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Heteroaromatic species are commonly found in complex gaseous mixtures, from tobacco smoke to petroleum and asphaltene combustion products. At high temperatures, C–H bond rupture produces various dehydro radical isomers. We have used the spin–flip formulation of equation-of-motion coupled cluster theory with single and double substitutions (EOM-SF-CCSD) to characterize the energies and wave functions of the lowest lying singlet and triplet states of the diradical (2,3), (2,4), (2,5), and (3,4) di-dehydro isomers of pyrrole, furan, and thiophene. In all cases, these diradicals are minima on the broken-symmetry ωB97X-D/cc-pVDZ potential energy surface. In most cases, the diradical geometries distort to enhance through-space or through-bond coupling in the singlet states and to avoid Coulombic or exchange repulsion in the triplet states. EOM-SF-CCSD results indicate that all diradical isomers are two-configurational, closed shell singlet states. The only exceptions to this are for (2,3) and (2,4) thiophene and (2,3) pyrrole, which each contain more than two configurations. In all cases, the leading term in the multiconfigurational diradical wave function doubly occupies the symmetric radical σ orbital, indicative of either through-space or 1,3 through-bond coupling. We utilized the nucleus-independent chemical shift (NICS) approach to qualitatively assess aromaticity and find that this property varies and may be related to the energetic splittings in these diradical isomers.
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Flash pyrolysis vacuum ultraviolet photoionization mass spectrometry of cycloheptane: A study of the initial decomposition mechanism
Thermal decomposition of cycloheptane was studied using flash pyrolysis coupled with vacuum ultraviolet (118.2 nm) single photon ionization time-of-flight mass spectrometry at temperatures ranging from 295 K to 1380 K. C-C bond breaking of cycloheptane leading to the 1,7-heptyl diradical was considered as the initiation step. The 1,7-heptyl diradical could readily isomerize to 1-heptene and decompose into several fragments, with dissociation to •C4H9and •C3H5as the predominant product channel. The 1,7-heptyl diradical could undergo direct dissociation, as evidenced by the production of the C5H10species. Quantum chemistry calculations at UCCSD(T)/cc-pVDZ//UB3LYP/cc-pVDZ level of theory on the initial reaction pathways of cycloheptane were also carried out to support the experimental observations. Other possible initiation channels, as well as some secondary reaction products, were also identified.
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- Award ID(s):
- 2155232
- PAR ID:
- 10404401
- Publisher / Repository:
- SAGE Publications
- Date Published:
- Journal Name:
- European Journal of Mass Spectrometry
- Volume:
- 29
- Issue:
- 2
- ISSN:
- 1469-0667
- Page Range / eLocation ID:
- p. 88-96
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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