44029-B4
Dynamic Processes in [4n] Annulenes
The grant money received during year 2 has been devoted to understanding several areas of annulene chemistry. All of the projects for this second annual report were initiated in the first grant year. Most of these are now complete and the work has been published or has been submitted. Highlights of each project are below.
A. Planar Bond Shifting in [4n]Annulenes
Undergraduate students Miles Braten and Gertrude Gutierrez finished their computational work on determining barrier heights for planar bond shifting in [8]-and [12]annulene as well as on bicyclic 1,7-methano[12]annulene. While the data collection phase of this project seems finished, the data still needs to be collated and written up in manuscript form. However, an oral presentation on this research was given by Gertrude Gutierrez at the Spring ACS Undergraduate Meeting in Santa Clara, May 2008.
B. [12]Annulene Global Minimum
The manuscript dealing with this project was completed and published in the Journal of Organic Chemistry, 2008. Undergraduate student Miles Braten also presented these results in a talk at the Spring ACS Undergraduate Meeting in Santa Clara, May 2008. In sum, a mono-trans isomer of [12]annulene was computationally located and determined to be lower in energy than the previously synthesized tri-trans-[12]annulene (CCSD(T)/cc-pVDZ//BHHLYP/6-311+G**). In addition, the computed barrier heights for several escape routes (ring closure) and dynamic processes (conformational automerization, bond shifting) for mono-trans-[12]annulene were all over 15 kcal/mol, suggesting that it should be stable enough at low temperatures to allow for complete characterization.
C. The Radical Anion of [12]Annulene
The bulk of the research done by undergraduates Braten and Gutierrez during the 2007-2008 academic year was related to interpreting the experimental data published regarding the putative synthesis of a [12]annulene radical anion. Specifically, the students needed to validate the computational method for computing hyperfine coupling constants (aH values). Their results on heptalene radical anion, [16]annulene radical anion, and tri-trans-[12]annulene radical anion indicated that ESR hyperfine coupling constants computed at the BLYP/EPR-III level on DFT geometries give much better agreement with experimental values than do those computed using B3LYP/6-31G* when looking at medium-sized annulenes and their valence isomers. Using this method, we were unable to locate C12H12•– isomer that could account for the ESR spectrum previously attributed to a highly twisted structure for 1,7-di-trans-[12]annulene radical anion.
A manuscript on this work is currently under review for the Journal of Organic Chemistry. Our key conclusions in the paper are: (1) when making structural assignments for medium-sized annulene radical anions and their valence isomers, it is essential to look at computed hyperfine coupling constants rather than computed spin densities; (2) when computing aH values, the BLYP functional is far superior to B3LYP, which performs very poorly; (3) when computing relative energies, UCCSD(T) results on are prone to high spin contamination, so restricted open-shell coupled cluster theory [RCCSD(T)] is recommended; (4) based on computed aH values and simulated spectra, none of the structures considered were consistent with the observed ESR spectrum previously assigned to 1,7-di-trans-[12]annulene radical anion; (5) in contrast with the neutral [12]annulene PES, in which mono-trans-[12]annulene is the most stable [12]annulene isomer, a di-trans-[12]annulene isomer is predicted to be the most stable monocyclic radical anion at the RCCSD(T)/cc-pVDZ level. Furthermore, this unusual, completely delocalized structure appears to lack a neutral conformational counterpart.
D. Substituent Effects on [12]Annulene Reactivity
Undergraduate Elizabeth Noey finished her computational project on teasing out the reasons for the different reaction paths for cyclohexeno-fused-[12]annulene and its parent counterpart (tri-trans-[12]annulene). Using B3PW91/6-31+G*//B3LYP/6-31G*, Liz determined that cyclohexeno substituents both hinder the second electrocyclization reaction and promote the intramolecular Diels-Alder reaction by causing increased puckering in the intermediate 8-membered ring. This work was published in 2008 in Organic Letters.
