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Spectroscopy and Photochemistry; General Theory
Direct Observation of Tetrahydrofuranyl and Tetrahydropyranyl Peroxy Radicals via Cavity Ring-Down Spectroscopy Hamzeh Telfah, Md. Asmaul Reza, Jahangir Alam, Anam C Paul, and Jinjun Liu J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01721 • Publication Date (Web): 23 Jul 2018 Downloaded from http://pubs.acs.org on July 25, 2018
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The Journal of Physical Chemistry Letters
Direct Observation of Tetrahydrofuranyl and Tetrahydropyranyl Peroxy Radicals via Cavity Ring-Down Spectroscopy
Hamzeh Telfah,1 Md Asmaul Reza,1 Jahangir Alam,1,† Anam C. Paul,1 and Jinjun Liu1,2,* 1. Department of Chemistry, University of Louisville, Louisville, Kentucky 40292, United States 2. Conn Center for Renewable Energy Research, University of Louisville, Louisville, Kentucky 40292, United States
† *
Current address: Brooks Instrument, 407 W. Vine St., Hatfield, PA 19440 E-mail:
[email protected]. 1 ACS Paragon Plus Environment
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Abstract Room-temperature cavity ring-down (CRD) spectra of the 𝐴𝐴̃ ← 𝑋𝑋� electronic transition of
tetrahydrofuranyl peroxy (THFOOˑ) and tetrahydropyranyl peroxy (THPOOˑ) radicals were
recorded. The peroxy radicals were produced by Cl-initiated oxidation of tetrahydrofuran and tetrahydropyran. Quantum chemical calculations of the lowest-energy conformers of all regioisomers of these two peroxy radicals have been carried out to aid the spectral simulation. Conformational identification and vibrational assignment were achieved by comparing the experimentally obtained spectra to the simulated ones. Absence of α-THPOOˑ absorption peaks in the CRD spectrum is attributed to ring opening due to its weak Cα’O bond.
TOC Graphics
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The Journal of Physical Chemistry Letters
Peroxy (ROOˑ) radicals are one of the most important families of reaction intermediates in low-temperature combustion of fossil fuels. Addition of O2 to a hydrocarbon radical (Rˑ) forms a peroxy radical and initiates the chain reactions1-2. Peroxy radicals also play a key role in atmospheric chemistry. The initial attack of the hydroxyl radicals (ˑOH) on hydrocarbons, which may be either biogenic or anthropogenic, and the subsequent addition of oxygen lead to the production of peroxy radicals. In the atmosphere peroxy radicals are converted to the alkoxy radicals (ROˑ) mainly via reaction with NO.3-4 In recent years, biofuels including alcohols, ethers, and esters have gained increasing interest. As alternatives to fossil fuels or as fuel additives, their advantages include regenerability and reduced climatic impact.5 In contrast to traditional petroleum-based fuels, most biofuels are oxygenated, containing oxygen as an additional element in their molecular structure. Specifically, prototypical cyclic ethers including tetrahydrofuran (THF) and tetrahydropyran (THP) are important structural building blocks for lignocellulosic biofuels. Therefore, the study of the combustion chemistry of THF and THP as well as other oxygenated biofuels has significant scientific and societal importance. Compared to cyclic alkanes such as cyclopentane and cyclohexane, the combustion chemistry of THF and THP are complicated by several thermodynamic features. Dissociation energies of C−O bonds in the tetrahydrofuranyl (THF-yl) and tetrahydropyranyl (THP-yl) radicals are weaker than C-C bands in the cyclopentyl and cyclohexyl radicals. THF-yl and THP-yl are therefore susceptible to ring opening. The oxygen atom also weakens the adjacent C-H bonds causing the β-scission pathways to have lower barriers than corresponding cyclic alkanes. Lowtemperature oxidation of THF and THP has been studied in combined experimental and computational investigations.6-8 Mass spectra and photoionization efficiency spectra of many 3 ACS Paragon Plus Environment
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intermediates and products in THF6 and THP7 combustion have been reported. Recently, oxygen addition reaction of the THF-yl radical was investigated by monitoring time-resolved infrared absorption of the ˑOH and HO2ˑ radicals.8 However, many unanswered questions invite further spectroscopic and kinetic studies. In particular, none of the combustion reaction intermediates of THF and THP have been observed with spectral resolution. Consequentially, no conformerselective kinetic measurements have been performed. The present Letter reports laser spectroscopic investigations of the tetrahydrofuranyl peroxy (THFOOˑ) and tetrahydropyranyl peroxy (THPOOˑ) radicals, important reaction intermediates in the oxidation of THF and THP, respectively. The cavity ring-down (CRD) technique9 was employed for the direct detection of these peroxy radicals. Aided by quantum chemical calculations, spectral simulation of partially resolved vibronic structure in the experimentally obtained CRD spectra allows identification of molecular carriers of observed transitions. Our CRD spectroscopy apparatus is described in the Supporting Information (see Section S.1). In the present work, THFOOˑ and THPOOˑ radicals were produced in a room-temperature gasflow reaction cell (total pressure ~150 Torr) by Cl-initiated oxidation of THF and THP. Chlorine atoms were produced by 193-nm or 266-nm photolysis of oxalyl chloride (COCl)2:10 ℎ𝜈𝜈 (𝜆𝜆 = 193 or 266 nm)
(COCl)2 �⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯� 2CO + 2Cl ∙
(1)
THF-yl and THP-yl radicals were produced via hydrogen abstraction of corresponding precursors (THF or THP) by chlorine attack,11-12 R − H + Cl ∙→ R ∙ + HCl
(2)
Reaction (2) was followed by oxygen addition to form the peroxy radicals: 4 ACS Paragon Plus Environment
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The Journal of Physical Chemistry Letters
R ∙ + O2 → ROO ∙
(3)
The 𝐴𝐴̃ ← 𝑋𝑋� electronic transitions of THFOOˑ and THPOOˑ in the near IR region of 1.15-1.55
μm were recorded by CRD spectroscopy. Absorbance recorded with the photolysis laser off, i.e., absorbance due to precursors, was subtracted from that with the photolysis laser on to obtain net absorbance due to the produced radicals. Kinetic measurements at observed spectral peaks were also performed. In kinetic measurements, delay time between the photolysis laser pulse and the CRD laser pulse was varied, and absorption was recorded as a function of the delay time. The heterocyclic nature of the target radicals leads to complicated conformational structure. Conformational behaviors of both five-13-16 and six-membered rings17 have been studied extensively. Fewer studies have been done on the conformational behaviors of heterocyclic rings including THF and THP (vide infra). In the case of THFOOˑ and THPOOˑ peroxy radicals, addition of the dioxygen moiety leads to conformational landscapes with labyrinthine complexity. To the best of our knowledge, no quantum chemical calculations on these two radicals have been reported. In the present work, a comprehensive survey of the conformational landscapes of THFOOˑ and THPOOˑ was not attempted. Instead, the goals of the computational investigation were to determine the lowest-energy stable conformers, to optimize their geometries, and to calculate their molecular constants for spectral simulation. Conformational identification and vibrational assignment were done by comparing the experimentally observed spectra with those simulated using the calculated geometries and molecular parameters. The conformational search was done as follows. First, conformational structures of the ring were taken into account. Then the regioisomers were determined according to different positions for addition of the dioxygen moiety on the ring with respect to the ring oxygen atom. Next, 5 ACS Paragon Plus Environment
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possible orientations of the peroxy group with respect to the ring (axial and equatorial) were considered. Finally, three possible orientations of the OOCH dihedral angle were included: gauche with the terminal peroxy oxygen towards the ring oxygen (G), trans (T), and gauche with the terminal oxygen away from the ring oxygen (G’). The detailed procedure and results of the conformational research are as follows. THFOOˑ: Conformational behavior of THF is a subject of extensive experimental and computational studies. It has been generally accepted that THF has two optically distinguishable stable conformers, namely, the bent and twisted structures, although there are disagreements on their relative energies.18-20 The bent and twisted structures are derivatives from the “envelope” and the “half-chair” ring structures of cyclopentane, respectively. The former conformer of THF belongs to the C2 point group, while the latter belongs to the Cs point group. They are close in energy and connected by pseudo-rotation with low barriers (
