Magnetic Circular Dichroism of Nonaromatic Cyclic π-Electron Systems. 5. Biphenylene and Its Aza Analogues

Abstract

The algebraic form of the perimeter model for nonaromatic cyclic π-electron systems developed in parts 1−4 of this series is used to analyze the previously reported magnetic circular dichroism (MCD) of biphenylene (1) and its aza analogues, to classify its excited states, and to relate them to those of other nonaromatic cyclic π systems. The observed MCD signs are interpreted in terms of relative sizes of orbital energy differences and the resulting configuration energy ordering. These require deviations from the alternant pairing associated with the simplest classical description, which are attributed to the increased negative magnitude of the diagonal resonance integrals in the four-membered ring. The interpretation of the UV and MCD spectra of 1 is confirmed by the observed effects of aza substitution, and predictions for other types of substitution follow. The magnetic field induced state mixing deduced from the perimeter model is supported by computations by the linear combination of orthogonalized atomic orbitals (LCOAO), time-dependent density functional theory (TD DFT), and symmetry-adapted cluster configuration interaction (SAC-CI) methods.

MCD of non‐aromatic cyclic π‐electron systems. Part 6: Pentalenes and heptalenes

“…The ordinary absorption spectra of 2 7 and of alkyl and aryl derivatives of 1 8, 9 have been recorded, as has the UV absorption spectrum of 1 in matrix isolation 5. The results are in qualitative agreement with expectations based on the simple 4 N ‐electron perimeter model, introduced in Parts 1–4 of this series 2, 10–12 whose hallmarks are a low‐energy region with an electric‐dipole–forbidden but magnetic‐dipole–allowed state (S) followed by a high‐energy region containing two weakly (N 1 , N 2 ) and two strongly (P 1 , P 2 ) electric‐dipole–allowed states and an entirely forbidden doubly excited state (D).…”

“…As a result, nonvanishing values for the critical orbital energy differences can and do result already in the PMO, Hückel, or Pariser, Parr, and Pople (PPP) approximations, permitting back‐of‐the‐envelope MCD sign predictions. In contrast, it is much more difficult to predict correctly the relative magnitude of Δ HL and Δ HSL and the negative sign of the former for biphenylene 1.…”

“…Perimeter model predictions are depicted in Figures 6 and 7 2, 10–12. Taking either of the two planes of symmetry perpendicular to the molecular plane at the D 2h geometry, which we consider first in this simple approximation, we note that the symmetries of the molecular orbitals h − and l − are the same.…”

“…In Part 5 of this series 1, we examined the magnetic circular dichroism (MDC) of biphenylene and its heterocyclic analogs. The non‐alternant pentalene ( 1 ) and heptalene ( 2 ) are additional prototype hydrocarbons of the “unaromatic” 2 kind, derived from antiaromatic 4 N ‐electron perimeters by a cross‐linking perturbation that significantly splits the nonbonding orbital pair.…”

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

“…LUMO (s À !s + ) transition. This type of forbidden transition is characteristic of unaromatic cyclic p-electron systems such as biphenylenes, [16] pentalenes, [17] and heptalenes. [17] 4N-Electron perimeter model analysis: According to the 4N-electron perimeter model, six frontier p-orbitals are of particular importance.…”