Theoretical and Experimental Investigations of High Spin Ionic Intermediates
Falvey, Daniel E
In order to identify high-spin organic intermediates that could potentially be used as building blocks for the construction of high-spin organic ferromagnets, density functional theory (DFT) computations were performed to assess the singlet-triplet state energy gaps for a number of substituted aryl ionic intermediates. The quantitative accuracy of these DFT computations was benchmarked by high-level multireference second order perturbation theory (CASPT2) computations for representative species. These computations led to the discovery of a novel meta pi donor effect, wherein substituting the meta positions of aryl cationic species such as arylnitrenium ions (Ar-N-H+), arylsilylenium ions (Ar-SiH2+), aryloxenium ions (Ar-O+), and benzyl cations (Ar-CH2+), with pi donors stabilizes a π,π* triplet state analogous to the electronic state of the m-xylylene diradical. Two of these benzylic cations were generated experimentally through photochemical methods and analyzed by laser flash photolysis, chemical trapping studies, and product analysis. The experimental results were consistent with the existence of low-energy triplet states. Additionally, species with an inverted connectivity (e.g. an anionic donor with two pi electron withdrawing groups) were also found to have low-energy triplet states by density functional theory computations. These anions were generated chemically and studied by NMR and EPR spectroscopy as well as quenching studies. The preliminary results of the experimental studies are consistent with the intermediacy of triplet ground state benzyl anions, in line with the theoretical predictions. Vinyl cations substituted with β pi donors were also found to have triplet ground states, as computed by DFT and CBS-QB3 methods. In many cases, the singlet vinyl cations are anticipated to have facile rearrangement pathways, but incorporating the pi donors into rings appears to discourage obvious rearrangement pathways. To permit the photogeneration of congested arylnitrenium ions, a new method for photochemically generating these species was developed through photolysis of protonated 1,1-diarylhydrazines. Additionally, the carbazolyl nitrenium ion was generated photochemically and studied by laser flash photolysis, chemical trapping studies, product analysis and computational studies. This nitrenium ion is found to be more short-lived and reactive than similar diarylnitrenium ions as a likely result of destabilizing antiaromatic character.