Ring-Current Maps for Benzenoids: Comparisons, Contradictions, and a Versatile Combinatorial Model | JoVE Visualize

Area of Science:

* Theoretical Chemistry
* Computational Chemistry
* Organic Chemistry

Background:

* Induced ring current is a key diagnostic property of benzenoids and polycyclic hydrocarbons.
* Existing calculation methods include grid-based (ipsocentric ab initio, pseudo-π) and graph-based (conjugated-circuit, Hückel-London) models.
* Assessing the physical relevance of different current maps is crucial for understanding molecular properties.

Purpose of the Study:

* To evaluate and compare the physical relevance of various induced ring current calculation models for benzenoids.
* To identify limitations of existing grid-based and graph-based models.
* To develop an improved model for accurate ring current calculations.

Main Methods:

* Devised a protocol for graph-reduction and comparison of current maps derived from different models.
* Analyzed pseudo-π grid maps and compared them with Hückel-London (HL) model maps.
* Compared various conjugated-circuit (CC) models against HL maps using error norms.

Main Results:

* Graph reduction revealed similarities and systematic differences between pseudo-π and HL maps, attributed to proximity limitations and fixed bond issues in pseudo-π models.
* Published CC models reasonably approximate HL maps but show physically implausible currents for systems with fixed bonds.
* The new combinatorial Model W, based on cycle decomposition, significantly improves adherence to HL maps for Kekulean benzenoids (reducing rms discrepancy from 11% to 5%) and performs well for non-Kekulean systems (4% rms discrepancy).

Conclusions:

* Model W offers a significant improvement over existing CC models for calculating induced ring currents in benzenoids, particularly those with fixed bonds.
* The new model accurately describes currents in both Kekulean and non-Kekulean benzenoids, including nanographenes.
* Consideration of largest and next-to-largest matchings is a valuable strategy for modeling and interpreting ring currents in complex polycyclic systems.