The Negative Ion Photoelectron (NIPE) spectrum of 1,2,4,5-tetraoxatetramethylenebenzene (TOTMB) 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 gives a value of -?EST = 3.5 kcal/mol in TOTMB. (10/10)CASPT2 calculations are successful in predicting this value of -?EST almost exactly. The same types of calculations predict -?EST = 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 à TOTMB is predicted to result in a decrease in the size of -?EST is discussed.
Revised: October 1, 2015 |
Published: July 22, 2015
Citation
Hrovat D., G. Hou, X.B. Wang, and W. Borden. 2015.Negative Ion Photoelectron Spectroscopy Confirms the Prediction that 1,2,4,5-Tetraoxatetramethylenebenzene Has a Singlet Ground State.Journal of the American Chemical Society 137, no. 28:9094-9099.PNNL-SA-109953.doi:10.1021/jacs.5b04416