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44029-B4
Dynamic Processes in [4n]Annulenes
The grant money received during year 1 has been devoted to computational studies of a variety of dynamic processes in annulenes. Specifically, working with undergraduates, we have determined barriers for planar bond shifting in two [12]annulene isomers: tri-trans-[12]annulene and 1,7-methano[12]annulene. We elaborated on this work by exploring barriers to bond shifting in all-cis-[12]annulene (two half-twists, thus formally Hueckel topology) and in Moebius [14]annulene, both of which proceed via transition states with diradical character. We located numerous conformational processes and valence isomerizations for a new global minimum (mono-trans) isomer of [12]annulene. We probed the differences in reactivity of parent [12]annulene vs. substituted [12]annulenes. Finally, we have performed numerous calculations on various [12]annulene radical anion isomers and their interconversions, with the goal of assessing whether or not any had been made, as claimed in the literature.
Planar Bond Shifting and Similar Diradical Processes
As mentioned in our proposal, we were interested in determining why the barrier to planar bond shifting in tri-trans-[12]annulene was so high (>17 kcal/mol), compared to other [4n]annulenes (e.g. ca 9 kcal/mol for [16]annulene). We were also intrigued by the possibility that the transition states for planar bond shifting in [4n]annulenes might generally violate Hund's rule, as in the case of cyclooctatetraene (COT). One undergraduate student (Gertrude Gutierrez) spent her first summer of research (2006) revisiting the case of COT with numerous computational methods, such as UB3LYP, UBH&HLYP, CASSCF, and CASPT2. Somewhat surprisingly, the CASPT2 method gave barriers that were too low, though after correcting for known systematic errors in the method, the agreement with experiment improved. Another undergraduate student, Miles Braten, tackled planar bond shifting in tri-trans-[12]annulene during summer 2006, also using a variety of methods. In this case, after adjustment for systematic errors, the CASPT2 method gave good agreement with experiment. Miles also teased out the important factors contributing to the high bond-shifting barrier in tri-trans-[12]annulene, and found that steric interactions among the inner hydrogens accounts for only ca. 3 kcal/mol, and complete delocalization of the pi electrons accounts for 5 kcal/mol. Thus, most of the barrier (ca. 10 kcal/mol) is presumably due to torsion about the C-C bonds during bond shifting. Another undergraduate, Joseph Moll, applied similar methods to solve the problem of configuration change in [14]annulene, and determined that for this process the bond-shifting transition state must have Moebius topology, and hence be anti-aromatic with diradical character. All three of these students have presented their results at ACS National Meetings (Chicago 2007, Boston 2007). We presented results from this part of our project at the San Francisco ACS Meeting (2006) and at the 4th Heron Island Conference on Reactive Intermediates and Unusual Molecules (2007).
[12]Annulene Radical Anion
Undergraduates Miles Braten and Gertrude Gutierrez have also performed a series of calculations (DFT and coupled-cluster) on numerous isomers of [12]annulene radical anion, with the aim of explaining some perplexing experimental observations reported in the literature. Their results to date––including relative energies, interconversion barriers, and computed ESR hyperfine splitting constants––are not consistent with the literature assignments for the di-trans-[12]annulene radical anion. Currently we are trying to determine what species was actually prepared.
The [12]Annulene PES
Interconversions of neutral [12]annulenes continue to fascinate us. Recently, on a suggestion from Prof. Rainer Herges that a mono-trans isomer was the [12]annulene global minimum, we studied its conformational processes, valence isomerizations, and bond-shifting reactions, to assess the feasibility of its synthesis and characterization. Miles Braten found that numerous escape routes for this species all have substantial barriers (>15 kcal/mol), suggesting that this isomer should be stable enough for characterization. A manuscript describing this work is currently in preparation in collaboration with Rainer Herges.
Another area of focus involves clarifying why the parent [12]annulene does not undergo the same valence isomerization reactions as cyclohexeno- and benzo-fused analogues, as reported recently by the Vollhardt group. Our undergraduate, Elizabeth Noey, has computed barriers for the parent system for reactions analogous to those observed for the substituted systems synthesized by Vollhardt. Her initial results indeed suggest large differences in barriers for the analogous processes, and we are currently working to understand the reasons for these differences.
