Abstract
We show that natural bond orbital (NBO) and natural resonance theory (NRT) analysis methods provide both optimized Lewis-structural bonding descriptors for ground-state electronic properties as well as suitable building blocks for idealized “diabatic” two-state models of the associated spectroscopic excitations. Specifically, in the framework of single-determinant Hartree-Fock or density functional methods for a resonance-stabilized molecule or supramolecular complex, we employ NBO/NRT descriptors of the ground-state determinant to develop a qualitative picture of the associated charge-transfer excitation that dominates the valence region of the electronic spectrum. We illustrate the procedure for the elementary bond shifts of SN2-type halide exchange reaction as well as the more complex bond shifts in a series of conjugated cyanine dyes. In each case, we show how NBO-based descriptors of resonance-type 3-center, 4-electron (3c/4e) interactions provide simple estimates of spectroscopic excitation energy, bond orders, and other vibronic details of the excited-state PES that anticipate important features of the full multi-configuration description. The deep 3c/4e connections to measurable spectral properties also provide evidence for NBO-based estimates of ground-state donor-acceptor stabilization energies (sometimes criticized as “too large” compared to alternative analysis methods) that are also found to be of proper magnitude to provide useful estimates of excitation energies and structure-dependent spectral shifts.
Highlights
Chemical reactivity of a molecular or supramolecular species is often associated with characteristic features of its electronic spectroscopy [1]
(3) Resonance-structural depiction of the secondary bonding pattern that results from the e-pair type are well-known to profoundly influence the structural, reactive, and spectroscopic properties of bond shift in the parent natural Lewis structure (NLS)
Starting from the simplest form of two-state model in the framework of single-determinant Kohn-Sham density functional theory (KS-DFT) [43], we show how natural bond orbital (NBO)-based deletion techniques [44] can be used to construct suitable diabatic models for the two-state secular determinant that couples KS-DFT description of the ground state to that of a target bond-shifted “mirror state” in the excitation spectrum
Summary
Chemical reactivity of a molecular or supramolecular species is often associated with characteristic features of its electronic spectroscopy [1]. {bAB}and resonance weightings {wR} that allow continuous tracing of a bond shift from one limit to πCN bond shift, with concomitant nn → nO lone pair shift and charge transfer; another along a reactive pathway Such resonance-type NBO/NRT bond shifts of nN-π*CO or πCC-π*CC (3) Resonance-structural depiction of the secondary bonding pattern that results from the e-pair type are well-known to profoundly influence the structural, reactive, and spectroscopic properties of bond shift in the parent NLS. The envisioned electronic responses to a distortion of nuclear geometry are quantified by the NBO/NRT-based descriptors of the ground-state PES, where the actual symmetry-breaking effect is observed.
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