Photoisomerization of furan, pyrrole and thiophene: The intermediate formation of their Dewar and ring contracted forms

Rendall, W. A.; Torres, M. Rosario; Lown, E. N.; Strausz, O. P.

doi:10.1007/bf03155664

Cited by 14 publications

(6 citation statements)

References 38 publications

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“…The existence of pyrrolenine as an intermediate in the isomerization of pyrrole was suggested by Mackie et al and raised as a possibility in several thermal , and photochemical rearrangement studies of pyrrole and substituted pyrroles. The pyrrole ⇔ pyrrolenine tautomerism was also a subject of an ab initio investigation, , where an activation barrier of 44.5 kcal/mol for 1,2 H-atom shift was computed at the MP2/6-31G* level of theory .…”

Section: Resultsmentioning

confidence: 96%

Isomerization of Pyrrole. Quantum Chemical Calculations and Kinetic Modeling

Dubnikova

Lifshitz

1998

J. Phys. Chem. A

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Density functional theory (DFT) calculations including configuration interaction (CI) were carried out to investigate the pathways of the unimolecular isomerizations of pyrrole. Vibrational frequencies calculated with the DFT method were used to estimate transition-state theory frequency factors. The potentional energy surface of the overall isomerization of pyrrole is composed of pyrrolenine, two biradical intermediates, and five transition states, in addition to pyrrole and its stable isomers. The first step of the isomerization is a fast transition from pyrrole to pyrrolenine. These two species reach a state of equilibrium that is maintained during the entire process. There is no direct path that leads from pyrrole to its stable isomers, which are produced only from pyrrolenine. Two biradical intermediates that are very similar in their structure and energetics and that isomerize to one another by a low-barrier rotation are involved in the process of pyrrole isomerization. One intermediate forms only cis-crotonitrile, and the other intermediate forms only vinylacetonitrile. These two biradical intermediates and several transition states have resonance structures. There is no direct route that leads from pyrrolenine to trans-crotonitrile. The latter is formed from the cis isomer by cis → trans isomerization. RRKM calculations were carried out to transfer values of A ∞ and E ∞ of the rate constants of two high-barrier steps in the isomerizations to A and E corresponding to the experimental conditions. Kinetic modeling, which uses the calculated rate constants, gives a very good agreement between the calculated and the experimental yields of the isomerization products.

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Section: Resultsmentioning

confidence: 96%

Isomerization of Pyrrole. Quantum Chemical Calculations and Kinetic Modeling

Dubnikova

Lifshitz

1998

J. Phys. Chem. A

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“…Patterson et al [23] pyrolyzed N-deuterated pyrrole under similar conditions to the N-alkyl rearrangements and found 2-deuterated pyrrole. Although pyrrolenines have not been isolated from the pyrolysis products of simple N-substituted pyrroles, 2H-pyrroles have been observed in the photolysis of N-alkyl ring substituted pyrroles [20]. There is now considerable indirect evidence for the presence of pyrrolenine intermediates in both thermal and photochemical rearrangements of 1H-pyrroles.…”

Section: Thermochemical Considerationsmentioning

confidence: 97%

“…Clearly, uncertainties in the thermochemistry make it difficult to eliminate a ring scission mechanism completely, but our analysis, coupled with the subsequent arguments, lead us to a different proposal for pyrrole pyrolysis. Pyrroles are known to rearrange thermally at temperatures between 500-600°C [20]. Patterson et al [21-231 studied the thermal rearrangement of several N-substituted pyrroles to the corresponding 2-substituted pyrrole.…”

Section: Thermochemical Considerationsmentioning

confidence: 99%

Shock tube pyrolysis of pyrrole and kinetic modeling

Mackie

Colket²,

Nelson

et al. 1991

Int J of Chemical Kinetics

124

122

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The kinetics of pyrolysis of pyrrole dilute in argon have been studied in a single pulse shock tube, using capillary column GC, together with GUMS and FTIR for product identification, over the temperature range 1200-1700 K, total pressures of 7.5-13.5 atm and nominal mixture compositions of pyrrole of 5000 and 700 ppm (nominal concentrations of 5 x and 7 x lo-* mol ~m -~) .Time-resolved measurements of the rate of disappearance of pyrrole behind reflected shock waves have been made by absorption spectroscopy at 230 nm, corresponding to the lowest In-* + In-transition of pyrrole at pressures of 20 atm and mixture compositions between 1000-2000 ppm pyrrole (1.7-3.0 x mol ~m -~) over the temperature range of 1300 to 1700 K. At the lower end of the studied temperature range, the isomers of pyrrole, allyl cyanide and cis-and trans-crotononitrile, were the principal products, together with hydrogen cyanide and propyne/allene. At elevated temperatures, acetylene, acetonitrile, cyanoacetylene, and hydrogen became important products.The rate of overall disappearance of pyrrole, as measured by absorption spectrometry, was found to be first order in pyrrole concentration, with a rate constant kdi,(pyrrole) = 10'4.'*0.7 exp(-74.1 2 3.0 kcal mol-'/RT) s-l between 1350-1600 K and at a pressure of 20 atm. First order dependence of pyrrole decomposition and major product formation was also observed in the single pulse experiments over the range of mixture compositions studied.A 75-step reaction model is presented and shown to substantially fit the observed temperature profiles of the major product species and the reactant profile. In the model the initiation reaction is postulated to be the reversible formation of pyrrolenine, (2H-pyrrole). Pyrrolenine can undergo ring scission at the C2 -N bond forming a biradical which can rearrange to form allyl cyanide and crotononitrile or undergo decomposition to form HCN and C3H4 or acetylene and a precursor of acetonitrile. The model predicts an overall rate of disappearance of pyrrole in agreement with the experimental measurements.

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“…Pyrrole is known to rearrange thermally at temperature between 773 and 873 K, and tautomerization reaction initiates pyrolysis process. 32 Mackie et al 10,11 postulated that pyrolysis procedure was initiated most readily by the lowest pathway of the reversible formation of pyrrolenines, among which 2Hpyrrole has been detected by Tian et al in the premixed pyrrole flames. 33 The following ring scission forms an open chain biradical intermediate, which will readily undergo hydrogen transfer reaction to form two highly stable butenenitrile isomers of pyrrole, cis-crotononitrile and allyl cyanide.…”

Section: Mole Fraction Profiles Of Other Pyrolysis Productsmentioning

confidence: 99%

An Experimental and Theoretical Study of Pyrrole Pyrolysis with Tunable Synchrotron VUV Photoionization and Molecular-Beam Mass Spectrometry

Hong

Zhang

et al. 2009

J. Phys. Chem. A

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The pyrolysis of pyrrole (6.46% pyrrole in argon) has been performed with the tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry (MBMS) technique. The experiment was carried out over the temperature range of 1260-1710 K at a pressure of 267 Pa. About 30 intermediates have been identified by near-threshold measurements of photoionization mass spectra, and the corresponding mole fractions versus temperatures have been obtained. Moreover, the isomers of some pyrolysis products have been identified by the measurement of photoionization efficiency spectrum. The major products are H(2), C(2)H(2), HCN, C(3)H(4) (propyne), and C(2)H(3)N (acetonitrile). Meanwhile, some new intermediates, such as C(4)H(4)N (cyanoallyl radical) and C(2)H(2)N (cyanomethyl radical), have been determined. The major pyrolysis channels have been provided with the high-level ab initio G3B3 calculation and are well consistent with the experimental observation. The formation pathway of HCN via the cyclic carbene tautomer has been proved to be the lowest formation pathway, which is in accordance with previous theoretical work. The potential pathways of the early formed C(4)H(4)N species together with their subsequent consumption to C(2)H(2)N and C(2)H(2) have been discussed in detail. Also, the formation pathways of the major products of C(2)H(3)N and C(2)H(2) have been investigated as well.

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Photoisomerization of furan, pyrrole and thiophene: The intermediate formation of their Dewar and ring contracted forms

Cited by 14 publications

References 38 publications

Isomerization of Pyrrole. Quantum Chemical Calculations and Kinetic Modeling

Isomerization of Pyrrole. Quantum Chemical Calculations and Kinetic Modeling

Shock tube pyrolysis of pyrrole and kinetic modeling

An Experimental and Theoretical Study of Pyrrole Pyrolysis with Tunable Synchrotron VUV Photoionization and Molecular-Beam Mass Spectrometry

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