Multi-reference configuration interaction, MR-CI (including extensivity
corrections, named +Q) calculations have been performed on
S0 to 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 have been employed, comprising MR-CI expansions as large
as ~ 374 x 106 configuration state
functions. The nature of the studied excited states has been
characterized. Vertical excitation energies (ΔE) and oscillator
strengths (f) have been computed. The most intense transitions
(S0→S2 for 24 and
S0→S3 for 25) do not change
with the wavefunction, although a variation as large as
~ 1 eV has been obtained for the S3
state of 24. On the other hand, ΔE changes at most
~ 0.15 eV for 25, as the wavefunction changes.
The S1 state of both thiones has 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 has been obtained. S2 and
S3 are in the range from ~ 3.5 to 5.2
eV, at the highest level (MR-CI+Q). It is the first time that the
excited states of the title molecules are studied. The computed results
agree with the experimental onsets of photoreactions of thiones
24 and 25 found by Reva et. al. (Phys.
Chem. Chem. Phys. 2015, 17, 4888).