Electronic structure, cycloaromatization and effective fragment potential
Abstract
Equation of motion coupled cluster with single and double excitations (EOM-CCSD) method and time-dependent density functional theory (TD-DFT) are used to investigate electronic states of Diphenylmethane. The S1 and S2 potential energy surfaces with regard to two torsional angles are constructed. The TD-DFT and EOM-CCSD surfaces are qualitatively similar; however, both methods overestimate the experimentally observed S 1–S2 splitting of 123 cm-1 by four to five times. EOM-Spin-Flip-CCSD method accurately describes diradical states and is used to determine vertical and adiabatic singlet–triplet energy splittings in the substituted cyclobutadienes. The adiabatic singlet–triplet gaps decrease upon substituent addition, but the singlet states remain lower in energy. However, the results are affected by spin contamination of the reference state and deteriorate when an unrestricted HF reference is employed. A MP2/cc-pVTZ intrinsic reaction pathway was constructed from a dialkynylcyclobuta-1,3-diene derivative undergoing thermal ring expansion to an eight-membered ring via either Cope-type-sigmatropic rearrangement or Bergman cyclization followed by ring opening. These findings raise questions concerning the ring coalescence and annealing model for carbon condensation and fullerene formation which does not include either of these reaction pathways. The exchange-repulsion term or effective fragment potential (EFP), a first-principles based model potential developed as a nonempirical alternative to force-field based QM/MM methods, was parameterized in order to increase the speed of EFP calculations by a factor of ten. The expression for exchange-repulsion involves overlap and kinetic energy integrals between pairs of localized orbitals. These integrals make the exchange-repulsion the most computationally expensive part of EFP energy calculations in moderately sized systems.
Degree
Ph.D.
Advisors
Slipchenko, Purdue University.
Subject Area
Physical chemistry
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