Spiro[4.4]nonatetraene and its Positive and Negative Radical Ions: Molectronic Structure Investigations

Abstract

Dedicated to Edgar Heilbronner on the occasion of his 80th birthdayThe electronic structure of spiro [4.4]nonatetraene 1 as well as that of its radical anion and cation were studied by different spectroscopies. The electron-energy-loss spectrum in the gas phase revealed the lowest triplet state at 2.98 eV and a group of three overlapping triplet states in the 4.5 ± 5.0 eV range, as well as a number of valence and Rydberg singlet excited states. Electron-impact excitation functions of pure vibrational and triplet states identified various states of the negative ion, in particular the ground state with an attachment energy of 0.8 eV, an excited state corresponding to a temporary electron attachment to the 2b 1 MO at an attachment energy of 2.7 eV, and a core excited state at 4.0 eV. Electronic-absorption spectroscopy in cryogenic matrices revealed several states of the positive ion, in particular a richly structured first band at 1.27 eV, and the first electronic transition of the radical anion. Vibrations of the ground state of the cation were probed by IR spectroscopy in a cryogenic matrix. The results are discussed on the basis of density-functional and CASSCF/ CASPT2 quantum-chemical calculations. In their various forms, the calculations successfully rationalized the triplet and the singlet (valence and Rydberg) excitation energies of the neutral molecule, the excitation energies of the radical cation, its IR spectrum, the vibrations excited in the first electronic absorption band, and the energies of the ground and the first excited states of the anion. The difference of the anion excitation energies in the gas and condensed phases was rationalized by a calculation of the Jahn-Teller distortion of the anion ground state. Contrary to expectations based on a single-configuration model for the electronic states of 1, it is found that the gap between the first two excited states is different in the singlet and the triplet manifold. This finding can be traced to the different importance of configuration interaction in the two multiplicity manifolds.1. Introduction. ± Spiroconjugation, a concept first introduced in 1967 [1] [2], results in many interesting spectroscopic and chemical properties. As a consequence, spiroconjugated compounds have often been studied experimentally and theoretically, and they find applications in technology (e.g., as materials for organic light-emitting diodes [3] or nonlinear optical materials).

Matrix Isolation

“…Although the introduction of these parameters gives this method a semiempirical flavor, its quantitative accuracy in the prediction of spectra appears to be comparable to that of CASPT2, at a fraction of the expense of computational and human effort. 142 In fact, DFT-MRCI has even yielded predictions in excellent agreement with experiment even in a recent case where CASPT2 failed entirely. 148…”

“…Nevertheless, if applied with due circumspection, CASPT2 is a very reliable method and has been applied succesfully to the modeling of electronic spectra of a wide variety of compounds, including many matrix-isolated reactive intermediates in the author's laboratory. 8,[134][135][136][137][138][139][140][141][142] An entirely different approach to ab initio calculations of excited states is to look at the response of the ground-state electron distribution to oscillating electric fields of different frequencies. If the frequency of such an oscillation is in resonance with an electronic excitation, then the polarizability will increase discontinuously, that is, the frequency-dependend polarizability will show a pole whose intensity is proportional to the electronic transition moment.…”

“…Since the excited-state electron density is modeled within the framework of a singles-CI, the results of TD-DFT calculations also lend themselves to a transparent interpretation in terms of electronic configurations. The TD-DFT theory also has recently been applied successfully to matrix-isolated reactive intermediates, 142,145,146 but the experiences made in our laboratory are mixed and the jury is perhaps still out on how reliable this promising new method really is.…”

Oligounsaturated Five‐Membered Carbocycles – Aromatic and Antiaromatic Compounds in the Same Family

“…Rather recently, the electronic structure of spiro [4.4]nonatetraene (31), as well as those of its radical anion and radical cation, respectively, have been probed by a variety of spectroscopic methods (electron transmission and energy loss in the gas phase, electronic absorption in cryogenic matrices) [24]. The extension of this concept of spiroconjugation to higher analogues of spiro [4.4]nonatetraene 31 would lead to the hydrocarbons 32 and 33.…”

Spiro-Conjugated Molecular Junctions: Between Jahn–Teller Distortion and Destructive Quantum Interference

“…Jahn-Teller Distortion.-Populating degenerate electronic states, however, typically results in Jahn-Teller (JT) distortion [44]. This is also the case in the SNT molecule where, upon charging, the symmetry reduces from D 2d to D 2 as the two conjugated moieties undergo a twist away from the 90 • angle (b 1 -type distortion) [32]. As shown in Fig.…”