Photoelectron Spectroscopy of m-Xylylene Anion

Abstract

The 351-nm photoelectron spectrum of the negative ion of 1,3-benzoquinodimethane (m-xylylene) is reported. Features are observed in the photoelectron spectrum corresponding to formation of the 3B2, 1A1, and 1B2 states of m-xylylene. The electron affinity of the triplet ground state is found to be 0.919 ± 0.008 eV, and vibrational frequencies of 290, 540, and 1500 cm-1 are obtained. The active modes are assigned to α-carbon bending, ring deformation, and methylene bending, respectively. The 1A1 state is found to lie 9.6 ± 0.2 kcal/mol higher in energy than the ground state, in good agreement with theoretical predictions. Vibrational frequencies of 265, 1000, and 1265 cm-1 are found for this state. The 1B2 is estimated to be <21.5 kcal/mol higher in energy than the ground state. Density functional calculations have been carried out on the negative ion, indicating that the 2B1 ion is a minimum on the potential energy surface, lying 2.9 kcal/mol lower in energy than the 2A2 ion, which is a transition state.

The synergy between qualitative theory, quantitative calculations, and direct experiments in understanding, calculating, and measuring the energy differences between the lowest singlet and triplet states of organic diradicals

“…17, although the relative energies of the two lowest singlet states are greatly affected by this substitution, the energy difference between the lower of the two singlets ( 1 A 1 in MBQDM and 1 B 2 in MBQ) and the 3 B 2 ground state is calculated to be largely unaffected. 97 The prediction, 97 that the value of E ST in MBQ will be comparable in size to the value of E ST = 9.6 ± 0.2 kcal/mol that has already been measured in MBQDM, 98 will, hopefully, soon be subjected to an experimental test by NIPES, since the generation of MBQ…”

“…51,52 The ability of the phenyl group to delocalize the nonbonding electron on nitrogen in the open-shell singlet state of PhN, thus confining this  electron to a region of space different from that occupied by the  electron of opposite spin, is responsible for all three of these calculated and observed properties of PhN. 51,52 The minimization of the Coulombic repulsion between electrons of opposite spin, by confining them to different regions of space, is also responsible a number of predictions and observations about non-Kekulé hydrocarbon diradicals These include (a) the difference between the planar geometry of triplet TMM and the preferred geometry of the singlet, which has one CH 2 group twisted out of conjugation, 1,28,29,87,88 (b) the triplet ground states calculated and found for non-Kekulé diradicals with non-disjoint NBMOs (e.g., TMM 1, 2, 28, 29, 80-83, 86, 98 and MBQDM 84,[96][97][98] ) and (c) the singlet ground states calculated and found for non-Kekulé diradicals with disjoint NBMOs (TME [105][106][107][108][109]115 and TMB 110,111,[122][123][124].…”

“…110 Another NIPE spectrum of considerable interest is that of MBQ•-, in order to see whether, as predicted, 97 the oxygen atoms in MBQ have little effect on changing the value of E ST from that in MBQDM. 98 However, there are other molecules, not discussed in this Perspective, for which the measurement of E ST would be of great interest. One such molecule is cyclobutanetetraone (CO) 4 .…”

5‐Dehydro‐1,3‐quinodimethane: A Hydrocarbon with an Open‐Shell Doublet Ground State

“…The lowest energy closed-shell type doublet (i.e., doublet with one of the triradical orbitals being occupied by a pair of electrons) is the 1 2 A 1 state, which resembles the lowest-energy singlet in MX. [4] The net conclusion is that state-of-the-art calculations carried out by using the SF approach confirm the prediction of the open-shell doublet ground state for DMX.…”

“…The MX biradical has a triplet ground state [2][3][4] with two ferromagnetically coupled electrons in two p molecular orbitals ( Figure 1). DHT, also shown in Figure 1, is a s 1 p 1 biradical [5] with singlet-coupled unpaired electrons, a result of spin polarization that favors antiferromagnetic coupling.…”