Characterizing nitrilimines with nuclear magnetic resonance spectroscopy. A theoretical study

Area of Science:

• Computational Chemistry
• Organic Chemistry
• Quantum Chemistry

Background:

• Nitrilimines are versatile organic compounds with unique electronic structures.
• Understanding the factors influencing their spectroscopic properties, like 13C chemical shifts, is crucial for characterizing these molecules.
• Previous studies have indicated a carbene-like component in nitrilimine electronic structure.

Purpose of the Study:

• To calculate and analyze 13C chemical shifts in nitrilimines and related compounds using computational methods.
• To investigate the impact of substituent effects on the 13C chemical shifts of nitrilimines.
• To explore the electronic structure of nitrilimines by examining their shielding tensors and comparing them to singlet carbenes.

Main Methods:

• Density-functional theory (DFT) calculations employing the PBE0 functional.
• Gauge-including atomic orbital (GIAO) method for accurate chemical shift prediction.
• Analysis of the diagonalized 13C shielding tensor components.

Main Results:

• Substituents deshield the carbon nucleus in nitrilimines in the order CH3 < NH2 < OH < F.
• Terminal N-substitution has a greater impact on chemical shifts than C-substitution due to shielding anisotropy.
• Diaminonitrilimine (H2N-CNN-NH2) is predicted to be a stable acyclic, unsaturated carbene.
• Increased dipolar character in nitrilimines leads to carbon nucleus shielding, while increased allenic or carbenic character causes deshielding.

Conclusions:

• The study provides a detailed computational analysis of 13C chemical shifts in nitrilimines, correlating them with electronic structure.
• The findings suggest diaminonitrilimine as a potential novel stable carbene, expanding the scope of known carbene structures.
• The relationship between dipolar, allenic, and carbenic character and 13C chemical shifts is elucidated.