Correlation Between NMR Coupling Constants and σ-Donating Properties of N-Heterocyclic Carbenes and Their Derivatives

Is the 1JC-H coupling constant for protonated carbene a relevant measure of its σ-donation ability? This paper addresses this question by comparing calculated 1JC-H values with various experimental and theoretical approaches across a broad spectrum of carbene compounds. We have examined not only Arduengo-type NHCs based on the 2-imidazolylidene scaffold but also many other derivatives with modified frameworks, such as carbenes with extended, saturated, or conjugated rings, those with reduced heteroatom stabilization, those stabilized by alternative heteroatoms, permanently charged carbenes, acyclic carbenes, amidocarbenes, and cyclic amino(alkyl/aryl) carbenes, as well as carbodicarbenes and carbodiphosphoranes. Our findings reveal a nuanced relationship between different parameters associated with σ-interaction, such as 1JC-H, Huynh electronic parameter (HEP), σ-donation from ETS-NOCV, and lone pair energy. Notably, the best correlation was observed between 1JC-H and the ETS-NOCV method, particularly for mono- and diaminocarbenes, highlighting the utility of 1JC-H in comparing σ-donation among structurally similar carbene types. However, the use of 1JC-H as a universal measure across all carbene classes appears limited, especially when considering carbenes with significantly different structural frameworks. While HEP is less effective for carbenes with diverse structural backbones, our study suggests that 1JC-H has potential across a broader range of systems. Additionally, the analysis demonstrates that lone pair energy reflects basicity rather than σ-donor ability.