Negative
Ion Photoelectron Spectroscopy Confirms the
Prediction that 1,2,4,5-Tetraoxatetramethylenebenzene Has a Singlet
Ground State
- Publication date
- Publisher
Abstract
The negative ion photoelectron (NIPE)
spectrum of 1,2,4,5-tetraoxatetramethylenebenzene
radical anion (<b>TOTMB</b><sup>•–</sup>) shows
that, like the hydrocarbon, 1,2,4,5-tetramethylenebenzene (<b>TMB</b>), the <b>TOTMB</b> diradical has a singlet ground state and
thus violates Hund’s rule. The NIPE spectrum of <b>TOTMB</b><sup>•–</sup> gives a value of −Δ<i>E</i><sub>ST</sub> = 3.5 ± 0.2 kcal/mol for the energy
difference between the singlet and triplet states of <b>TOTMB</b> and a value of <i>EA</i> = 4.025 ± 0.010 eV for the
electron affinity of <b>TOTMB</b>. (10/10)CASPT2 calculations
are successful in predicting the singlet–triplet energy difference
in <b>TOTMB</b> almost exactly, giving a computed value of −Δ<i>E</i><sub>ST</sub> = 3.6 kcal/mol. The same type of calculations
predict −Δ<i>E</i><sub>ST</sub> = 6.1–6.3
kcal/mol in <b>TMB</b>. Thus, the calculated effect of the substitution
of the four oxygens in <b>TOTMB</b> for the four methylene groups
in <b>TMB</b> is very unusual, since the singlet state is selectively
destabilized relative to the triplet state. The reason why <b>TMB</b> → <b>TOTMB</b> is predicted to result in a decrease
in the size of −Δ<i>E</i><sub>ST</sub> is discussed