How do ring currents affect (1)h NMR chemical shifts?

Area of Science:

• Quantum chemistry
• Nuclear Magnetic Resonance (NMR) spectroscopy

Background:

• Traditional explanations for proton NMR chemical shifts, particularly concerning aromatic systems, are widely accepted but lack fundamental validation.
• The concept of 'ring current effects' has been a cornerstone in interpreting the chemical shifts of protons in molecules like benzene, naphthalene, and anthracene.

Purpose of the Study:

• To fundamentally revise conventional explanations of proton NMR chemical shifts.
• To investigate the electronic origins of chemical shifts in various organic molecules using advanced computational methods.

Main Methods:

• Utilizing *ab initio* calculations, specifically the Individual Gauge for Localized Orbitals (IGLO) method.
• Analyzing the contributions of different electronic factors (e.g., pi electron systems, sigma CC influences) to the total magnetic shielding.

Main Results:

• The downfield chemical shift of benzene's proton (¹H) is not primarily caused by deshielding ring current effects; its shielding is less than the pi contribution in vinyl protons.
• Enhanced deshielding from sigma CC influences explains the downfield shifts observed for the inner protons of naphthalene and anthracene.
• Double pi effects contribute to shielding in ethynyl protons, contradicting the notion of a special 'ring current influence'.

Conclusions:

• The prevailing 'ring current effect' theory for explaining proton NMR chemical shifts is inadequate and requires substantial revision.
• Electronic factors such as sigma CC influences and pi electron interactions are more critical in determining chemical shifts than previously understood.
• This study provides a new framework for interpreting NMR chemical shifts, emphasizing *ab initio* computational insights over traditional models.