Part I. A carbon-13 NMR labeling study of the Wessling polymerization. Part II. Using DFT methods to probe open shell molecules: Problems and prospects
Abstract (Summary)Part I. A 13 C-NMR labeling study of the Wessling polymerization. The Wessling polymerization was studied by 13 C-NMR techniques. Using commercially available para -xylene-Ã?Â±-Ã?Â± ' -di-13 C, the synthesis of 1,4-bis(bromo- 13 C-methyl) benzene was accomplished and subsequently converted to 1,4-bis(tetrahydrothiophenio-13 C-methyl)benzene dibromide as a water-soluble monomeric salt, which was treated with base to give the water soluble polyelectrolyte precursor of poly(para -phenylene vinylene). Calibration reactions of varying concentrations of precursors via the copolymerization of 1,4-bis(tetrahydrothiophenio-13 C-methyl)benzene dibromide and 1,4-bis(tetrahydrothiopheniomethyl)benzene dibromide were carried out. Copolymerization of 1,4-bis(tetrahydrothiophenio-13 C-methyl)benzene dibromide with 1,4 bis(tetrahydrothiopheniomethyl)-3,5-dichlorobenzene dibromide, 1,4-bis(tetrahydrothiopheniomethyl)-3,5-dimethoxybenzene, 1,4-bis(tetrahydrothiopheniomethyl)-3,5-dibromobenzene, 1,4-bis(tetrahydrothiopheniomethyl)-2-nitrobenzene dibromide, and 2,5-bis(tetrahydrothiopheniomethyl)thiophene dibromide were completed. NMR analysis of polyelectrolytes of 1,4-bis(tetrahydrothiophenio- 13 C-methyl)benzene dibromide reacted with 1,4-bis(tetrahydrothiopheniomethyl)benzene dibromide and 1,4-bis(tetrahydro-thiophenio-13 C-methyl)benzene dibromide reacted with 2,5-dimethyl thiophene tetrahydrothiophenium bromide displayed 13 C-13 C coupling consistent with end polymerization or highly blocky copolymerization. All other copolymer cases showed a lack of 13 C-13 C coupling consistent with random copolymerization. Part II. Using DFT methods to probe open shell molecules : Problems and prospects. A computational study of the effects of structure on the ground state multiplicities of extended phenylenedinitrenes was carried out. The systems evaluated were ortho -phenylenedinitrene, meta -phenylenedinitrene, para -phenylenedinitrene, biphenyl-4,4' -dinitrene, stilbene-4,4' -dinitrene, (E,E)-bis(p )-nitrenophenyl)-1,3-butadiene, 1,6-bis(4-nitrenophenyl)-(E,E,E)-1,3,5-hexatriene, bis(4-nitrenophenyl)diazene, 1,4-dinitrenonaphthalene, 1,5-dinitrenonaphthalene, as well as 3,5-dicyano-2,4,6-trinitrenopyridine and 3,5-dichloro-2,4,6-trinitrenopyridine. Geometry optimizations employing density functional theorem (DFT) functionals along with restricted Hartree-Fock (RHF), restricted open-shell Hartree-Fock (ROHF), unrestricted open-shell Hartree-Fock (UHF)--as well as post Hartree-Fock configuration interaction and complete active space self-consistent field (CASSCF) wavefunctions--were utilized for singlet and triplet biradicals and for quintet states to investigate and predict their ground and excited states. All methods predicted bond lengths consistent with aromatic rings for the quintet states and quinonoidal geometries for the singlet and triplet states. DFT methods for para -phenylenedinitrene are found to predict large singlet-triplet energy gaps (which are inconsistent with experimental values and post HF calculations), when singlet states are represented by spin restricted DFT methods. Singlet ground states are predicted for para - and ortho -phenylenedinitrene, biphenyl-4,4' -dinitrene, stilbene-4,4' -dinitrene, (E,E)-bis(p -nitrenophenyl)-1,3-butadiene, 1,6-bis(4-nitrenophenyl)-(E,E,E)-1,3,5-hexatriene. All are predicted to have nearly degenerate singlet and triplet state energies using multiconfigurational scf (MCSCF) methods. A singlet ground state is also predicted for bis(4-nitrenophenyl)diazene with the triplet first excited state within 2 kcal/mole. DFT state ordering agreed with MCSCF methods when state mixing was allowed. Unrestricted singlet DFT energies were calculated to be lower in energy than triplet states when but were heavily spin contaminated.
School Location:USA - Massachusetts
Source Type:Master's Thesis
Date of Publication:01/01/2000