The LCAO version of the perimeter model with overlap through second order is used to treat the ππ* electronic absorption and magnetic circular dichroism (MCD) of low-symmetry molecules with a closed-shell ground state and no degenerate states (no threefold or higher order axis) derived from biradical (antiaromatic) parent 4N-electron [n]annulene perimeters by structural perturbations. If a symmetry plane perpendicular to the molecular plane is present, simple explicit algebraic solutions are obtained. Rules are derived for predicting the intensities, polarizations, and MCD signs of low-energy transitions in this class of molecules from the knowledge of relative magnitudes of MO energy differences, which can be frequently deduced by mere inspection of molecular formulas. On the basis of the results, a generalized nomenclature is proposed for low-energy electronic excited states of all even-electron cyclic π systems with a single perimeter.
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.
A computational approach to predict structures of rhodopsin-like G protein-coupled receptors (GPCRs) is presented and evaluated by comparison to the X-ray structural models. By combining sequence alignment, the rhodopsin crystal structure, and point mutation data on the b 2 adrenoreceptor (b2ar), we predict a (À)-epinephrine-bound computational model of the b 2 adrenoreceptor. The model is evaluated by molecular dynamics simulations and by comparison with the recent X-ray structures of b2ar. The overall correspondence between the predicted and the X-ray structural model is high. Especially the prediction of the ligand binding site is accurate. This shows that the proposed dynamic homology modelling approach can be used to create reasonable models for the understanding of structure and dynamics of other rhodopsin-like GPCRs.
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