Time-Resolved Studies of CN Radical Reactions and the Role of Complexes in Solution

Time-resolved studies using 100 fs laser pulses generate CN radicals photolytically in solution and probe their subsequent reaction with solvent molecules by monitoring both radical loss and product formation. The experiments follow the CN reactants by transient electronic spectroscopy at 400 nm and monitor the HCN products by transient vibrational spectroscopy near 3.07 mu m. The observation that CN disappears more slowly than HCN appears shows that the two processes are decoupled kinetically and suggests that the CN radicals rapidly form two different types of complexes that have different reactivities. Electronic structure calculations find two bound complexes between CN and a typical solvent molecule (CH2Cl2) that are consistent with this picture. The more weakly bound complex is linear with CN bound to an H atom through the N atom, and the more strongly bound complex has a structure in which the CN bridges Cl and H atoms of the solvent. Fitting the transient absorption data with a kinetic model containing two uncoupled complexes reproduces the data for seven different chlorinated alkane solvents and yields rate constants for the reaction of each type of complex. Depending on the solvent, the linear complex reacts between 2.5 and 12 times faster than the bridging complex and is the primary source of the HCN reaction product. Increasing the Cl atom content of the solvents decreases the reaction rate for both complexes.