The negative ion photoelectron (NIPE) spectrum of 1,2,4,5-tetraoxatetramethylenebenzene radical anion (TOTMB center dot-) shows that, like the hydrocarbon, 1,2,4,5-tetramethylenebenzene (TMB), the TOTMB diradical has a singlet ground state and thus Violates Hund's rule. The NIPE spectrum of TOTMB center dot- gives a value of -Delta E-ST = 3.5 +/- 0.2 kcal/mol for the energy difference between the singlet and triplet states of TOTMB and a value of EA = 4.025 +/- 0.010 eV for the electron affinity of TOTMB. (10/10)CASPT2 calculations are successful in predicting the singlet-triplet energy difference in TOTMB almost exactly, giving a computed value of -Delta E-ST = 3.6 kcal/mol. The same type of calculations predict -Delta E-ST, = 6.1-6.3 kcal/mol in TMB. Thus, the calculated effect of the substitution of the four oxygens in TOTMB for the four methylene groups in TMB is very unusual, since the singlet state is selectively destabilized relative to the triplet state. The reason why TMB -> TOTMB is predicted to result in a decrease in the Size of -Delta E-ST is discussed.