Computational Studies of the Chemistry of Reactive Intermediates
This group is interested in the application of computational tools to questions of chemical reactivity and the structures of reactive intermediates. They are using MSI for two projects:
- Photochemistry of N-oxides. The photochemistry of compounds as simple as pyridine N-oxide is still not well understood and quite complex: ring fragmentation and expansion reactions have been observed, as well as triplet oxygen ejection and electron transfer reactions. The mechanisms involved, and intermediates formed are still a matter of debate. The situation becomes more complex with fused bicyclic systems. Perhaps surprisingly, no computational study of N-oxide photochemistry has been published with an attempt to elucidate these matters - this may be in part due to the fact that many of the proposed intermediates have diradical character. These researchers are carrying out such a study with a view to providing additional clarity regarding the mechanism of reaction - they will be utilizing a combination of DFT and CASSCF theories to do so. The approach will be similar to those they and others have employed in the past to investigate nitrene photochemistry and rearrangements - mechanistically the proposed N-oxide reactions bear some similarity to rearrangements of phenylnitrene, even though the systems are not isoelectronic.
- Phosphazide/Iminophosphorane reactivity. Phosphazides are generated by the reaction of organic azides with phosphines, and are important intermediates in aza-Wittig chemistries such as the Staudinger reduction or Staudinger ligation. Generally, these are believed to undergo nitrogen extrusion to generate iminophosphoranes, which in the case of the Staudinger reduction undergo hydrolysis to yield an amine and phopshine oxide. These researchers have determined that in some cases, however, the phosphazide is hydrolyzed directly without the intermediacy of the iminophosphorane. The formation of iminophosphorane requires conformational change in the phosphazide. The researchers are interested in probing the conformational space of the phosphazide to determine the rotational barriers to iminophosphorane formation, and also in using computational methods to investigate the viability of various potential mechanisms for phosphazide hydrolysis. In these cases, conformational searching will be performed using low level HF, DFT or even simpler empirical methods, with the structures of critical points (conformers and rotational transition states) refined at higher levels of theory. Mechanistic studies will be performed at moderate levels of DFT modeling with single point energy refinement using higher levels of DFT, or with composite methods in association with appropriate solvation models (such as the COSMO approach used in the ADF suite).
Dr. James Poole