UV photochemistry of 1,3-cyclohexadiene isolated in solid parahydrogen

Miyazaki, J.; Toh, Shin Yi; Moore, Brendan; Djuricanin, Pavle; Momose, Takamasa

doi:10.1016/j.molstruc.2020.128986

Cited by 4 publications

(5 citation statements)

References 39 publications

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“…Ketenes are generated from 3F-24CHD or 5F-24CHD by means of ring-opening reactions involving cleavage of the weaker C–C bond at the α-position with respect to the carbonyl group, i.e., that connecting this group to CH 2 , as seen in Scheme 2 . This reaction is similar to the well-known ring-opening reaction of 1,3-cyclohexadiene yielding 1,3,5-hexatriene [ 65 ]. Note that in 3F-25CHD, the two C–C bonds at the α-position with respect to the C=O group are not particularly prone to be broken, meaning that this species is expected to be much less photoreactive than the other two isomers.…”

Section: Resultssupporting

confidence: 65%

Infrared Spectra and Phototransformations of meta-Fluorophenol Isolated in Argon and Nitrogen Matrices

et al. 2022

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Monomers of meta-fluorophenol (mFP) were trapped from the gas phase into cryogenic argon and nitrogen matrices. The estimated relative energies of the two conformers are very close, and in the gas phase they have nearly equal populations. Due to the similarity of their structures (they only differ in the orientation of the OH group), the two conformers have also similar predicted vibrational signatures, which makes the vibrational characterization of the individual rotamers challenging. In the present work, it has been established that in an argon matrix only the most stable trans conformer of mFP exists (the OH group pointing away from the fluorine atom). On the other hand, the IR spectrum of mFP in a nitrogen matrix testifies to the simultaneous presence in this matrix of both the trans conformer and of the higher-energy cis conformer (the OH group pointing toward the fluorine atom), which is stabilized by interaction with the matrix gas host. We found that the exposition of the cryogenic N2 matrix to the Globar source of the infrared spectrometer affects the conformational populations. By collecting experimental spectra, either in the full mid-infrared range or only in the range below 2200 cm−1, we were able to reliably distinguish two sets of experimental bands originating from individual conformers. A comparison of the two sets of experimental bands with computed infrared spectra of the conformers allowed, for the first time, the unequivocal vibrational identification of each of them. The joint implementation of computational vibrational spectroscopy and matrix-isolation infrared spectroscopy proved to be a very accurate method of structural analysis. Some mechanistic insights into conformational isomerism (the quantum tunneling of hydrogen atom and vibrationally-induced conformational transformations) have been addressed. Finally, we also subjected matrix-isolated mFP to irradiations with UV light, and the phototransformations observed in these experiments are also described.

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Section: Resultssupporting

confidence: 65%

Infrared Spectra and Phototransformations of meta-Fluorophenol Isolated in Argon and Nitrogen Matrices

et al. 2022

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“…According to the results recently reported for the photochemistry of 1,3-CHD, the open-ring photoproducts that appear during the initial stages of the UV irradiation correspond to the Z-type isomers around the central double bond . Another important consideration is that the open-ring ketene isomers 5 (Z-type) of cyclohexadienones exist in photoequilibrium (see Scheme ) with their closed-ring counterparts 3 .…”

Section: Resultsmentioning

confidence: 85%

“…Therefore, only the Z-type of ketene isomers (note the Z-orientations around the green double bonds in Scheme ) was here considered as candidates for photoproducts 5a and 5b . There is experimental evidence that 1,3-CHD converts rapidly to Z-1,3,5-HT and slowly to E-1,3,5-HT. , Apparently, a slow accumulation of the E-isomer of open-ring carvacrol is not competitive with other observed UV-induced isomerizations and other exit channels (decarbonylation, see below).…”

Section: Resultsmentioning

confidence: 99%

“…To the best of our knowledge, only a few monomeric monoterpenes have been objects of photochemical studies in cryogenic matrices. − Among these, α-terpinene and α-phellandrene , have the closest structural proximity with carvacrol. They represent alkyl-substituted derivatives of 1,3-cyclohexadiene (1,3-CHD, see Scheme ), and their main photochemical reactions resemble the photochemistry of 1,3-CHD, which consists of a ring-opening reaction with production of the respective 1,3,5-hexatriene (1,3,5-HT) or its alkyl-substituted analogues. − …”

Section: Introductionmentioning

confidence: 99%

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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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“…For species 5, only the structures adopting Z-configuration around the central C=C bond have been considered in the calculations. This assumption was based on the results of a recent study on the photochemistry of 1,3-cyclohexadiene isolated in solid parahydrogen, where only the Z-type isomers of the open-ring photoproducts were found to appear during the initial stages of the UV-irradiations [57]. The range of wavenumbers predicted for the ν as (C=C=O ) mode of 5 was 2106-2084 cm -1 (A th = 637-1485 km mol -1 ) and for the ν(C=O ) mode of 3 was 1679-1654 cm -1 (A th = 165-318 km mol -1 ).…”

Section: Uv-induced Transformationsmentioning

confidence: 99%

Conformational Structure, Infrared Spectra and Light-Induced Transformations of Thymol Isolated in Noble Gas Cryomatrices

2022

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The conformational space of the natural product thymol (2-isopropyl-5-methylphenol) was investigated using quantum chemical calculations at the B3LYP and MP2 levels, which revealed the existence of four types of conformers differing in the orientation of the isopropyl and hydroxyl groups. Thymol monomers were isolated in noble gas (Ar and Xe) matrices (at 15 K) and characterized by IR spectroscopy. With the support of B3LYP harmonic vibrational calculations, the two most stable trans-OH-conformers, differing in the isopropyl orientation, were identified in the cryomatrices. The two less stable cis-OH conformers were not detected as they shall undergo fast tunneling to the most stable ones. Annealing experiments in a Xe matrix up to 75 K did not lead to any conversion between the two isolated conformers, which is in accordance with the significative energy barrier computed for rotamerization of the bulky isopropyl group (~24 kJ mol−1). Vibrational excitation promoted by broadband or by narrowband irradiation, at the 2ν(OH) frequencies of the isolated conformers, did not lead to any conversion either, which was interpreted in terms of a more efficient energy transfer to the hydroxyl rotamerization (associated with a lower energy barrier and a light H-atom) than to the isopropyl rotamerization coordinate. Broadband UV irradiation experiments (λ > 200 nm) led to a prompt transformation of matrix isolated thymol, with spectroscopic evidence suggesting the formation of isomeric alkyl-substituted cyclohexadienones, Dewar isomers and open-chain conjugated ketenes. The photochemical mechanism interpretation concords with that reported for analogous phenol derivatives.

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UV photochemistry of 1,3-cyclohexadiene isolated in solid parahydrogen

Cited by 4 publications

References 39 publications

Infrared Spectra and Phototransformations of meta-Fluorophenol Isolated in Argon and Nitrogen Matrices

Infrared Spectra and Phototransformations of meta-Fluorophenol Isolated in Argon and Nitrogen Matrices

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

Conformational Structure, Infrared Spectra and Light-Induced Transformations of Thymol Isolated in Noble Gas Cryomatrices

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