Ketene Thermochemistry

Simmie, John M.; Metcalfe, Wayne K.; Curran, Henry J.

doi:10.1002/cphc.200800003

Cited by 11 publications

(11 citation statements)

References 24 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…For formaldehyde, our calculated value of −26.6 kcal mol −1 agrees relatively well with a recent value of −26.05 kcal mol −1 obtained from a reanalysis of experimental reaction thermochemistry . The calculated value for the HO 2 radical heat of formation (3.3 kcal mol −1 ) is also in good agreement with two recent experimental values (3.2 kcal mol −1 and 2.94 kcal mol −1 ). , There is some uncertainty in the literature as to the true heat of formation for ketene (CH 2 CO), with most studies suggesting values in the range of −13 to −11 kcal mol −1 , consistent with our calculated value of −12.5 kcal mol −1 . There is also reasonable agreement between our calculated heats of formation for CH 2 CHO (3.3 kcal mol −1 ) and O 2 (0.9 kcal mol −1 ) and literature values (2.5 kcal mol − 1 and 0 kcal mol −1 , respectively).…”

Section: Resultssupporting

confidence: 88%

Reaction of the i-C₄H₅ (CH₂CCHCH₂) Radical with O₂

et al. 2011

View full text Add to dashboard Cite

The resonantly stabilized radical i-C(4)H(5) (CH(2)CCHCH(2)) is an important intermediate in the combustion of unsaturated hydrocarbons and is thought to be involved in the formation of polycyclic aromatic hydrocarbons through its reaction with acetylene (C(2)H(2)) to form benzene + H. This study uses quantum chemistry and statistical reaction rate theory to investigate the mechanism and kinetics of the i-C(4)H(5) + O(2) reaction as a function of temperature and pressure, and unlike most resonantly stabilized radicals we show that i-C(4)H(5) is consumed relatively rapidly by its reaction with molecular oxygen. O(2) addition occurs at the vinylic and allenic radical sites in i-C(4)H(5), with respective barriers of 0.9 and 4.9 kcal mol(-1). Addition to the allenic radical form produces an allenemethylperoxy radical adduct with only around 20 kcal mol(-1) excess vibrational energy. This adduct can isomerize to the ca. 14 kcal mol(-1) more stable 1,3-divinyl-2-peroxy radical via concerted and stepwise processes, both steps with barriers around 10 kcal mol(-1) below the entrance channel energy. Addition of O(2) to the vinylic radical site in i-C(4)H(5) directly forms the 1,3-divinyl-2-peroxy radical with a small barrier and around 36.8 kcal mol(-1) of excess energy. The 1,3-divinyl-2-peroxy radical isomerizes via ipso addition of the O(2) moiety followed by O atom insertion into the adjacent C-C bond. This process forms an unstable intermediate that ultimately dissociates to give the vinyl radical, formaldehyde, and CO. At higher temperatures formation of vinylacetylene + HO(2), the vinoxyl radical + ketene, and the 1,3-divinyl-2-oxyl radical + O paths have some importance. Because of the adiabatic transition states for O(2) addition, and significant reverse dissociation channels in the peroxy radical adducts, the i-C(4)H(5) + O(2) reaction proceeds to new products with rate constant of around 10(11) cm(3) mol(-1) s(-1) at typical combustion temperatures (1000-2000 K). For fuel-rich flames we show that the reaction of i-C(4)H(5) with O(2) is likely to be faster than that with C(2)H(2), bringing into question the importance of the i-C(4)H(5) + C(2)H(2) reaction in initiating ring formation in sooting flames.

show abstract

Section: Resultssupporting

confidence: 88%

Reaction of the i-C₄H₅ (CH₂CCHCH₂) Radical with O₂

et al. 2011

View full text Add to dashboard Cite

show abstract

“…The calculated maximum energetically allowed relative translational energy for the dimethylcarbene + CO channel is E max = 60 kJ/mol at 355.1 nm excitation. As described in the Supporting Information, this maximum value was calculated by using the known photon energy and the 0 K enthalpies of formation of dimethylketene, , dimethylcarbene, and CO . This calculated maximum corresponds to the limiting case in which all energy in excess of the CC bond dissociation energy appears as CH 3 CCH 3 + CO relative translational energy.…”

Section: Results and Analysismentioning

confidence: 99%

Dimethylcarbene versus Direct Propene Formation in Dimethylketene Photodissociation

Datta

Davis

2021

J. Phys. Chem. A

View full text Add to dashboard Cite

Highly reactive carbenes are usually produced by photolysis of ketenes, diazoalkanes, or diazirines. Sequential kinetic pathways for deactivation of nascent carbenes usually involve bimolecular reactions in competition with isomerization producing stable products such as alkenes. However, the direct photolytic production of stable products, effectively bypassing formation of free carbenes, has been postulated for over 50 years but remains very poorly understood. Often termed “rearrangement in the excited state” (RIES), examples include 1,2-hydrogen migration within photoexcited carbene precursors yielding alkenes and the Wolff rearrangement in photogenerated carbonyl-substituted carbenes producing ketenes. In this study, the two competing CO elimination channels from photoexcited gaseous dimethylketene, producing dimethylcarbene and propene, were studied as a function of electronic excitation energy, under collision-free conditions, by using photofragment translational energy spectroscopy with vacuum ultraviolet photoionization of the products. A significant fraction of the dimethylcarbene → propene isomerization exothermicity (∼300 kJ/mol) was released as propene + CO translational energy, indicating that propene is formed prior to or concurrent with CO elimination. An increase in the propene yield with increasing excitation energy suggests that the effective potential energy barrier for this channel lies ∼24 kJ/mol above the energetic threshold for dimethylcarbene formation via CC bond fission. Possible mechanisms for direct propene elimination are discussed in light of the observed energy dependence for the competing pathways.

show abstract

“…Given the reaction enthalpy above we compute Δ H f (298.15 K) = 125.4 ± 3.5 kJ mol –1 for the singlet α-carbene of 25DMF based on propyne and methylketene values of 185.21 ± 0.69 and −63.6 ± 2.8, respectively.…”

Section: Resultsmentioning

confidence: 99%

“…Given the reaction enthalpy above we compute ΔH f (298.15 K) = 125.4 ( 3.5 kJ mol À1 for the singlet R-carbene of 25DMF based on propyne 55 and methylketene 56 The calculated rate constant is k = 7.24 Â 10 14 exp(À16 417/ T) s À1 based on G3 energies and HF/6-31G(d) frequencies, rotational constants, and hindered rotor analysis. As a test, the same rate constant was computed but with CBS-QB3 energies and B3LYP/6-311G(d,p) data yielding k = 1.10 Â 10 15 exp-(À16 060/T) s À1 .…”

Section: Formation Of Carbenesmentioning

confidence: 99%

Ab Initio Study of the Decomposition of 2,5-Dimethylfuran

2011

Self Cite

View full text Add to dashboard Cite

The initial steps in the thermal decomposition of 2,5-dimethylfuran are identified as scission of the C-H bond in the methyl side chain and formation of β- and α-carbenes via 3,2-H and 2,3-methyl shifts, respectively. A variety of channels are explored which prise the aromatic ring open and lead to a number of intermediates whose basic properties are essentially unknown. Once the furan ring is opened demethylation to yield highly unsaturated species such as allenylketenes appears to be a feature of this chemistry. The energetics of H abstraction by the hydroxyl radical (and other abstracting species) from a number of mono- and disubstituted methyl furans has been studied. H-atom addition to 2,5-dimethylfuran followed by methyl elimination is shown to be the most important route to formation of the less reactive 2-methylfuran. Identification of 2-ethenylfuran as an C(6)H(6)O intermediate in 2,5-dimethylfuran flames is probably not correct and is more likely the isomeric 2,5-dimethylene-2,5-dihydrofuran for which credible formation channels exist.

show abstract

Ketene Thermochemistry

Cited by 11 publications

References 24 publications

Reaction of the i-C₄H₅ (CH₂CCHCH₂) Radical with O₂

Reaction of the i-C₄H₅ (CH₂CCHCH₂) Radical with O₂

Dimethylcarbene versus Direct Propene Formation in Dimethylketene Photodissociation

Ab Initio Study of the Decomposition of 2,5-Dimethylfuran

Contact Info

Product

Resources

About

Ketene Thermochemistry

Cited by 11 publications

References 24 publications

Reaction of the i-C4H5 (CH2CCHCH2) Radical with O2

Reaction of the i-C4H5 (CH2CCHCH2) Radical with O2

Dimethylcarbene versus Direct Propene Formation in Dimethylketene Photodissociation

Ab Initio Study of the Decomposition of 2,5-Dimethylfuran

Contact Info

Product

Resources

About

Reaction of the i-C₄H₅ (CH₂CCHCH₂) Radical with O₂

Reaction of the i-C₄H₅ (CH₂CCHCH₂) Radical with O₂