Negative Ion Photoelectron Spectroscopy Confirms the Prediction that 1,2,4,5-Tetraoxatetramethylenebenzene Has a Singlet Ground State

Abstract

The negative ion photoelectron (NIPE) spectrum of 1,2,4,5-tetraoxatetramethylenebenzene radical anion (<b>TOTMB</b>^•–) 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>^•– gives a value of −Δ<i>E</i>_ST = 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>_ST = 3.6 kcal/mol. The same type of calculations predict −Δ<i>E</i>_ST = 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>_ST is discussed