INFRARED-LASER CHEMISTRY OF LARGE MOLECULES

We have recently discovered a surprisingly efficient CO2 laser-induced reaction in the largest and most complex molecularsystem studied to date. The results obtained in this work suggest that the multiple photon excitation that drives this reaction can be localized long enough to lead to a spatial and site-selective unimolecular dissociation. The experiments are performed on a volatile uranyl complex. UO2(hfacac)2∙THF, which consists of 44 atoms and has some 126 normal modes. The dissociation products are UO2(hfacac)2 and THF. A model is proposed which requires T1 of the pumped mode to be shorter than 10-9 s in order to absorb the five to ten photons required to overcome an approximately 30 kcal/mol activation barrier with laser intensities of only a fraction of a megawatt/cm2. The observed absorption line shape appears homogeneous and the dissociative yield can be as high as 100%. The critical energy migration from the excited uranyl moiety to the bond broken probably occurs in a time longer than 1 ps and shorter than 1 ns. Collisions with argon during the excitation process are found to disrupt the process with extreme efficiency. Our results suggest that the reaction pathway induced by the laser can be predetermined by careful design of the structure and bonding in a molecule. This selective vibrational heating process is applicable to molecules which approach the size and complexity of molecules of biological importance. © 1979, American Chemical Society. All rights reserved.