Assessment of ring current models for monocycles.

Area of Science:

• Theoretical Chemistry
• Computational Chemistry
• Organic Chemistry

Background:

• Ring current models (RCMs) are crucial for interpreting chemical shifts in cyclic molecules.
• The standard reference RCM, the infinitely thin circular loop of current (ICLOC), has seen limited improvement.
• Understanding π and σ current contributions is key to accurate chemical shift prediction.

Purpose of the Study:

• To evaluate and compare six analytical RCMs, including three novel models.
• To assess the ability of these RCMs to reproduce ab initio ring current strengths and magnetic shielding scans.
• To determine the best RCMs for describing π and σ current contributions in monocyclic systems.

Main Methods:

• Development and discussion of six analytical RCMs.
• Comparison of RCMs based on their reproduction of π and σ current strengths.
• Analysis of NICS∥ (nuclear independent chemical shift parallel component) scans for 33 organic and inorganic monocycles.
• Evaluation of the impact of loop displacement and toroidal widening on model accuracy.

Main Results:

• The ICLOC2 model, featuring two vertically displaced ICLOCs, effectively reproduces π current scans.
• The Farnum and Wilcox toroidal loop model showed no significant improvement for π currents.
• The novel ICLOC2C model, comprising two concentric ICLOCs, demonstrated superior performance for σ currents.
• An off-plane NICS∥ extremum is not a universal indicator of π-aromaticity, being absent in large rings but present in small rings and σ scans.

Conclusions:

• The ICLOC2 and ICLOC2C models offer significant improvements for describing π and σ ring currents, respectively.
• The interpretation of NICS∥ scans requires careful consideration of ring size and current type (π vs. σ).
• Novel RCMs provide a more accurate framework for understanding magnetic shielding in cyclic systems.