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 (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 of TOTMB(•-) gives a value of -ΔEST = 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 -ΔEST = 3.6 kcal/mol. The same type of calculations predict -ΔEST = 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 -ΔEST is discussed.

Negative ion photoelectron spectroscopy of P2N3−: electron affinity and electronic structures of P2N3˙

“…Resonant photodetachment was previously used to explain the difference between the experimental and simulated NIPE spectra in our previous NIPES study of the TOTMB radical anion. 46 …”

“…We have shown previously that electrospray ionization has the ability to retain the solid-state structure of the molecules in the gas phase 40 and that NIPES is an excellent technique for investigating the electronic structures of cyclic organic molecules, such as (CO) n , 41 – 43 (CS) 4 , 44 meta -benzoquinone (MBQ), 45 and 1,2,4,5-tetraoxatetramethylenebenzene (TOTMB). 46 In this paper we describe and analyze the NIPE spectra of P 2 N 3 – .…”

A Joint Experimental and Computational Study of the Negative Ion Photoelectron Spectroscopy of the 1-Phospha-2,3,4-triazolate Anion, HCPN3–

“…Resonant photodetachment 37 via some excited state of HCPN 3 − , may give rise to an underlying band in this region, which could account for the differences between the experimental and simulated NIPE spectra at EBE > 4.0 eV. Resonant photodetachment was also suggested to occur in our previous NIPES studies of the 1,2,4,5-tetraoxatetramethylenebenzene (TOTMB) radical anion 38 • radical are close in energy. This conjecture is supported by the results of (U)B3LYP, CASPT2, and (U)CCSD(T) calculations, which all find that these two electronic states of the radical do, indeed, have very similar energies.…”

Negative Ion Photoelectron Spectroscopy Confirms the Prediction of a Singlet Ground State for the 1,8-Naphthoquinone Diradical

“…The single-triplet energy separations in diradicals have been of interest for many decades. [1][2][3][4][5][6][7][8][9][10][11][12] Among the diradicals that have been studied, the isomeric benzoquinones (BQs) have been of particular interest. [13][14][15][16][17][18][19][20] Both o-and p-BQ have a singlet ground state; whereas, the ground state of m-BQ is a triplet.…”