Substituent effects on stability and isomerization energies of isocyanides and nitriles

Homodesmotic reactions were designed to study substituent effects on stability and isomerization energies of isocyanides and nitriles. Good correlations between isocyanide and nitrile stabilization energies and Taft's dual-substituent parameters have been found. As to substituent effects of isocyanides, σ-donating N-substituents stabilize isocyanides while σ- accepting N-substituents destabilize them; π-donating N-substituents destabilize isocyanides while π-accepting N-substituents stabilize them. Regarding substituent effects of nitriles, σ-donating substituents stabilize nitriles while σ-accepting substituents destabilize them; π-donating substituents stabilize nitriles while π-accepting substituents destabilize them. Sensitivity of nitrile stability to substituent resonance and inductive effects is much less than that of isocyanides does. Isocyanide-nitrile isomerization energies have a good correlation with Taft's dual-substituent parameters; σ-donating substituents disfavor isomerization of isocyanides to nitriles while σ-accepting substituents favor it; π-donating substituents favor the isomerization while π-accepting substituents disfavor it. The isomerization of isocyanides to nitriles is exothermic, except for isocyanides with highly electropositive substituents such as Li, MgH, Na, AlH₂, and BeH.