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Molecular modeling of para-benzyne and a series of push-pull [14]-pyridoannulenes

By Evan B. Wang


Molecular modeling is vital for gaining insight into experimental results and discovering molecular properties not yet characterized by experiment. Depending on the type of molecule under investigation, certain methods will yield more accurate and revealing answers. In the first project, a state-averaged, multireference complete active space (CAS) approach was used for the determination of the vertical excitation energies of valence and Rydberg states of para-benzyne. Orbitals were generated with a 10- and 32-state averaged multiconfigurational self consistent field (MCSCF) approach. Electron correlation was included using multireference configuration interaction with singles and doubles (MR-CISD), including Pople correction for size extensivity (MR-CISD+Q), averaged quadratic coupled cluster (MR-AQCC) and MR-AQCC based on linear response theory (MR-AQCC-LRT). There is a very high density of electronic states in this diradical system - there are more than seventeen states within 7 eV of the ground state including two 3s Rydberg states. Of the thirty-two states characterized, fifteen were multi-configurational, including the ground 1Ag state, providing further evidence for the necessity of a multireference approach for p-benzyne. In the second project, three new isomeric dipyridoannulenes have been synthesized and characterized. These molecules possess differing conjugation pathways between the substituent alkoxy donating groups and the pyridyl acceptor groups. Optical absorption and emission properties of the dipyridoannulenes and their corresponding acyclic precursors were measured and correlated to structural differences, and used to evaluate conjugation effectiveness and charge-transfer pathways. Optical properties of protonated dipyridoannulenes were also measured and found to be somewhat insensitive to the degree of protonation. Density functional studies of these systems at the B3LYP/6-31G* level provided insight into the stabilities, polarities and quinoidal character. An analysis of the HOMO and LUMO molecular orbitals provided further information regarding charge - transfer behavior. These systems are good metal binding candidates as the pyridine moiety can act as both an electron acceptor and a site for metal coordination

Topics: Molecules, Models, Chemistry, Physical Sciences and Mathematics
Publisher: UR Scholarship Repository
Year: 2009
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