From carbenes to cumulenes : application and investigation of Density Functional Theory
Woodcock, Henry Lee
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In Chapter 2 Density functional theory (DFT) was used to evaluate the unusual stability of triplet over singlet 2,6-dibromo-4-tert-butyl-2',6'-bistrifluoromethyl-4'- isopropyl-diphenylcarbene (1). The triplet, computed at B3LYP/6-311G(d,p), had a slightly bent carbene angle of 167° with a singlet angle of 134°. The singlet – triplet energy separation was 12.2 kcal/mol whereas the parent, diphenylcarbene, was computed to have a 5.8 kcal/mol .Est. Using an isodesmic reaction we found that both singlet and triplet 1 is destabilized by 6.3 and 12.5 kcal/mol, respectively, due to multiple steric effects. Our computations confirm experimental results that triplet diarylcarbene, ortho substituted with bulky CF3 and Br substituents, is persistent due to steric protection of the diradical center. In Chapter 3 a systematic investigation of DFT was performed to evaluate the erroneous relative energy separations observed when computations on cumulenes and poly-ynes are executed. Isomeric energy separations, .E, were studied via three schemes: standard optimizations and energy determinations (CCSD(T)/cc-pVXZ//MP2/cc-pVTZ), heats of formation predictions via isodesmic and homodesmotic reactions (CCSD(T)/ccpVTZ// MP2/cc-pVTZ), and BHLYP functional analysis fitted to experimental data. Results from the BXLYP (X being a variable to describe the amount of HF exchange included) and energy decomposition analyses in conjunction with previous studies lead to the conclusions that gradient corrected functionals are not properly constructed to handle delocalized cumulenes and that they tend to overstabilize them.