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ABSTRACT The energies and geometries of the lowest lying singlet and triplet states of the four diradicals formed by removing two H atoms from thiophene have been characterized. We utilized the highly correlated, multireference methods configuration interaction with single and double excitations with and without the Pople correction for size‐extensivity (MR‐CISD+Q and MR‐CISD) and averaged quadratic coupled cluster theory (MR‐AQCC). CAS (8,7) and CAS (10,8) active spaces involving σ, σ*, π, and π* orbitals were employed along with the cc‐pVDZ and cc‐pVTZ basis sets. The larger active space included the two electrons in the nonbonding sp2hybrid orbital on sulfur. We find that all didehydro isomers exist as planar, stable ground state singlets. The singlet‐triplet (S‐T) adiabatic gaps range from 15 to 25 kcal/mol while the vertical splittings are 21–35 kcal/mol. The 2,3 isomer has the lowest absolute ground state singlet energy and the largest adiabatic and vertical S‐T splitting. The ground states of the 2,3‐, and 2,5‐didehydrothiophene isomers are predicted to exhibit the smallest and largest diradical character, respectively, based on their electronic structures, spin densities and bonding analysis. To our knowledge, no experimental excitation energies of any of the didehydrothiophene isomers are available, and our computed MR‐AQCC/cc‐pVTZ data are believed to be among the most accurate computed results. This extensive study shows a competitive performance between MR‐AQCC and MR‐CISD+Q.
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A spin–flip study of the diradical isomers of pyrrole, furan, and thiophene
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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- Award ID(s):
- 2320718
- PAR ID:
- 10634168
- Publisher / Repository:
- AIP
- Date Published:
- Journal Name:
- The Journal of Chemical Physics
- Volume:
- 161
- Issue:
- 15
- ISSN:
- 0021-9606
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1063/5.0233736
