Unimolecular thermal fragmentation ofortho-benzyne

Abstract

The ortho-benzyne diradical, o-C(6)H(4) has been produced with a supersonic nozzle and its subsequent thermal decomposition has been studied. As the temperature of the nozzle is increased, the benzyne molecule fragments: o-C(6)H(4)+Delta--> products. The thermal dissociation products were identified by three experimental methods: (i) time-of-flight photoionization mass spectrometry, (ii) matrix-isolation Fourier transform infrared absorption spectroscopy, and (iii) chemical ionization mass spectrometry. At the threshold dissociation temperature, o-benzyne cleanly decomposes into acetylene and diacetylene via an apparent retro-Diels-Alder process: o-C(6)H(4)+Delta-->HC triple bond CH+HC triple bond C-C triple bond CH. The experimental Delta(rxn)H(298)(o-C(6)H(4)-->HC triple bond CH+HC triple bond C-C triple bond CH) is found to be 57+/-3 kcal mol(-1). Further experiments with the substituted benzyne, 3,6-(CH(3))(2)-o-C(6)H(2), are consistent with a retro-Diels-Alder fragmentation. But at higher nozzle temperatures, the cracking pattern becomes more complicated. To interpret these experiments, the retro-Diels-Alder fragmentation of o-benzyne has been investigated by rigorous ab initio electronic structure computations. These calculations used basis sets as large as [C(7s6p5d4f3g2h1i)H(6s5p4d3f2g1h)] (cc-pV6Z) and electron correlation treatments as extensive as full coupled cluster through triple excitations (CCSDT), in cases with a perturbative term for connected quadruples [CCSDT(Q)]. Focal point extrapolations of the computational data yield a 0 K barrier for the concerted, C(2v)-symmetric decomposition of o-benzyne, E(b)(o-C(6)H(4)-->HC triple bond CH+HC triple bond C-C triple bond CH)=88.0+/-0.5 kcal mol(-1). A barrier of this magnitude is consistent with the experimental results. A careful assessment of the thermochemistry for the high temperature fragmentation of benzene is presented: C(6)H(6)-->H+[C(6)H(5)]-->H+[o-C(6)H(4)]-->HC triple bond CH+HC triple bond C-C triple bond CH. Benzyne may be an important intermediate in the thermal decomposition of many alkylbenzenes (arenes). High engine temperatures above 1500 K may crack these alkylbenzenes to a mixture of alkyl radicals and phenyl radicals. The phenyl radicals will then dissociate first to benzyne and then to acetylene and diacetylene.

Isomerization and fragmentation pathways of 1,2‐azaborine

“…We also considered the concerted fragmentation of 4 to give acetylene and a BN substituted diacetylene 14 (Scheme ) . The reaction is energetically uphill by 112.5 kcal mol −1 .…”

“…Meta ‐ and para ‐benzyne are known to undergo a ring opening reaction to 3‐hexene‐1,5‐diyne ( 3 ) (Scheme ) . Another known reaction of 1 is the direct fragmentation to acetylene and diacetylene at high temperatures as was shown by Zhang et al in 2007 . However, the proposed concerted mechanism for the decomposition has an activation barrier of 90 kcal mol −1 a, and is higher in energy than the pathway that includes the isomerization of 1 to meta ‐benzyne …”

The problematic C2H4+F2 reaction barrier

“…Cartesian coordinates for each stationary point are provided in Table S1 of supplementary material. 67 The empirical r e structural parameters for F 2 and C 2 H 4 , obtained by correcting experimental rotational constants with CCSD͑T͒/cc-pVQZ vibration-rotation interaction constants, are r e ͑F u F͒ = 1.4124 Å, r e ͑C v C͒ = 1.3307 Å, r e ͑C u H͒ = 1.0807 Å, and e ͑H u C u H͒ = 117.14°. Furthermore, the largest coupled-cluster T 2 amplitudes ͑t 2,max ͒ of F 2 ͑0.095͒, C 2 H 4 ͑0.083͒, and the TS ͑0.16͒ indicate that single-reference methods are suitable for this study, particularly if dynamical electron correlation is treated to high order.…”

Orbital-optimized MP2.5 and its analytic gradients: Approaching CCSD(T) quality for noncovalent interactions

“…The CCSD(T)/cc-pCVQZ level of theory was shown to be quite accurate for equilibrium geometries of molecules without a strong multi-reference character. [67][68][69][70][71] Hence, MP2, MP2.5, OMP2.5, and CCSD results are compared with those of CCSD(T)/cc-pCVQZ. Optimized geometries of the molecules considered in hydrogen-transfer reactions were taken from previous studies, 11,12,[72][73][74][75][76] while those for openshell noncovalent interactions (the O23 database) were taken from our recent study.…”