VIBRATIONAL OVERTONE ACTIVATION OF THE ISOMERIZATION OF METHYL ISOCYANIDE

The photoisomerization of methyl isocyanide to form acetonitrile induced by excitation into the fourth (approximately 1 kcal/mol above the activation barrier) and fifth (approximately 8 kcal/mol above the barrier) C-H stretch vibrational overtones is reported. The ratio of the collisional deactivation rate constant to the unimolecular rate coefficient, k(epsilon), was determined by a Stern-Volmer analysis plotting the inverse apparent rate constant against the total pressure. The unimolecular rate coefficients increase monotonically with increasing excitation energies across the rotational band contours. The experimental k(epsilon) agree with RRKM calculated values. The Stern-Volmer plots are nonlinear at low pressure: the fourth overtone excitation shows negative curvature (decreasing slope with increasing pressure) and the fifth overtone shows positive curvature (increasing slope with increasing pressure). The magnitude and direction of this curvature agree with the calculated Stern-Volmer plots in earlier work using a master equation simulation. In these vibrational overtone activation studies, the collisional deactivation efficiency of argon is 0.3 of that of the self-collider.