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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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Photoelectron spectroscopy and thermochemistry of o -, m -, and p -methylenephenoxide anions
The anionic products following (H + H + ) abstraction from o -, m -, and p -methylphenol (cresol) are investigated using flowing afterglow-selected ion flow tube (FA-SIFT) mass spectrometry and anion photoelectron spectroscopy (PES). The PES of the multiple anion isomers formed in this reaction are reported, including those for the most abundant isomers, o -, m - and p -methylenephenoxide distonic radical anions. The electron affinity (EA) of the ground triplet electronic state of neutral m -methylenephenoxyl diradical was measured to be 2.227 ± 0.008 eV. However, the ground singlet electronic states of o - and p -methylenephenoxyl were found to be significantly stabilized by their resonance forms as a substituted cyclohexadienone, resulting in measured EAs of 1.217 ± 0.012 and 1.096 ± 0.007 eV, respectively. Upon electron photodetachment, the resulting neutral molecules were shown to have Franck–Condon active ring distortion vibrational modes with measured frequencies of 570 ± 180 and 450 ± 80 cm −1 for the ortho and para isomers, respectively. Photodetachment to excited electronic states was also investigated for all isomers, where similar vibrational modes were found to be Franck–Condon active, and singlet–triplet splittings are reported. The thermochemistry of these molecules was investigated using FA-SIFT combined with the acid bracketing technique to yield values of 341.4 ± 4.3, 349.1 ± 3.0, and 341.4 ± 4.3 kcal mol −1 for the o -, m -, and p -methylenephenol radicals, respectively. Construction of a thermodynamic cycle allowed for an experimental determination of the bond dissociation energy of the O–H bond of m -methylenephenol radical to be 86 ± 4 kcal mol −1 , while this bond is significantly weaker for the ortho and para isomers at 55 ± 5 and 52 ± 5 kcal mol −1 , respectively. Additional EAs and vibrational frequencies are reported for several methylphenyloxyl diradical isomers, the negative ions of which are also formed by the reaction of cresol with O − .
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- Award ID(s):
- 1734006
- PAR ID:
- 10092011
- Date Published:
- Journal Name:
- Physical Chemistry Chemical Physics
- Volume:
- 20
- Issue:
- 39
- ISSN:
- 1463-9076
- Page Range / eLocation ID:
- 25203 to 25216
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/C8CP05403G
