Influence of Ether Functional Group on Ketohydroperoxide Formation

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A: Kinetics, Dynamics, Photochemistry, and Excited States

Influence of Ether Functional Group on Ketohydroperoxide Formation in Cyclic Hydrocarbons: Tetrahydropyran and Cyclohexane Jacob C Davis, Alanna L Koritzke, Rebecca L. Caravan, Ivan O. Antonov, Matthew G Christianson, Anna C Doner, David L. Osborn, Leonid Sheps, Craig Allen Taatjes, and Brandon Rotavera J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b12510 • Publication Date (Web): 13 Mar 2019 Downloaded from http://pubs.acs.org on March 13, 2019

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The Journal of Physical Chemistry

Influence of Ether Functional Group on Ketohydroperoxide Formation in Cyclic Hydrocarbons: Tetrahydropyran and Cyclohexane Jacob C. Davis1, Alanna L. Koritzke2, Rebecca L. Caravan3, Ivan O. Antonov3, Matthew G. Christianson2, Anna C. Doner2, David L. Osborn3, Leonid Sheps3, Craig A. Taatjes3, and Brandon Rotavera1, 2 1College

of Engineering, University of Georgia, Athens, GA 30602 of Chemistry, University of Georgia, Athens, GA 30602 3Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94551 2Department

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Abstract

12 13 14 15 16 17 18 19 20 21 22 23 24 25

1. Introduction 2. Experimental and computational approach 2.1. Multiplexed photoionization mass spectrometry (MPIMS) measurements 2.2. Initial radical distribution from tetrahydropyran + Ċl reactions 2.3. ab initio calculations 3. Results 3.1. Ketohydroperoxides 3.2. -Q̇OOH ring-opening 3.2.1. pentanedial + ȮH 3.2.2. vinyl formate + ethene + ȮH 3.2.3. 3-butenal + formaldehyde + ȮH 4. Discussion 5. Conclusion 6. References

26 27

Corresponding author. University of Georgia, College of Engineering | Department of Chemistry, Athens, GA, 30602,

USA Tel: (706) 542-1801; Email: [email protected].

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Abstract

2

Photolytically initiated oxidation experiments were conducted on cyclohexane and tetrahydropyran using

3

multiplexed photoionization mass spectrometry (MPIMS) to assess the impact of the ether functional group

4

in the latter species on reaction mechanisms relevant to autoignition. Pseudo-first-order conditions, with

5

[O2]0:[Ṙ]0 > 2000, were used to ensure that Ṙ + O2 → products were the dominant reactions. Quasi-

6

continuous, tunable vacuum ultraviolet light from a synchrotron was employed over the range 8.0 – 11.0

7

eV to measure photoionization spectra of the products at two pressures (10 Torr and 1520 Torr) and three

8

temperatures (500 K, 600 K, and 700 K).

9

Photoionization spectra of ketohydroperoxides were measured in both species and were qualitatively

10

identical, within the limit of experimental noise, to those of analogous species formed in n-butane oxidation.

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However, differences were noted between the two species in the temperature dependence of

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ketohydroperoxide formation. While the yield from cyclohexane is evident up to 700 K,

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ketohydroperoxides in tetrahydropyran were not detected above 650 K. The difference indicates reaction

14

mechanisms change due to the ether group the likely affecting the requisite Q̇OOH + O2 addition step.

15

Branching fractions of nine species from tetrahydropyran were quantified with the objective of determining

16

the role of ring-opening reactions on diminished ketohydroperoxide. The results indicate that products

17

formed from unimolecular decomposition of Ṙ and Q̇OOH radicals via concerted C–C and C–O -scission

18

are pronounced in tetrahydropyran and are insignificant in cyclohexane oxidation. The main conclusion

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drawn is that, under the conditions herein, ring-opening pathways reduce the already low steady-state

20

concentration of Q̇OOH, which in the case of tetrahydropyran prevents Q̇OOH + O2 reactions necessary

21

for ketohydroperoxide formation. Carbon balance calculations reveal that products from ring-opening of

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both Ṙ and Q̇OOH, at 700 K, account for >70% at 10 Torr and >55% at 1520 Torr. Three pathways

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confirmed that contribute to the depletion of Q̇OOH in tetrahydropyran include: (i) -Q̇OOH → pentanedial

24

+ ȮH, (ii) -Q̇OOH → vinyl formate + ethene + ȮH, (iii) -Q̇OOH → 3-butenal + formaldehyde + ȮH.

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Analogous mechanisms in cyclohexane oxidation leading to similar intermediates are compared, and on the

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basis of mass spectra results confirm that no such ring-opening reactions occur. The implication from the

27

comparison to cyclohexane is that the ether group in tetrahydropyran increases the propensity for ring-

28

opening reactions and inhibits the formation of ketohydroperoxide isomers that precede chain-branching.

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On the contrary, the absence of such reactions in cyclohexane enable ketohydroperoxide formation up to

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700 K and perhaps higher temperature.

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1. Introduction

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Understanding fundamental chemical kinetics of cyclic ethers is important for two primary reasons. First,

3

as products of degenerate chain-branching mechanisms during low-temperature combustion of alkanes,

4

insight into the formation and decomposition of cyclic ethers play a central role in modeling autoignition

5

chemistry1-2. Second, owing to advances in catalysis and synthetic chemistry, several cyclic ethers including

6

tetrahydrofuran3-7,

7

dimethyltetrahydrofuran6-7, 10, 15-16, and tetrahydropyran17 may substantively contribute to next-generation

8

biofuel objectives. The latter species, tetrahydropyran, is a lignocellulosic-derived biofuel17 analogous to

9

cyclohexane by replacement of a methylene group (–CH2–) with an ether functional group (–O–). The

10

presence of the ether group creates three distinct abstraction sites, labeled , , and  in Figure 1. Upon H-

11

abstraction, three distinct initial radicals are produced from tetrahydropyran (-tetrahydropyranyl, -

12

tetrahydropyranyl, and -tetrahydropyranyl), in contrast to cyclohexane for which only one distinct radical

13

is formed (cyclohexyl). The higher number of distinct initial radicals adds significant complexity to

14

understanding reaction mechanisms of tetrahydropyran due to the sheer increase in the number of reactions

15

and to the role of the ether functional group. Oxygen content in hydrocarbons is known to affect low-

16

temperature oxidation mechanisms and impacts autoignition chemistry as a result, e.g. in ethers

17

alcohols

18

dissociation energies on carbon sites adjacent to functional groups (i.e.  carbon), which enables facile H-

19

abstraction 24-25.

20-21,

2-methyltetrahydrofuran3-14,

and esters

22-23.

3-methyltetrahydrofuran7,

9,

11,

2,5-

18-19,

Part of the influence of oxygen in biofuels is due to lower C–H bond

O



 

20 21

Figure 1. Molecular structures of cyclohexane and tetrahydropyran with H-abstraction sites labeled.

22

Autoignition in combustion systems at temperatures below 1000 K is initiated by the reaction of O2 with

23

an organic radical Ṙ formed via H-abstraction from the fuel (RH), primarily by ȮH 1. The next step in the

24

process is an isomerization reaction of the resultant peroxy radical ROȮ into a carbon-centered

25

hydroperoxy-substituted radical, Q̇OOH. The propensity of hydrocarbons to undergo low-temperature

26

chain-branching depends on the fate of Q̇OOH, specifically on the balance between inhibiting reactions,

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which yield HOȮ, propagation reactions, which yield ȮH, and branching reactions derived from a second-

28

O2-addition step, Q̇OOH + O2. In the latter, upon isomerization of the adduct (ȮOQOOH) when the

29

abstracted H atom is  to the –OOH group, scission of that O–O bond yields ȮH and forms a carbonyl

30

group leading to the formation of ketohydroperoxide, HOOQ'=O. Figure 2 illustrates the molecular

31

structure of ketohydroperoxide isomers in cyclohexane and tetrahydropyran oxidation. Chain-branching Page 3 of 28 ACS Paragon Plus Environment


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subsequently unfolds from a second O–O bond-scission step, producing a second ȮH radical and an oxy

2

radical ȮQ'=O. Figure 3 outlines the degenerate chain-branching mechanisms for hydrocarbons. Reactions

3

that impede either the formation of Q̇OOH or that facilitate unimolecular decomposition reactions of

4

Q̇OOH lowers the rate of second-O2-addition and, ultimately, the degree of chain-branching during

5

combustion. O

OH

OH

O

O

(a)

O

(b) 2-hydroperoxy-cyclohexa-1-one (m/z 130)

6 7

O

5-hydroperoxy-2-oxotetrahydropyran (m/z 132)

Figure 2. Representative ketohydroperoxide isomers and corresponding mass-to-charge ratios from (a) cyclohexane oxidation, derived from a -Q̇OOH radical, and (b) from tetrahydropyran oxidation, derived from a -Q̇OOH radical. RH - H2O

+ OH R

alkene + R

+ O2 ROO

conjugate alkene + HOO (chain-inhibition)

QOOH + O2 OOQOOH

cyclic ether/carbonyl + OH (chain-propagation)

-scission products + OH (chain-propagation)

HOOQOOH

HOOQ=O + OH

OQ=O + OH + OH

(chain-branching)

8 9 10 11 12

Figure 3. Hydrocarbon oxidation paradigm depicting the degenerate chain-branching mechanism mediated by reactions involving Q̇OOH radicals. The bold, blue-colored text indicates a reaction pathway that depletes Q̇OOH radicals that may otherwise react with O2 to form ketohydroperoxide species (HOOQ'=O) leading subsequently to chain-branching. Analysis of products from Q̇OOH -scission is the focus of the present work.

13

Reaction pathways in Figure 3 are influenced by molecular structure including functional groups common

14

to biofuels such as alcohols, ketones, esters, and ethers. For example, the distribution of radicals from

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initiation reactions such as RH + ȮH → Ṙ + H2O favors the formation of  isomers in ethers because of Page 4 of 28 ACS Paragon Plus Environment

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the lower C–H bond energies 24-27. Depending on the type of functional group, the unpaired electron on 

2

carbon creates resonance-stabilized radicals, as in ketones 28-29 and esters, 30-31 that disfavor reactions with

3

O2, or facilitates chain-inhibiting steps, as in alcohols 32 and ethers 24, 26. One example of the latter, in the

4

case of cyclic ethers, is an increase in ring-opening rates of  radicals in tetrahydropyran 24, 27 relative to

5

ring-opening rates of initial radicals in cyclohexane.

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While several experimental and computational studies on cyclohexane oxidation exist at temperatures

7

below 1000 K 24, 33-37, a limited number of combustion-relevant studies exist for tetrahydropyran. Telfah et

8

al.

9

radicals, produced using Cl-initiated oxidation, to resolve the geometries of structural conformers. Quantum

10

chemical calculations were conducted to determine Franck−Condon factors for vibronic transitions and

11

indicated that ~80% of the peroxy radicals are conformers of -tetrahydropyranylperoxy, the majority of

12

which are axial. Dagaut et al. 39 measured ignition delay times in a shock tube and species profiles in a jet-

13

stirred reactor and also developed the first detailed chemical kinetics model for pressure and temperature

14

ranges of 2 – 10 atm and 800 – 1700 K, respectively. However, peroxy radical chemistry was not included.

15

Labbe et al.

16

pyrolysis and high-temperature ignition and flame properties of tetrahydropyran, and also constructed a

17

comprehensive chemical kinetics model covering a broader parameter space in temperature, pressure, and

18

fuel concentration than in Dagaut et al. 39. Under both pyrolysis and oxidation conditions, H-abstraction

19

reactions were found to be the principal removal pathway of tetrahydropyran, with abstraction favored at

20

the  carbon. The resultant radicals were postulated to decompose via -scission through a number of

21

channels, as evidenced by the detection of corresponding stable species using GC/MS.

22

Rotavera et al.

23

tetrahydropyran on chain-termination in comparison to cyclohexane. Below 700 K and 10 Torr, chain-

24

termination is more favorable in tetrahydropyran due to coupled effects of (1) lower C–H bond energy of

25

the -carbon, which leads to -tetrahydropyranyl being the dominant initial radical, and (2) the barrier to

26

direct HOȮ formation on the -tetrahydropyranyl + O2 surface being lower by approximately 5 kcal/mol.

27

With increasing temperature, however, competition from -tetrahydropyranyl ring-opening reduced the

28

flux through Ṙ + O2 and subsequent product formation thereafter – an effect compounded by the fact that

29

abstraction at the weakest C–H bond produces the initial radical most amenable to ring-opening.

30

The present work expands on Rotavera et al. 24 by quantifying branching fractions of intermediates formed

31

via Q̇OOH ring-opening reactions (cf. bold text in Figure 3) that diminish the overall production of

32

ketohydroperoxides in tetrahydropyran oxidation. The results are contextualized by comparison to

38

measured cavity ring-down spectra of the Ã ← X̃ electronic transitions of tetrahydropyranylperoxy

40

conducted speciation measurements in low-pressure flames. Tran et al.

24

27

studied the

employed MPIMS experiments to determine the influence of the ether group in



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analogous reactions in cyclohexane oxidation, for which intermediates from such reactions were not

2

detected. Three specific ring-opening mechanisms for tetrahydropyran are discussed, each of which are

3

derived from -Q̇OOH radicals. Other, minor reaction pathways related to Q̇OOH ring-opening are also

4

discussed.

5 6

Branching fractions were measured in chlorine atom-initiated oxidation experiments conducted using

7

MPIMS on tetrahydropyran and cyclohexane at 10 and 1520 Torr, where [O2] = 1.9 ∙ 1016 and 3.0 ∙ 1018

8

molecules cm–3, respectively, and from 500 – 700 K. More broadly, the aim is to examine the influence of

9

an ether functional group in cyclic hydrocarbons on the formation of ketohydroperoxides and related impact

10

on chain-branching propensity. Connecting product formation to specific Ṙ and Q̇OOH ring-opening

11

reactions provides the basis for determining the impact of the ether group.

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2. Experimental and computational approach

13

MPIMS experiments were conducted on cyclohexane and tetrahydropyran to quantify branching fractions

14

of initial radicals forming species from ring-opening reactions of either Ṙ or Q̇OOH radicals via chain-

15

propagating steps coincident with ȮH. Species concentrations were quantified and the results were utilized

16

to calculate the flux of initial radicals forming ring-opened products. In the case of tetrahydropyran, because

17

of the three distinct radicals formed, structure-activity relations (SAR) were applied to the RH + Ċl → Ṙ +

18

HCl reaction in order to estimate the branching ratio of , , and  radicals. Complementary ab initio

19

calculations consisting of stationary point energies on pentanedial were also conducted to draw inference

20

into the experimental observations on dissociative ionization and fragment ion structure.

21

2.1. Multiplexed photoionization mass spectrometry (MPIMS) measurements

22

The MPIMS experiments were conducted in a 1.05-cm diameter slow-flow quartz reactor 41 using pulsed-

23

photolytic chlorine atom-initiated oxidation, highly diluted in He, with constant initial reactant number

24

densities (Table 1) at two pressures (10 Torr and 1520 Torr) and three temperatures (500 K, 600 K, and

25

700 K). High-purity O2 and He and were co-flowed with separate gas-phase mixtures of He with reactant

26

(i.e. cyclohexane or tetrahydropyran) and with oxalyl chloride, (COCl)2. Concentrations in the reactor were

27

controlled by flow rates of the four gases that were defined using thermal-based mass flow controllers.

28

Photolysis of oxalyl chloride, using unfocused 248-nm light from an excimer laser, generated Ċl atoms

29

homogeneous along both the radial and longitudinal axes of the reactor. The Ċl atoms react via RH + Ċl →

30

Ṙ + HCl to form the initial Ṙ radicals, which subsequently undergo pseudo-first-order reaction with O2. To

31

minimize side chemistry unrelated to Ṙ + O2, pseudo-first-order conditions were employed for RH + Ċl


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1

such that Ṙ + Ċl reactions remain negligible, and the > 103 excess of [O2] relative to [Ċl] forces Ṙ + O2 to

2

be the dominant reaction.

3 4 5

Table 1. Initial number densities of reactants (molecules · cm–3) and pressures utilized for Cl-atom-initiated oxidation experiments on tetrahydropyran and cyclohexane. [He] dilution balance is not listed. Pseudo-first-order conditions were utilized. [RH]0:[Ċl]0 > 20 and [O2]0:[Ṙ]0 > 2000.

pressure

[RH]0

[O2]0

[Ċl]0

10 Torr

1.9 · 1014

1.9 · 1016

8.8 · 1012

1520 Torr

5.0 · 1015

3.0 · 1018

9.8 · 1013

6 7

Ċl atoms were produced using 248-nm photolysis of oxalyl chloride (𝜎248 nm = 2.66 10–19 cm2 molec.–1 42-

8

43),

9

instantaneously with a quantum yield of one by (COCl)2 + h → Ċl + CO + ClCO*, and the second Ċl atom

10

is generated from dissociation of chemically activated ClCO via ClCO* → Ċl + CO on timescales

11

appreciably short relative to the oxidation timescales of interest. Under the experimental conditions herein,

12

collisional quenching of ClCO* is unimportant (i.e. the ClCO* dissociation rate is shorter than the time

13

between collisions) and the timescale of the second step is ~1 s. The co-product of (COCl)2 photolysis,

14

CO, is unreactive at the temperatures and pressures herein and is not relevant to the oxidation reactions

15

studied. Depletion timescales of Ċl by RH under the concentrations listed in Table 1 are ~ 20 s at 10 Torr

16

and ~ 1 s at 1520 Torr, assuming kRH + Ċl = 2.2 · 10–10 cm3 · molec.–1 · s–1 44. Initial depletion percentages

17

of [RH]0 by Ċl in both cyclohexane and tetrahydropyran remained near 5%, determined using 13C time

18

profiles (Supplemental Material S1). Secondary depletion timescales of RH, predominantly by OH

19

radicals formed from R + O2, were evident in the 13C time profiles and led to additional RH consumption

20

of approximately 5 – 10%.

21

The majority of the photoionization experiments were conducted at the Chemical Dynamics Beamline of

22

the Advanced Light Source 45-46 over a photon energy range of 8.0 – 11.0 eV using 50-meV intervals for

23

tetrahydropyran oxidation, and similarly for cyclohexane oxidation with the exception of a lower bound of

24

8.5 eV. Measurements were also conducted at Sandia National Laboratories using the Lyman-alpha line

25

from a hydrogen-discharge lamp (nominally 10.2 eV). Products from the Ċl-initiated oxidation reactions

26

exit the quartz reactor through a 600-m side orifice into a detector region maintained at ~10–8 Torr forming

27

a near-effusive molecular beam, which is then collimated by a 1.5-mm diameter skimmer positioned

28

approximately 20 mm downstream from the side orifice. Cations, consisting of both parent and fragment

29

ions, are formed by orthogonally intersecting the collimated molecular beam with quasi-continuous photons

which occurs via a single-photon process and a two-step mechanism 42. The first Ċl atom is generated



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produced either from tunable synchrotron radiation or from the H2-discharge lamp and then detected using

2

an orthogonal-acceleration time-of-flight mass spectrometer equipped with microchannel plates.

3

Photoionization mass spectra were recorded at 20-s time intervals over a span of 170 ms (20-ms pre-

4

photolysis and 150-ms post-photolysis) and normalized to photocurrent, which changes with photon energy.

5

The time-dependent mass spectra were measured using a 4-Hz photolysis repetition rate of the excimer

6

laser and signal-averaged at a given photon energy. In order to ensure that no residual gases from the

7

previous reaction remained, flow rates were set such that the volume inside of the reactor was completely

8

replenished with reactants in between laser pulses, a process verified by observing a baseline pre-photolysis

9

signal of zero for m/z corresponding to product masses. Sequential recording of the time-dependent mass

10

spectra at discrete photon energies led to a three-dimensional measurement (m/z, time, photon energy) of

11

products from the oxidation reactions. The time dimension comes from the position of the reacting gases

12

relative to the side orifice at the instant the Ċl atoms are generated in the reactor, and the photon energy

13

dimension comes from the range and energy step size set for the experiment. Background-subtraction is

14

applied to the mass spectra to remove non-time-resolved pre-photolysis signals, producing difference mass

15

spectra. Integration of the difference mass spectra at a given m/z over a selected range of time yields

16

photoionization spectra, and integration over a selected photon energy range yields time histories.

17

In order to quantify species concentrations and assign isomeric contributions to the peaks in the mass

18

spectra, absolute photoionization cross-sections 𝜎(𝐸) were measured using 25-meV steps for several

19

species in separate experiments as reference spectra (Supplementary Material S2): cyclohexane,

20

cyclohexene, tetrahydropyran, 3,4-dihydro-2H-pyran, 3,6-dihydro-2H-pyran, 3-butenal, and pentanedial.

21

With the exception of pentanedial, for which quantification required additional analysis (Supplemental

22

Material S3), binary mixtures with He were prepared and delivered via flow controllers. Cross-sections

23

were measured at 600 K in all cases with the exception of pentanedial for which 500 K and 700 K

24

measurements were taken to check for any temperature dependence of fragment ion formation. The 𝜎(𝐸)

25

of species herein are defined relative to propene in accord with the procedure described in 47, where the

26

absolute photoionization cross-section from Person and Nicole

27

(10.325 eV) = 10.473 Mb. Because the normalization is the same for the spectra in both the reference

28

measurements and oxidation measurements, an energy-dependent scaling factor is not used.

29

Branching fractions are defined as the concentration of a given species relative to the initial radical number

30

density [Ṙ]0, and were measured using least-squares fitting of the corresponding absolute photoionization

31

spectra time-integrated 30 ms post-photolysis (i.e. 30 ms of reaction time). The equation for quantifying

32

branching fractions is derived in Supplemental Material S4. Because of the pseudo-first-order conditions

48

at a single photon energy is used: 𝜎


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1

employed, [RH]0:[Ċl]0 = 60, the initial depletion of both tetrahydropyran and cyclohexane is due to Ċl

2

atoms being consumed via abstraction exclusively by RH to produce Ṙ, i.e. [Ṙ]0 ≌ [Ċl]0. Using a laser

3

power of 1.5 W (photolysis flux ~124 mJ cm–2) and cross-section of oxalyl chloride 42-43, [Ṙ]0 ~ 8.8 · 1012

4

at 10 Torr and ~9.8 · 1013 at 1520 Torr.

5

2.2. Initial radical distribution from tetrahydropyran + Cl reactions

6

Structure-activity relationship (SAR) calculations were conducted for tetrahydropyran + Ċl → Ṙ + HCl in

7

order to approximate the distribution of initial Ṙ radicals , , and  (cf. Figure 1). The approximation

8

relies on the modification of a base rate coefficient using empirically derived substituent factors F(x) to

9

account for the influence of moieties adjacent to the site of H-abstraction. The rate coefficient at 298 K for

10

cyclohexane + Ċl → cyclohexyl + HCl of Aschmann and Atkinson

11

substituent factors for F(–CH2–) of 0.79 49 and 3.72 for F(–O–) were used. The latter substituent factor is

12

derived from Aschmann and Atkinson 50 assuming the same reduction as for F(–CH2–) when H-abstraction

13

from an ether group is by Ċl compared to ȮH 49. The SAR-predicted rate coefficient for tetrahydropyran +

14

Ċl → Ṙ + HCl (4.78 · 10–10 molec. cm–3) is within a factor of approximately two compared with the

15

measurements of Giri and Roscoe 51 and Alwe et al. 52, and the distribution of Ṙ is dominated by  radicals,

16

which is consistent with the ab initio calculations of Ballesteros et al. 44. On a per-C–H bond basis, the

17

branching ratio for tetrahydropyranyl radicals is approximated as :: = 0.76:0.16:0.08, which is similar

18

to the distribution in Tran et al. 27. The results are also consistent with the spectroscopic results from Telfah

19

et al. 38 in which ~80% of the peroxy radicals were -ROȮ. Using a different method of approximation, the

20

 radical remains dominant in H-abstraction reactions from tetrahydropyran by ȮH 53. Since H-abstraction

21

reactions by Ċl are weakly dependent on temperature 54 and pressure 55-56, the abovementioned branching

22

ratio is similar over the range of experimental conditions herein.

23

2.3. ab initio calculations

24

Stationary point energies were calculated for isomerization of pentanedial cation to examine

25

photodissociation pathways leading to observed fragment ions. The saddle point geometries and vibrational

26

analyses were conducted with Gaussian 09 57 using M06-2x functional with a 6-311++G(d,p) basis set. The

27

energies of all reactants, saddle points, and products were also calculated and conformational searches were

28

executed to determine the lowest energy structures including axial and equatorial conformers. Confirmation

29

of saddle point geometries rested on verification of single imaginary frequencies and on intrinsic reaction

30

coordinate (IRC) calculations showing maximum energy at the coordinates of the optimized geometry.

31

Adiabatic ionization energies were calculated for several species, including ketohydroperoxide isomers,

32

(Table S1) using the CBS-QB3 composite method within Gaussian 09 57.

49

(3.07 · 10–10 molec. cm–3) and



1

3. Results

2

The difference mass spectra in Figure 4, measured from Cl-initiated oxidation of tetrahydropyran at 600 K

3

and 1520 Torr, highlights the primary mass peaks of interest. Mass spectra for all conditions are provided

4

in the Supplemental Material (S5). The negative signals in the difference mass spectra are due to

5

background subtraction 20-ms pre-photolysis and reflect depleted tetrahydropyran [C5H10O]+ and related

6

fragment ions. Chlorinated products from Ċl-addition to R (i.e. R35Cl and R37Cl) were not observed in either

7

case, confirming that peaks appearing in the mass spectra are from product formation via Ṙ + O2 and

8

associated fragment ions or by other relevant reactions. Calibration of the mass spectra was performed using

9

species of known molecular formula and exact mass, spanning m/z 28 (ethene) to m/z 118 (ROOH). Exact

10

mass determinations for other species, where the molecular formula was unknown, used calibrated

11

coefficients that convert time-of-flight to mass. 1.0 600 K 2 atm

Ion Signal (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 29

m/z 72 m/z 100 0.5 m/z 70 m/z 30

m/z 132

0.0 20

40

60

80

100

120

140

m/z

12 13 14 15

Figure 4. Difference mass spectrum produced from Ċl-initiated oxidation of tetrahydropyran at 600 K and 2 atm, integrated 30-ms post-photolysis. The photon energy range is 8.5 – 11.0 eV. m/z 132 is ketohydroperoxide ion signal, identified by exact mass 132.042 (C5H8O4).

16

In addition to the oxidation experiments, separate measurements were conducted in absence of oxygen flow

17

to assess whether any formation of products resulted from ring-opening of initial radicals (Figure S6). With

18

oxygen flow restricted, nominal oxygen concentration in the flow cell is estimated at 1012 using the

19

minimum pressure (10–7 Torr) and 21% O2 from ambient air, which is more than 104 lower than in the

20

concentration level used in the oxidation experiments. Three species in particular that are connected to ring-

21

opening reactions of Q̇OOH radicals, discussed in Section 3.2, are ethene, formaldehyde, and 3-butenal.

22

All were observed in absence of O2 flow and the time profile of formaldehyde displayed a significant

23

dependence on [O2] indicating that some portion quantified in the oxidation experiments is attributed to

24

ring-opening of initial radicals, in addition to oxidation reactions (i.e. Q̇OOH ring-opening). The time


Page 11 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

profiles of ethene and 3-butenal were identical over the 1012 – 1017 range of oxygen concentration (Figure

2

S7).

3

Branching fractions of intermediates that are formed via ring-opening reactions in tetrahydropyran

4

oxidation were quantified, including ethene, formaldehyde, propene, acrolein, 1-butene, and 3-butenal

5

(Table 2). All of the quantified species exhibit a positive temperature dependence, with the exception of

6

3,4-dihydro-2H-pyran due to ring-opening reactions of -tetrahydropyranyl becoming competitive with

7

O2-addition above ~650 K 24. Similar results were determined for cyclohexane (Table S2), although the

8

only main product was cyclohexene (i.e. no ring-opening products). Ion signal at m/z 70, potentially

9

indicative of 3-butenal, was also detected. However, low signal-to-noise ratios in the photoionization

10

spectra prevented definitive assignment and time profiles suggested secondary chemistry as the source

11

(Supplemental Material S8). The focus on ring-opening products provides a means of identifying

12

reactions that inhibit Q̇OOH + O2, and thereby diminish ketohydroperoxide formation either through ring-

13

opening of Ṙ or of Q̇OOH. In the ensuing sections, reaction mechanisms that explain several of the main

14

peaks observed in Figure 4 are discussed in the context of ring-opening reactions. Results from the

15

mechanism analysis are then compared to analogous reactions in cyclohexane that hypothetically lead to

16

the same products, yet are not observed. The comparison highlights the effect of the ether functional group

17

on ring-opening reactions and subsequent implications for ketohydroperoxide formation.

18 19 20 21 22

Table 2. Branching fractions of species quantified (with absolute uncertainties of ±20%) relative to total initial concentration of tetrahydropyranyl radicals. 3,4- and 3,6-dihydro-2H-pyran form via Ṙ + O2 reactions 24, while all other species form via ring-opening reactions. Carbon balance calculations reflect the percentage of products quantified relative to the total carbon in the initial radical population. Branching fractions were calculated based on ion signals integrated 30- and 60-ms post-photolysis in the 10 Torr and 2 atm experiments, respectively. 500 K 10 Torr (2 atm)

600 K 10 Torr (2 atm)

700 K 10 Torr (2 atm)

methyl (m/z 15)

0.1 (0.0)

0.1 (0.0)

0.3 (0.0)

ethene (m/z 28)

0.3 (0.1)

2.0 (0.2)

19.2 (1.6)

formaldehyde (m/z 30)

1.4 (0.4)

6.9 (0.9)

29.8 (19.1)

propene (m/z 42)

0.0 (0.0)

0.0 (0.0)

7.8 (17.1)

acrolein (m/z 56)

0.0 (0.0)

0.0 (0.2)

1.5 (0.4)

1-butene (m/z 56)

0.0 (0.0)

0.0 (0.0)

0.7 (0.3)

3-butenal (m/z 70)

1.0 (0.5)

6.6 (1.6)

9.2 (18.4)

3,4-dihydro-2H-pyran (m/z 84)

5.2 (0.4)

15.9 (4.5)

13.4 (8.5)

3,6-dihydro-2H-pyran (m/z 84)

0.4 (0.2)

3.6 (0.7)

4.3 (0.2)

carbon balance

8.3% (1.6%)

35.1% (8.0%)

86.2% (65.6%)

species formed via ring-opening:

2.7% (1.1%)

15.6% (2.8%)

68.5% (56.9%)

species



1 2

3.1. Ketohydroperoxides

3

Time profiles of ketohydroperoxide ion signals produced from both tetrahydropyran and cyclohexane

4

oxidation at 2 atm are compared in Figure 5. Similar to observations in n-butane 58-62, ketohydroperoxides

5

are not formed at 500 K over the 60-ms timeframe of the experiment, from either species, because of the

6

multiple elementary reactions that are involved (i.e. two isomerization steps, second-O2-addition, -

7

scission). Additionally, in the case of tetrahydropyran, the barrier height on the -tetrahydropyranyl + O2

8

surface favors direct formation of conjugate alkene + HOȮ, which is enhanced at 500 K and consumes Ṙ

9

radicals that may otherwise undergo second-O2-addition reactions 24. With increasing temperature, the rates

10

of elementary reactions leading to ketohydroperoxide increase and, at 600 K, both tetrahydropyran and

11

cyclohexane produce time-resolved ion signal at m/z 132 and m/z 130, which upon exact mass analysis

12

indicates molecular formulae C5H8O4 and C6H10O3, respectively. At the highest temperature (700 K),

13

ketohydroperoxide formation remains evident in cyclohexane oxidation with a notable increase in the total

14

decomposition rate (Figure 5b). However, the ion signal from tetrahydropyran oxidation at m/z 132

15

becomes zero, indicating that one or more of the elementary steps preceding the formation of

16

ketohydroperoxide are inhibited including ring-opening of Ṙ or of Q̇OOH radicals. Other possibilities

17

include increased rates of reverse reactions, i.e. ȮOQOOH ⇌ Q̇OOH + O2, although ring-opening reactions

18

were responsible for a shift in the reaction mechanism of HOȮ formation in tetrahydropyran 24. Despite the

19

absence of cross-sections, assuming similar magnitudes and noting that the ion signals in Figure 5 are

20

normalized to the total number of laser photolysis shots, the higher signal-to-noise ratio in Figure 5b

21

implies that ketohydroperoxide concentration is higher in cyclohexane oxidation than in tetrahydropyran

22

indicating potential mechanistic differences connected to the ether functional group in the latter species. 0.6

0.4

2 atm

0.2

0.0

0

1.0

(a)

500 K 600 K 700 K

m/z 130 Ion Signal (a.u.)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 29

20

40

60

0.8 0.6

2 atm

0.4 0.2 0.0

0

Time (ms)

23 24 25

(b)

500 K 600 K 700 K

20

40

60

Time (ms)

Figure 5. Ion signals of ketohydroperoxide time histories in oxidation of (a) tetrahydropyran and (b) cyclohexane at 2 atm. Ketohydroperoxides are not detected in tetrahydropyran oxidation at 700 K, indicating a mechanistic difference with cyclohexane.


Page 13 of 29

1

In order to examine the temperature dependence of the m/z 132 ion signal from tetrahydropyran on a more-

2

discretized scale, a series of experiments were conducted in 25-K increments from 500 – 700 K using a

3

hydrogen-discharge lamp. Time-dependent ion signal was measured for the same reactant conditions as in

4

Table 1. Total ion counts (Figure 6) were determined from signal integration over 131.94 < m/z < 132.14

5

(m of 0.1 amu around the nominal 132.04) 15-ms post photolysis. For each temperature, integrated ion

6

counts were normalized to the number of photolysis laser shots to avoid bias in overall signal level.

7

Consistent with the time profiles in Figure 5, ion signal is absent at 500 K and 700 K. Notably, the peak in

8

ketohydroperoxide formation occurs near 625 K – identical to the peak temperature in n-butane 60.

500 Ion Counts (Normalized)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400 300

m/z 132 (C5H8O4) tetrahydropyran 3 · 1018 [O2] 2 atm

200 100 0 500

550

600

650

700

Temperature (K)

9 10 11 12

Figure 6. Laser-shot-normalized integrated ion counts for m/z 132 in tetrahydropyran oxidation indicating peak ketohydroperoxide formation at 625 K.

13

Explicit determination of the isomeric structure of ketohydroperoxide, from either tetrahydropyran or

14

cyclohexane, was not possible since photoionization spectra are unknown. In order to provide some

15

inference as to potential contributors to the m/z 132 and m/z 130 signal, adiabatic ionization energy

16

calculations were conducted at the CBS-QB3 level for all ten isomers derived from tetrahydropyran and for

17

all three derived from cyclohexane (S9). However, the calculated ionization thresholds depended strongly

18

on ring conformation as well as on the axial/equatorial position and dihedral angle of the –OOH group. For

19

example, the two structures in Figure 7 span a range of ionization energies of ~500 meV. Similarly, for the

20

ortho- isomer in cyclohexane, i.e. where the carbonyl and –OOH groups are located on adjacent carbon

21

atoms, a difference of ~300 meV was calculated (S10).



(a)

1 2 3

(b)

Figure 7. Optimized structures for two conformers of a ketohydroperoxide isomer, 3-hydroperoxy-2oxotetrahydropyran. Adiabatic ionization energies are (a) 8.98 eV (b) 9.49 eV, which indicates that the range of ~500 meV is due to the spatial orientation of the –OOH group.

4 5

Photoionization spectra of the ketohydroperoxide isomers were measured in 50-meV steps over the range

6

8.0 – 11.0 eV for tetrahydropyran oxidation and similarly from 8.5 – 11.0 eV for cyclohexane oxidation.

7

The spectra from both systems overlap closely with one another and with ketohydroperoxide signal from

8

n-butane in Eskola et al. 60 and Battin-Leclerc et al. 62 (Figure 8). Cyclohexene oxidation also produces a

9

nearly identical photoionization spectrum of ketohydroperoxide 63. The signal-to-noise ratio in the present

10

work precludes definitive assignment of onset energy for either signal. However, ion signal first exceeds

11

the noise level near ~9.2 eV. Above the ionization threshold, the similarity in the spectra is evident and may

12

indicate that the electron removed from the neutral species during ionization is derived from either the

13

carbonyl group or the –OOH group, which is common among the different species. Ketohydroperoxide Ion Signal (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 29

14

10

tetrahydropyran cyclohexane n-butane (Eskola et al.) n-butane (Battin-Leclerc et al.)

5

0 8.5

9.0

9.5

10.0

10.5

11.0

Photon Energy (eV)

15 16 17 18

Figure 8. Photoionization spectra of ketohydroperoxide isomers formed in tetrahydropyran and cyclohexane oxidation. 1520 Torr, 600 K, [O2] = 3.0 ∙ 1018 molecules cm–3. The spectra exhibit similarity to ketohydroperoxides produced from n-butane oxidation under similar conditions in both Eskola et al. 60 and in Battin-Leclerc et al. 62.

19

Comparison of the results in Figure 5 between cyclohexane and tetrahydropyran indicates a shift in the

20

reaction mechanism in the latter species due to the presence of the ether functional group. More specifically,

21

the absence of ketohydroperoxide at 700 K is likely due to the presence of reactions facilitated by the ether Page 14 of 28 ACS Paragon Plus Environment

Page 15 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

group that reduce the already low steady-state concentration of Q̇OOH, which subsequently reduces the

2

reaction rate of the second-O2-addition step. The sections below describe several reaction mechanisms

3

involving Ṙ and Q̇OOH radicals that prevent ketohydroperoxide formation. In all cases, the branching

4

fractions for the species involved increase with temperature and are appreciable at 700 K, where the carbon

5

balance of products from ring-opening accounts for >70% at 10 Torr and >55% at 1520 Torr (cf. Table 2).

6

3.2. -Q̇OOH ring-opening

7

The depletion of Q̇OOH radicals restricts the formation of ketohydroperoxide, and in the case of

8

tetrahydropyran occurs via ring-opening pathways. Three ring-opening mechanisms involving -Q̇OOH,

9

produced via favorable six-membered transition states in ROȮ isomerization, are discussed: (i) -Q̇OOH

10

→ pentanedial + ȮH, (ii) -Q̇OOH → vinyl formate + ethene + ȮH, (iii) -Q̇OOH → 3-butenal +

11

formaldehyde + ȮH. A fourth channel is also postulated involving acrolein, although likely a minor channel

12

given the small branching fraction of ~1% (cf. Table 2): -Q̇OOH → acrolein + oxirane + ȮH. With the

13

exception of one mechanism, all are initiated from -tetrahydropyranyl, which is consistent with the SAR

14

predictions (Section 2.1) of initial radical yields. The nomenclature in the ensuing sections designating the

15

location of the –OOH group and the unpaired electron adopts the labeling in Figure 1 and specifies the

16

location of the former first and the latter second. For example, -'-Q̇OOH designates the Q̇OOH species

17

where both the –OOH group and the unpaired electron are adjacent to the ether functional group and the

18

prime notation indicates the latter is on the opposite side of the ring.

19

3.2.1. pentanedial + ȮH

20

Ion signal at m/z 100 is identified by the molecular formula C5H8O2, which includes cyclic species (ethers,

21

ketones, and unsaturated alcohols) and (acyclic) pentanedial. Figure 9 compares the ion signal measured

22

in the tetrahydropyran oxidation experiments at 10 Torr for all three temperatures to the photoionization

23

spectrum of pentanedial, which forms via Q̇OOH ring-opening (Figure 10) and inhibits second-O2-addition

24

and subsequent ketohydroperoxide formation. Similar results were obtained at 2 atm (Figure S11). Evident

25

in Figure 9 is the temperature independence of the m/z 100 spectrum, implying either that a small number

26

of species are present or a significant degree of similarity in the photoionization spectra of the 14 potential

27

C5H8O2 isomers. Ion signal is apparent near 9.0 eV and unaccounted for up to ~9.8 eV. While the species

28

producing the signal are unknown, CBS-QB3 calculations indicate that two cyclic ethers ionize near the

29

experimental threshold of ~9.0 eV, accounting for the ~0.08 eV uncertainty in the method – the three-

30

membered bicyclic 2,7-dioxabicyclo[4.1.0]heptane (9.13 eV) and the four-membered bicyclic 2,7-

31

dioxabicyclo[4.1.0]heptane (9.12 eV). The complete list of ionization energies for cyclic ethers of

32

tetrahydropyran is included in the Supplemental Material (S12). Because of unknown photoionization



1

spectra for the remaining m/z 100 isomers, branching fractions of pentanedial were not determined.

2

However, while the photoionization spectrum of m/z 100 is not solely reproduced by pentanedial,

3

significant overlap is evident, which lends confidence that the ring-opening mechanism is consuming the

4

-'-Q̇OOH radical. A similar reaction was a dominant pathway in tetrahydrofuran oxidation, leading to

5

butanedial + ȮH 26. 10 m/z 100 Ion Signal (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 29

500 K 600 K 700 K pentanedial 10 Torr

5

0 8.5

9.0

6 7 8

9.5

10.0

10.5

11.0

Photon Energy (eV)

Figure 9. Photoionization spectra of m/z 100 produced from tetrahydropyran oxidation at 10 Torr overlaid with the spectrum for pentanedial. O

O

OH O

O

+

OH

9 10

Figure 10. Ring-opening mechanism of -'-Q̇OOH forming pentanedial and ȮH.

11

In addition to the overlap of the spectra in Figure 9, confirmation of pentanedial formation is provided

12

from photoionization cross-section measurements that revealed significant photofragmentation, particularly

13

at m/z 72. Other fragment ions appear at m/z 54, 56, 57, 58, 67, 68, 81, 82, and 83. At the highest photon

14

energy utilized in the oxidation experiments, 11.0 eV, the peak ratio in the mass spectrum between the

15

fragment ion at m/z 72 and the parent ion at m/z 100 is approximately ~60:1 (Figure 11). Accounting for

16

the abundance of other fragment ions, some of which are of similar magnitude to m/z 72 (Figure S13), the

17

decomposition of pentanedial cation is a dominant process that results in appreciably small ion signal at the

18

parent mass (m/z 100).


Page 17 of 29

1.0

500 K 5 Torr

H

O

Ion Signal (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.5 O

O

0.0 70

100

102

m/z

1 2 3

72

Figure 11. Photoionization mass spectrum of pentanedial at 11.0 eV (500 K, 5 Torr) highlighting the parent ion signal at m/z 100 and the fragment ion signal at m/z 72.

4 5

To explain the photodissociation propensity of pentanedial, stationary point energies were calculated to

6

determine the barrier height leading to the production of a fragment ion of m/z 72 from pentanedial, which

7

requires the loss of a neutral moiety of m/z 28. Considering the loss of carbon monoxide (CO, m/z 28), a

8

fragment ion of the molecular formula C4H8O (m/z 72) forms. The mechanism responsible for facile

9

pentanedial fragmentation into m/z 72 is depicted in Figure 12, which is supported by quantum chemical

10

calculations. A barrier height of 3.0 kcal/mol is calculated at the M06-2x/6-311++G(d,p) level of theory for

11

isomerization of pentanedial cation to m/z 100 cation [H⋯OCH(CH2)3Ċ=O]+, which is 7.5 kcal/mol more

12

stable. The isomerized cation decomposes readily to form [H⋯OCHCH2CH2ĊH2]+ (m/z 72) and CO, similar

13

to the pathway leading to pent-1-al-1-yl from ring-opening the neutral radical -tetrahydropyranyl 24.

3 kcal/mol

14 15 16

Figure 12. Pentanedial cation isomerization via a 3.0 kcal/mol barrier leading to the formation of fragment ion [H⋯OCHCH2CH2ĊH2]+ (m/z 72) and neutral CO (m/z 28).

17 18

The photoionization cross-section experiments on pentanedial were conducted at several temperatures and

19

fragment ion spectra at m/z 72 did not display temperature dependence. Photoionization spectra of the m/z

20

72 fragment ion at 500 K and 700 K are overlaid in Figure 13 against the ion signal at m/z 72 from the

21

oxidation experiments. Noting that ion signal in the latter case shifts towards lower energies with increasing



1

temperature, and that fragment ion spectra are temperature independent, the signal from ~9.3 – 10.2 eV in

2

Figure 13 is due to either to neutral species or to different fragment ions from species that are not produced

3

at 500 K. However, at 500 K, pentanedial fragment ion is the sole source of m/z 72 signal, which indicates

4

that Q̇OOH ring-opening occurs despite the relatively low temperature. 10


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 29

pentanedial fragment ion (500 K) pentanedial fragment ion (700 K) 500 K 600 K 700 K

5

0 9.0

9.5

10.5

11.0

Photon Energy (eV)

5 6 7

10.0

Figure 13. Temperature dependence of relative photoionization spectrum for m/z 72 fragment ion measured in tetrahydropyran oxidation at 10 Torr; identical spectra were measured at 2 atm.

8 9

3.2.2. vinyl formate + ethene + ȮH

10

One plausible contribution to the m/z 72 ion signal below 10.2 eV at 600 K and 700 K in Figure 13 is vinyl

11

formate (C3H4O2, m/z 72.021), which may form coincident with ethene and ȮH via the Q̇OOH ring-opening

12

mechanism in Figure 14. The photoionization spectrum of vinyl formate is overlaid in Figure 15 with ion

13

signal from the oxidation experiments and with the pentanedial fragment ion. The onset energy for vinyl

14

formate of ~9.6 eV coincides with the increase in ion signal at 600 K and given the spectral shape may also

15

contribute to ion signal at 700 K, particularly since the pathway in Figure 14 is expected to become

16

increasingly relevant with higher temperature. Lending additional support to the presence of vinyl formate

17

is the detection of ethene (Figure 16a), the co-product with vinyl formate, on the same kinetic timescale

18

(Figure 16b). Ethene is a significant product at 10 Torr where the initial radical population yields ~19%

19

and the branching fraction, at both pressures, exhibits positive temperature dependence (cf. Table 2). O

O

OH

O

O

+

+

OH

20 21

Figure 14. Ring-opening mechanism of -'-Q̇OOH yielding vinyl formate, ethene, and hydroxyl radical.


Page 19 of 29


6

vinyl formate pentanedial fragment ion 500 K 600 K 4 700 K 10 Torr O

O

9.5

10.0

10.5

H

11.0

Photon Energy (eV)

1 2 3

O

2

0 9.0

Figure 15. Photoionization spectra of m/z 72 from tetrahydropyran oxidation at 10 Torr overlaid with the vinyl formate and the m/z 72 pentanedial fragment ion spectra. 1.0

10

(b) Ion Signal (Normalized)

(a)

Ion Signal (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


m/z 28 ethene 5

0 9.5

600 K 10 Torr

0.8 0.6 m/z 28

0.4 m/z 72 0.2

600 K 10 Torr

0.0

10.0

10.5

11.0

0

Photon Energy (eV)

10

20

30

Time (ms)

4 5

Figure 16. (a) Photoionization spectrum of ethene overlaid with m/z 28 ion signal. (b) Correspondence of time profiles for m/z 28 (ethene) and m/z 72 indicating that both species form on the same kinetic timescale.

6

3.2.3. 3-butenal + formaldehyde + ȮH

7

A third significant ring-opening pathway affecting ketohydroperoxide formation in tetrahydropyran, either

8

by consuming Q̇OOH or Ṙ radicals, involves the formation of 3-butenal (m/z 70) in a chain-propagating

9

step. Figure 17a compares the photoionization cross-section of 3-butenal with m/z 70 ion signal at 10 Torr.

10

The comparison is identical at 2 atm, meaning that no other contributions are evident at either pressure and

11

3-butenal is the only species producing signal at m/z 70. Three possible channels for producing 3-butenal

12

are depicted in Figures 18a – 18c in which formaldehyde (CH2O) and ȮH are co-products. Additional

13

confirmation of the ring-opening pathways in Figure 18 is the identical time profiles of formaldehyde and

14

3-butenal (Figure 17b), indicating that both species form from the same reaction.



15

1.5

500 K 600 K 700 K 3-butenal

10

Ion Signal (a.u.)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a) O

5

0 8.5

9.0

9.5

10.0

10.5

600 K 10 Torr

1.0 (b) 0.5 formaldehyde 3-butenal 0.0

11.0

0

Photon Energy (eV)

1 2 3

Page 20 of 29

10

20

30

Time (ms)

Figure 17. (a) Photoionization spectrum of 3-butenal overlaid onto m/z 70 ion signal measured in tetrahydropyran oxidation at 10 Torr and time-integrated 30 ms post-photolysis. (b) Time histories of formaldehyde and 3-butenal; 600 K, 10 Torr.

4 O

O

5 6 7 8 9

O

OH

OH

O +

CH2O

+

OH

O

O

OH

O +

CH2O

+

OH

Figure 18b. Ring-opening mechanism of -Q̇OOH leading to 3-butenal (m/z 70) via unimolecular decomposition via concerted -scission reactions. O

O

O

13 14 15

O

Figure 18a. Ring-opening mechanism leading to 3-butenal (m/z 70). The reaction initiates via oxidation of pentanal5-yl, formed by ring-opening of -tetrahydropyranyl. The ring-opened radical subsequently reacts with O2 and isomerizes to a Q̇OOH via 6-membered transition state. The -Q̇OOH then undergoes concerted -scission reactions.

O

10 11 12

O

+ O2

OH

O

OH

O +

CH2O

+

OH

Figure 18c. Ring-opening mechanism of -Q̇OOH leading to 3-butenal (m/z 70) via unimolecular decomposition via concerted -scission reactions.

16 17

4. Discussion

18

The results above, namely the ketohydroperoxide time profiles and subsequent reaction mechanisms

19

consuming Ṙ and Q̇OOH radicals, points to mechanistic differences that restrict second-O2-addition

20

reactions in tetrahydropyran, as compared to cyclohexane, due to the ether functional group in the former. Page 20 of 28 ACS Paragon Plus Environment

Page 21 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

In the -tetrahydropyranyl radical, and similarly for Q̇OOH radicals when the unpaired electron occupies

2

the  carbon, the electron density increases on the radical site and decreases around the non-radical C atom.

3

The result is incipient C=O bond formation between the oxygen and the carbon radical site, and an increase

4

in the coulomb repulsive force between the C–O bond with the non-radical C atom. Combined, the change

5

in electronic structure results in ring-opened radicals that undergo subsequent oxidation reactions leading

6

to distinct products. CBS-QB3 calculations were conducted to determine the change in C–O bond length in

7

the optimized geometries of the three radicals. For reference, in closed-shell tetrahydropyran the C–O bond

8

length is 1.422 Å, which is identical in both - and -tetrahydropyranyl radicals. In -tetrahydropyranyl,

9

where the unpaired electron is on the  carbon, the C–O bond decreases by ~0.1 to 1.369 Å while the C–O

10

bond non-radical carbon is slightly longer (~0.02 Å). A similar effect is exhibited in Q̇OOH radicals. The

11

shortening of the C–O bond enables facile ring-opening reactions, which in the case of in -

12

tetrahydropyranyl radicals leads to pentanal-5-yl and in Q̇OOH radicals to the products in Figure 18.

13

Because the ether group enables facile ring-opening reactions, evident in chain-termination reactions 24 and

14

in the branching fractions in Table 2, Q̇OOH radicals that could otherwise react with O2 instead produce

15

small, unsaturated oxygenates in chain-propagating reactions. Because such electronic structure changes

16

does not occur in cyclohexane, Q̇OOH ring-opening reactions are not significant in cyclohexane below 700

17

K, which maintains the second-O2-addition step necessary for ketohydroperoxide formation. The ab initio

18

calculations in Knepp et al. 37 indicated that - and -Q̇OOH radicals of cyclohexane are thermalized up to

19

least 800 K (i.e. relaxation timescales are short relative to chemical timescales) and the remaining (-)

20

Q̇OOH is stable up to 650 K. -Q̇OOH radicals encounter a barrier height of ~12 kcal/mol to form cyclic

21

ether (1,2-epoxycyclohexane) + ȮH. However, -scission reactions lead either to cyclohexene + HOȮ or

22

to a hydroperoxy-substituted radical, H2C=CHCH(–OOH)CH2CH2ĊH2, the barriers for which are higher

23

than the cyclic ether channel.

24

An example comparison is confirmed upon analysis of the mass spectra for the various products. Figure

25

19 shows the ring-opening pathway postulated for tetrahydropyran (cf. Figure 18b) and an analogous

26

(hypothetical) reaction for cyclohexane. The two bond-breaking schemes are identical, yet lead to distinct

27

products depending on the starting reactant being a Q̇OOH derived from tetrahydropyran (top reaction) or

28

from cyclohexane (bottom reaction). The mass spectra in Figure S14 shows no evidence of m/z 30 from

29

cyclohexane oxidation, which precludes the latter reaction and confirms that the ring-opening pathway does

30

not occur. In addition, speciation studies on cyclohexane oxidation do not report 3-butenal as a product 35,

31

64-65.

Comparison of other pathways in Figure 18 affirms the absence of ring-opening in cyclohexane (S15).



O

O

O

OH

OH

O

O

O

OH

OH

Page 22 of 29

O +

CH2O

O +

+

OH

+

OH

1 2

Figure 19. Comparison of ring-opening pathways in tetrahydropyran and cyclohexane.

3

The main product from the cyclohexane oxidation experiments was cyclohexene. No ring-opening products

4

consistent with the bond-breaking routes occurring in tetrahydropyran were detected. Because of m/z 70

5

ion signal observed, 3-butenal was potentially formed from cyclohexane oxidation. However, least-squares

6

fitting of the photoionization spectra was inconclusive due to low signal-to-noise ratios at both 10 Torr and

7

2 atm (Supplemental Material S8). Because no clear pathway from cyclohexane to 3-butenal exists

8

without forming ethene, which is not detected in present work, the conclusion is drawn that the pathway

9

via the ring-opening mechanism in Figure 19 is not occurring. In addition, 3-butenal was not detected in

10

the 1-atm jet-stirred reactor experiments on cyclohexane in Serinyel et al. 35.

11

To expand on the potential for ring-opening in cyclohexane, reactions of hex-1-en-6-yl were examined.

12

Cyclohexyl ring-opens to hex-1-en-6-yl, the oxidation of which may lead via HOȮ-loss to 1,5-hexadiene

13

(m/z 82) or via Q̇OOH-mediated reactions to unsaturated cyclic ether + ȮH. As reported in Rotavera et al.

14

24

15

by cyclohexene. The absence of 1,5-hexadiene in the m/z 82 photoionization spectrum confirms that hex-

16

1-en-6-yl + O2 → 1,5-hexadiene + HOȮ is not occurring. m/z 98 ion signal in the present work may include

17

some contribution from unsaturated cyclic ether. However, since spectral deconvolution was not possible,

18

the formation of unsaturated cyclic ethers in cyclohexane oxidation remains an unknown.

19

Another possible channel for hex-1-en-6-yl is decomposition into ethene + but-1-en-4-yl. The alkenyl

20

radical may subsequently react with O2 and form 1,3-butadiene + HOȮ. However, while ion signal at m/z

21

54 near 9.1 eV, low signal levels prevented definitive assignment. Similarly, the time profile for m/z 54

22

appears delayed from time-zero, indicating secondary chemistry. Outside of forming a co-product of ethene,

23

which is not formed in the present work and necessary for a Q̇OOH-mediated reaction mechanism, a source

24

of 3-butenal from cyclohexane remains unclear.

25

The ether group in tetrahydropyran promotes ring-opening of the type in Figure 19, for both Ṙ and Q̇OOH,

26

and is a prominent feature in ether oxidation. Several minor species were detected and quantified in the

under the same conditions as the present work, m/z 82 ion signal from cyclohexane is reproduced solely


Page 23 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

tetrahydropyran experiments: acrolein, propene, 1-butene, and methyl were detected and quantified (cf.

2

Table 2). With the exception of one case (propene) all species were produced with branching fractions of

3

~1%. Acrolein is postulated to form from ring-opening of --Q̇OOH, which yields oxirane and ȮH as co-

4

products (Figure S16). The two alkenes likely arise from reaction of 1-propyl and 1-butyl, which are formed

5

from the linear ring-opened radical -tetrahydropyranyl 24. The branching fraction of propene was zero at

6

500 K and 600 K, for both pressures, yet at the highest temperature (700 K) rose to 7.8 10 Torr and 17.1 at

7

2 atm. The source of methyl radical is unclear.

8

The absence of significant ring-opening reactions in cyclohexane and the formation of ketohydroperoxides

9

from 600 – 700 K (cf. Figure 5) is consistent with distinct negative temperature coefficient (NTC) behavior

10

exhibited in ignition delay times, as reported in Lemaire et al. 66, Daley et al. 67, and Vranckx et al. 68, as

11

well as in speciation studies

12

1000 K, where peroxy radical chemistry dominates. In addition, no low-temperature chemistry models exist

13

for tetrahydropyran despite recent mechanism development 27, 39-40. Because the ether group enables ring-

14

opening reactions of Ṙ and Q̇OOH, which inhibit ketohydroperoxide formation, tetrahydropyran may prove

15

a useful biofuel blending agent for high-octane combustion systems. For such an application, the

16

development of a low-temperature chemical kinetics mechanism for tetrahydropyran is necessary. As a first

17

step, potential energy surface calculations of unimolecular Ṙ and Q̇OOH decomposition pathways, as well

18

as detailed Master Equation analysis, are required.

34-35.

No ignition delay time measurements exist for tetrahydropyran below

19 20 21

5. Conclusion

22

Reaction mechanisms of tetrahydropyran and cyclohexane were studied using Cl-initiated oxidation in

23

MPIMS experiments at 10 and 1520 torr and from 500 – 700 K. In the tetrahydropyran experiments, several

24

ring-opening mechanisms facilitated by the ether group were postulated to explain the absence of

25

ketohydroperoxide at 700 K, as indicated by time profiles at m/z 132. In total, at 700 K, ring-opening

26

reactions were significant in tetrahydropyran oxidation, involving both Ṙ and Q̇OOH and accounting for

27

>70% of product formation at 10 Torr and >55% at 1520 Torr. Three main pathways were confirmed via

28

photoionization spectra analysis: (i) -Q̇OOH → pentanedial + ȮH, (ii) -Q̇OOH → vinyl formate + ethene

29

+ ȮH, (iii) -Q̇OOH → 3-butenal + formaldehyde + ȮH. The flux of Q̇OOH radicals through unimolecular

30

decomposition channels reduces the overall rate of the second-O2-addition step and, by extension,

31

diminishes ketohydroperoxide formation owing to the presence of the ether group.



Page 24 of 29

1

6. Supporting Information

2

Supporting information including time profiles, photoionization cross-sections, derivation of branching

3

fraction equation, mass spectra, CBS-QB3 calculations (geometries, ionization energies), photoionization

4

spectra, and additional mechanistic analysis is available via http://pubs.acs.org.

5

7. Acknowledgements

6

The operation and maintenance of the Sandia multiplexed photoionization mass spectrometry apparatus

7

and the participation of RC, IA, DLO, LS and CAT are supported by the Office of Chemical Sciences,

8

Biosciences, and Geosciences and the Office of Basic Energy Sciences of the U.S. Department of Energy

9

(BES/USDOE). Sandia National Laboratories is a multi-mission laboratory managed and operated by

10

National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell

11

International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under

12

contract DE-NA0003525. The present work describes objective technical results and analysis. Any

13

subjective views or opinions that might be expressed in the paper do not necessarily represent the views of

14

the USDOE or the United States Government. The research herein used resources (Beamline 9.0.2) of the

15

Advanced Light Source, which is a DOE Office of Science User Facility under contract no. DE-AC02-

16

05CH11231. BR and AK acknowledge support from the U.S. Department of Transportation under award

17

693JJ31945035.

18

8. References

19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

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Influence of Ether Functional Group on Ketohydroperoxide Formation

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