Fulvenallene Decomposition Kinetics

Abstract

While the decomposition kinetics of the benzyl radical has been studied in depth both from the experimental and the theoretical standpoint, much less is known about the reactivity of what is likely to be its main decomposition product, fulvenallene. In this work the high temperature reactivity of fulvenallene was investigated on a Potential Energy Surface (PES) consisting of 10 wells interconnected through 11 transition states using a 1 D Master Equation (ME). Rate constants were calculated using RRKM theory and the ME was integrated using a stochastic kinetic Monte Carlo code. It was found that two main decomposition channels are possible, the first is active on the singlet PES and leads to the formation of the fulvenallenyl radical and atomic hydrogen. The second requires intersystem crossing to the triplet PES and leads to acetylene and cyclopentadienylidene. ME simulations were performed calculating the microcanonical intersystem crossing frequency using Landau-Zener theory convoluting the crossing probability with RRKM rates evaluated at the conical intersection. It was found that the reaction channel leading to the cyclopentadienylidene diradical is only slightly faster than that leading to the fulvenallenyl radical, so that it can be concluded that both reactions are likely to be active in the investigated temperature (1500-2000 K) and pressure (0.05-50 bar) ranges. However, the simulations show that intersystem crossing is rate limiting for the first reaction channel, as the removal of this barrier leads to an increase of the rate constant by a factor of 2-3. Channel specific rate constants are reported as a function of temperature and pressure.

The thermal decomposition of the benzyl radical in a heated micro-reactor. II. Pyrolysis of the tropyl radical

“…As shown in Eq. (4), C 5 H 5 decomposes to acetylene and propargyl radicals at 1300 K. Likewise the fulvenallenyl radical has been observed 63,64 to decompose to propargyl radical and diacetylene. This can be summarized, 37 C 5 H 4 -C≡ ≡CH(+ M) → HCCCH 2 + HC≡ ≡C-C≡ ≡CH, The presence of the cyclopentadienyl radical and HC≡≡CH is confirmed by PIMS, PIE, and IR spectroscopy.…”

Analysis of Some Reaction Pathways Active during Cyclopentadiene Pyrolysis

“…Several test calculations showed that relative energies on the C 10 H 11 PES computed through eq 1 are comparable within 1.5 kcal/mol with those determined with CCSD(T) energies extrapolated to the CBS limit using the much more computationally expensive cc-pVDZ/cc-pVTZ extrapolation scheme we recently adopted to investigate the C 7 H 6 PES. 31,32 To improve the level of accuracy of the calculations, the energies of the saddle points whose reaction fluxes are rate determining were also calculated using the ROCBS-QB3 methodology proposed by Wood et al 33 This computational approach was designed to improve the level of accuracy of reaction energies calculated using the CBS-QB3 approach when spin contamination is relevant, thus eliminating the need to include the CBS-QB3 empirical corrections factor for spin contamination. This is accomplished by performing the calculations using spin restricted wave functions.…”

The thermal decomposition of the benzyl radical in a heated micro-reactor. I. Experimental findings

“…Density functional theory (DFT) calculations 14 found that hydrogen addition to fulvenallene would result in C 5 H 5 + C 2 H 2 . Recently, there has been extensive work [15][16][17][18][19] performed on potential energy surfaces for each of the species C 7 H 7 , C 7 H 6 , and C 7 H 5 and their interplay during benzyl radical decomposition. Because of their likely importance during the decomposition of benzyl, both fulvenallene and fulvenallenyl radicals, 20,21 as well as the cycloheptatrienyl radical 22 have been detected and identified by PIMS.…”