quantum chemical calculations, which were also used to calculate the infrared spectra of the different forms. Calculations were also carried out for the higher-energy Z-enol-imine and keto-amine forms of the compound. Upon UV (230 nm) irradiation, -OCH 3 rotamerization was observed, leading to conversion of the lowest energy conformer, where the methoxy group is aligned with the plane of the ring, into the other two conformers initially present in the matrix, in which the OCH 3 group is out-of-the-plane of the ring. As for other phenolic compounds previously studied, spontaneous quantum mechanical tunneling conversion of the cis-OH conformers present in the gas-phase into the three observed conformers was found to take place during matrix deposition.

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“…The barrier height computed for the tc → tt conversion (corresponding to the internal rotation of the isopropyl group) amounts to ∼10 kJ mol –1 . It is known that energy barriers of this order of magnitude and involving the rearrangement of bulky fragments (such as isopropyl group) are difficult to be crossed in a low-temperature matrix. , Hence, besides the global minimum tt , the conformer tc should be also trapped in the deposited Ar matrix. Conformers cc and ct can be converted, respectively, to conformers tc and tt by internal rotation of the H atom of the OH group around the CO bond.…”

Section: Resultsmentioning

confidence: 99%

Conformational Space, IR-Induced, and UV-Induced Chemistry of Carvacrol Isolated in a Low-Temperature Argon Matrix

2021

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In this work, monomers of carvacrol (5-isopropyl-2-methylphenol), a natural monoterpene exhibiting wide range bioactivity, were trapped in a cryogenic argon matrix and characterized by infrared spectroscopy, while quantum chemical calculations at the B3LYP and MP2 levels were employed to characterize the conformational landscape of the isolated molecule. Four conformers have been localized on the potential energy surface, and the factors accounting for their relative stability were analyzed. The two most stable conformers of carvacrol, differing in the relative orientation of the isopropyl group and both having the OH group pointing away from the vicinal methyl fragment, were identified in the cryomatrix for the first time. The individual spectral signatures of the two conformers were distinguished based on the change in their relative abundance induced by exposing the matrix to broadband infrared light. Matrix-isolated carvacrol was also irradiated with broadband UV light (λ > 200 nm), which resulted in the cleavage of the OH group. Recombination of the released H atom at the ortho- or para-position of the ring resulted in the formation of alkyl-substituted cyclohexadienones. These were found to undergo subsequent valence and open-ring isomerizations, leading, respectively, to the formation of a Dewar isomer and open-chain conjugated ketenes. Decarbonylation of the photoproducts was also observed for longer irradiation times. A mechanistic analysis of the observed photochemical transformations is presented.

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

Cited by 12 publications

References 41 publications

Conformational Changes in 5-Methoxyindole: Effects of Thermal, Vibrational, and Electronic Excitations

Conformational Changes in 5-Methoxyindole: Effects of Thermal, Vibrational, and Electronic Excitations

UV-induced –OCH3 rotamerization in a matrix-isolated methoxy-substituted ortho-hydroxyaryl Schiff base

Conformational Space, IR-Induced, and UV-Induced Chemistry of Carvacrol Isolated in a Low-Temperature Argon Matrix

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