Abstract
AbstractMulti‐reference configuration interaction, MR‐CI (including extensivity corrections, named +Q), calculations were performed on the S0–S3 states of cyclohexa‐2,4‐diene‐1‐thione (thione 24) and cyclohexa‐2,5‐diene‐1‐thione (thione 25), which are thione isomers of thiophenol. Several types of uncontracted MR‐CIS and MR‐CISD wavefunctions were employed, comprising MR‐CI expansions as large as ~365 × 106 configuration state functions. The nature of the studied excited states was characterized. Vertical excitation energies (ΔE) and oscillator strengths (f) were computed. The most intense transitions (S0 → S2 for 24 and S0 → S3 for 25) did not change with the wavefunction, although a variation as large as ~1 eV was obtained for the S3 state of 24, at the highest (MR‐CI+Q) level. On the other hand, ΔE changed at most by ~0.56 eV for 25 as the wavefunction changes, at the same level. The S1 state of both thiones was found to have nπ* character and is in the visible region. For 24, S2 and S3 are ππ* and nπ* states, respectively, while for 25 the reverse order is obtained. S2 and S3 are in the range ~3.5 to 5.2 eV, again at the highest level. It is the first time that the excited states of the title molecules are studied. The computed results agree with the experimental onset of photoreactions of thiones 24 and 25 found by Reva et al (Phys. Chem. Chem. Phys., 2015, 17, 4888).
Highlights
Cyclohexa-2,4-diene-1-thione (24; Figure 1) has been identified in a ground state isomerization reaction of thiophenol, studied at the MP2 and QCISD(T) levels by AlMuhtaseb et al.[1]
The basis set effect has been taken into account at the CASSCF level, for both systems, and at the w1 and w4 levels for 25
As it can be seen from Tables and 3, the ΔE values obtained with both basis sets differ by at most 0.03 eV
Summary
Cyclohexa-2,4-diene-1-thione (24; Figure 1) has been identified in a ground state isomerization reaction of thiophenol, studied at the MP2 and QCISD(T) levels by AlMuhtaseb et al.[1] Those authors obtained a very large barrier of ~ 63 kcal/mol for the thiophenol 24 thiol-thione H-transfer reaction. Formation of the analogous cyclohexa-2,5-diene-1-thione (25; see Figure 1) has been studied, the authors did not find a direct pathway for formation of 25 from thiophenol.[1]. Reva et al.[2] have isolated thiophenol in cryogenic argon matrices and observed the reversible photochemical thiophenol 24 and 24 25 reactions. This was the first experimental observation of thione isomers 24 and 25.
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