The unimolecular isomerization of methyl isocyanide to methyl cyanide (acetonitrile)

The unimolecular isomerization of CH3NC→CH3 NC →CH3CN is studied by many-body (diagrammatic) perturbation theory. Using double-zeta plus polarization (DZP) contracted Gaussian basis sets (63 CGTO’s), correlated calculations are made by evaluating all second-, third-, and fourth-order diagrams that occur due to single, double, and quadruple excitations SDQ–MBPT(4). These diagrams involve ∼107 CI type configurations. The SDQ–MBPT(4) model predicts the enthalpy of isomerization to be −22.7 kcal/mole exp(−23.7 ±0.14). The activation barrier is found to be ∼41 kcal/mole (exp 38.4). The inclusion of quadrupole excitation diagrams has little effect on the enthalpy of isomerization but increases the activation barrier by 1 kcal/mole.