MCD of non-aromatic cyclic π-electron systems. Part 1. The perimeter model for antiaromatic 4N-electron [n]annulene biradicals 1

Application of the Perimeter Model to the Assignment of the Electronic Absorption Spectra of Gold(III) Hexaphyrins with [4n+2] and [4n] π‐Electron Systems

“…This is in contrast to the system derived from the (4N + 2)-electron perimeter model where the low-energy absorption bands consist of the electronic transitions involving four molecular porbitals. [9][10][11] Michl et al used a new classification of even-electron cyclic p-systems for the purposes of electronic and MCD spectroscopy, [12] and we follow their classification in the present study. Thus, the term "antiaromatic" is used only for even-electron cyclic p-electron systems that have very strong biradical-like character.…”

“…Thus, the term "antiaromatic" is used only for even-electron cyclic p-electron systems that have very strong biradical-like character. According to this definition, [12] the reduced form of the present hexaphyrins (Au 2 -R and Au-R) are classified into an "unaromatic" system, which describes cyclic p-electron systems derivable from a 4N-electron perimeter by perturbations that are strong enough to remove biradical-like character. In the case of the neutral forms (Au 2 -N and Au-N), we will consider the (4N + 2)-electron perimeter model, which has been shown to be useful for analyzing the electronic absorption and MCD spectra of aromatic porphyrinoids.…”

“…In recent years, Michl and co-workers reported that the spectral features and electronic states of small nonaromatic cyclic p-electron systems can be interpreted using the 4N-electron [n]annulene perimeter model. [12][13][14][15][16][17] The UV and MCD spectra of this type of compound, such as biphenylenes [16] and pentalenes, [17] were successfully interpreted on the basis of the perimeter model. Six frontier molecular porbitals (h À , h + , s À , s + , l À , and l + ) play an essential role in the low-energy singlet electronic states of the system derived from the 4N-electron perimeter model.…”

“…[9][10][11] We demonstrate in this study that the spectral features of the reduced form of the hexaphyrins (Au 2 -R and Au-R) can be rationalized by the 4N-electron version of the perimeter model. [12][13][14][15][16][17] Band assignments of porphyrinoids derived from (4N + 2)-electron annulenes have been extensively studied for over half a century. [18] However, to the best of our knowledge, an analogous treatment of "unaromatic" porphyrinoids derived from 4N-electron annulenes has not been available, so that this study is the first application of the 4N-electron perimeter model to porphyrinoid systems.…”

Electronic Structure, Spectra, and Magnetic Circular Dichroism of Cyclohexa‐, Cyclohepta‐, and Cyclooctapyrrole

“…[11][12][13][14][15][16][17] This approach, originally applied to understand the electronic spectra of aromatic hydrocarbons, was subsequently adapted and extended to encompass magnetic circular dichroism (MCD). The perimeter model correctly accounts for the electronic and MCD patterns in numerous compounds that can be derived from either 4N+2 [11][12][13][14] or 4N [15][16][17] p-electron perimeters. In several cases, the theoretical predictions [18] preceded the synthesis of the systems in question and hence necessarily antedated experimental confirmation.…”