Carbenic nitrile imines: properties and reactivity.

Area of Science:

• Computational Chemistry
• Organic Chemistry
• Quantum Chemistry

Background:

• Nitrile imines are reactive intermediates with ambiguous electronic structures.
• Previous studies lacked definitive methods to distinguish between propargylic, allenic, and carbenic forms.

Purpose of the Study:

• To computationally investigate the structures and properties of nitrile imines.
• To differentiate between propargylic, allenic, and carbenic electronic structures using advanced quantum chemical methods.
• To correlate spectroscopic properties with specific nitrile imine structures.

Main Methods:

• Density Functional Theory (DFT) calculations using the B3LYP functional.
• Coupled Cluster calculations with Single, Double, and Triple excitations (CCSD(T)).
• Natural Bond Orbital (NBO) analysis to probe electronic structure and bonding.

Main Results:

• B3LYP calculations suggested a propargylic structure, while CCSD(T) calculations clearly distinguished propargylic, allenic, and carbenic forms.
• Nitrile imines with strong IR absorptions above 2150 cm⁻¹ are propargylic (e.g., RCNNSiMe3).
• Nitrile imines with IR absorptions below 2150 cm⁻¹ are allenic (e.g., HCNNPh).
• Nitrile imines lacking significant IR absorptions in the 1900–2200 cm⁻¹ range are carbenic.
• Electronegative, lone pair-donating groups (NR2, OR, F) stabilize carbenic nitrile imines.
• Carbenic nitrile imines are predicted to undergo cycloadditions, form azoketenes, and cyclize to 1H-diazirenes.

Conclusions:

• CCSD(T) calculations provide a reliable method for classifying nitrile imine structures.
• The electronic structure of nitrile imines dictates their reactivity and potential synthetic applications.
• Carbenic nitrile imines exhibit versatile reactivity, accessible through photochemical and thermal pathways.