AB-INITIO STUDY OF THE PHOTOCHEMICAL DISSOCIATION OF METHYLAMINE

The threshold photochemistry of methylamine along three dissociation c hannels is examined using CASSCF and MRSDCI wave functions. Excitation of a nitrogen lone pair electron into a Rydberg s orbital produces an excited state that can decompose via NH rupture, CN rupture, or CH ru pture. In the case of NH and CN rupture the excited state and ground s tate surfaces cross as the molecule dissociates, forming a conical int ersection. The NH rupture excited state products lie above the excitat ion energy, and so dissociating molecules are funneled through the con ical intersection and emerge as ground state products. The CN rupture excited state products lie below the excitation energy, and dissociati on would proceed mostly on the excited state surface. In the case of C H rupture, the excited state products lie above the excitation energy, but since there is no conical intersection in this channel, CH ruptur e has to occur only after internal conversion to the ground state. The se three mechanisms are all consistent with recent experimental eviden ce.