Radical cations of tetrahydropyran and 1,4-dioxane revisited: quantum chemical calculations and low-temperature EPR results

Naumov, Sergej; Janovský, I.; Knolle, Wolfgang; Mehnert, R.

doi:10.1039/b303868h

Cited by 11 publications

(11 citation statements)

References 30 publications

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“…To perform IR spectroscopic investigations of nonaromatic molecular cations in the gas phase, IR predissociation spectroscopy based on the vacuum-ultraviolet (VUV) photoionization detection has been shown to be a powerful tool. − In the present study, we apply this technique to the THF and THP cations to study their hyperconjugation mechanisms. Because THF and THP are cyclic ethers, which are prototypes of sugar rings seen in biomolecules, there have been a number of spectroscopic and theoretical studies on these two molecules. − However, to our knowledge, no spectroscopic study has been performed on the gas-phase acidity in their cationic state.…”

Section: Introductionmentioning

confidence: 99%

Infrared Spectroscopic Investigation of Photoionization-Induced Acidic C–H Bonds in Cyclic Ethers

2015

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Infrared (IR) predissociation spectroscopy based on vacuum-ultraviolet photoionization detection is performed for the neutral and cationic tetrahydrofuran (THF) and tetrahydropyran (THP). The CH bonds in neutral THF and THP are regarded as aprotic, even though the CH bonds are weakened by the negative hyperconjugation. After 118 nm photoionization, however, the negative hyperconjugation changes to the positive hyperconjugation and their CH bond acidities remarkably increase. In the IR spectrum of the THF cation, an intense band is observed at ca. 2700 cm(-1). This band is assigned to the antisymmetric stretch vibration of the two CαH bonds next to the oxygen atom. The high intensity and low frequency of this band are due to the delocalization of the σ electrons of the two CαH bonds to the singly occupied molecular orbital (SOMO) through the hyperconjugation. In the IR spectrum of the THP cation, on the other hand, the stretch bands of the CαH bonds do not show obvious low-frequency shift and intensity enhancement, while the stretch band of the equatorial CγH bond, at the para-position to the oxygen atom, appears at 2855 cm(-1) with high intensity. This acidity enhancement of the equatorial CγH bond is attributed to the mutiple hyperconjugation among the CγH bond, two CαCβ bonds, and SOMO of the oxygen atom. These results suggest that the difference of the hyperconjugation mechanism between the THF and THP cations arises from their preferable conformations.

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Section: Introductionmentioning

confidence: 99%

Infrared Spectroscopic Investigation of Photoionization-Induced Acidic C–H Bonds in Cyclic Ethers

2015

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“…Before examining crown ether-containing IL systems (section ), it is important to first consider briefly the known radiation chemistry of neat CEs. − Ionization of unsubstituted CE ( 1 ) causes the formation of a radical cation 1 +• that promptly deprotonates (Scheme a). , The resulting radical 2 can also be generated via H + transfer to an excess electron, homolytic cleavage of the C−H bonds in an electronically excited CE molecule, or hydrogen abstraction by radiolytically generated H • atoms. This radical 2 (most likely, in the excited state precursor, see section ) undergoes C−O fragmentation yielding terminal radical 3 .…”

Section: Introductionmentioning

confidence: 99%

On the Radiation Stability of Crown Ethers in Ionic Liquids

2011

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Crown ethers (CEs) are macrocyclic ionophores used for the separation of strontium-90 from acidic nuclear waste streams. Room temperature ionic liquids (ILs) are presently being considered as replacements for traditional molecular solvents employed in such separations. It is desirable that the extraction efficacy obtained with such solvents should not deteriorate in the strong radiation fields generated by decaying radionuclides. This deterioration will depend on the extent of radiation damage to both the IL solvent and the CE solute. While radiation damage to ILs has been extensively studied, the issue of the radiation stability of crown ethers, particularly in an IL matrix, has not been adequately addressed. With this in mind, we have employed electron paramagnetic resonance (EPR) spectroscopy to study the formation of CE-related radicals in the radiolysis of selected CEs in ILs incorporating aromatic (imidazolium and pyridinium) cations. The crown ethers have been found to yield primarily hydrogen loss radicals, H atoms, and the formyl radical. In the low-dose regime, the relative yield of these radicals increases linearly with the mole fraction of the solute, suggesting negligible transfer of the excitation energy from the solvent to the solute; that is, the solvent has a "radioprotective" effect. The damage to the CE in the loading region of practical interest is relatively low. Under such conditions, the main chemical pathway leading to decreased extraction performance is protonation of the macrocycle. At high radiation doses, sufficient to increase the acidity of the IL solvent significantly, such proton complexes compete with the solvent cations as electron traps. In this regime, the CEs will rapidly degrade as the result of H abstraction from the CE ring by the released H atoms. Thus, the radiation dose to which a CE/IL system is exposed must be maintained at a level sufficiently low to avoid this regime.

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“…Calculations on the singlet ground state of oxane by Naumov et al . agree with the most stable conformer to be the chair, followed by the 2,5‐twisted boat and 1,4‐twisted boat. They also showed that according to frequency analysis, the boat and sofa are transition states, and the planar structure is a third‐order saddle point.…”

Section: Introductionmentioning

confidence: 56%

“…They also showed that according to frequency analysis, the boat and sofa are transition states, and the planar structure is a third‐order saddle point. DFT calculations by Smith agree that the three minima in oxane are one chair and two twisted boat conformations (2,5 and 1,4), which are calculated to lie at 23.6 and 25.9 kJ mol −1 above the chair, respectively …”

Section: Introductionmentioning

confidence: 91%

“…Although several theoretical studies on the conformation of oxane, the oxane radical cation, substituted oxanes, and related six‐membered structures have been reported, none has paid attention to the relationship between vibrational frequencies and the experimental IR spectrum of this molecule. An ab initio and DFT study of the conformers of oxane by Freeman et al .…”

Section: Introductionmentioning

confidence: 99%

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Determination of the relationship between theoretical vibrational frequencies and experimental IR absorbance bands in organic molecules: computational study of oxane, chromane and flavan

Kuo

Bezuidenhoudt

Conradie

2013

J. Phys. Org. Chem.

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The most stable isomers of oxane, chroman and flavan as basic building blocks for flavonoids were determined by density functional methods. Excellent agreement between the theoretical vibrational frequencies and the experimental infrared (IR) absorbance spectra were obtained. Similarities and differences between the spectra of chroman and flavan were identified. The difference in the IR spectra of the two conformers of flavan, mainly related to the vibration of atoms near the stereogenic centre, makes it possible to distinguish between them. Copyright © 2013 John Wiley & Sons, Ltd.

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Radical cations of tetrahydropyran and 1,4-dioxane revisited: quantum chemical calculations and low-temperature EPR results

Cited by 11 publications

References 30 publications

Infrared Spectroscopic Investigation of Photoionization-Induced Acidic C–H Bonds in Cyclic Ethers

Infrared Spectroscopic Investigation of Photoionization-Induced Acidic C–H Bonds in Cyclic Ethers

On the Radiation Stability of Crown Ethers in Ionic Liquids

Determination of the relationship between theoretical vibrational frequencies and experimental IR absorbance bands in organic molecules: computational study of oxane, chromane and flavan

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