Substituent effects on stability and isomerization energies of isocyanidesand nitriles

Homodesmotic reactions were designed to study substituent effects on stabil ity and isomerization energies of isocyanides and nitriles. Good correlatio ns between isocyanide and nitrile stabilization energies and Taft's dual-su bstituent parameters have been found. As to substituent effects of isocyani des, sigma-donating N-substituents stabilize isocyanides while sigma-accept ing N-substituents destabilize them; pi-donating N-substituents destabilize isocyanides while pi-accepting N-substituents stabilize them. Regarding su bstituent effects of nitriles, sigma-donating substituents stabilize nitril es while sigma-accepting substituents destabilize them; pi-donating substit uents stabilize nitriles while pi-accepting substituents destabilize them. Sensitivity of nitrile stability to substituent resonance and inductive eff ects is much less than that of isocyanides does. Isocyanide-nitrile isomeri zation energies have a good correlation with Taft's dual-substituent parame ters; sigma-donating substituents disfavor isomerization of isocyanides to nitriles while sigma-accepting substituents favor it; pi-donating substitue nts favor the isomerization while pi-accepting substituents disfavor it. Th e isomerization of isocyanides to nitriles is exothermic, except for isocya nides with highly electropositive substituents such as Li, MgH,-Na, AlH2, a nd BeH.