A theoretical study of compounds containing sulfur and nitrogen and a statistical assessment of errors in quantum chemistry procedures

Mawhinney, Robert Christopher
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University of Guelph

In this thesis the errors in commonly used computational quantum chemistry procedures were assessed using a linear regression analysis approach and error confidence intervals for predictions were obtained. Seven predicted properties were appraised: bond distances, harmonic vibrational frequencies, dipole moments, atomization energies, ionization potentials, electron affinities, and disproportionation energies. The relative errors for each of the properties were combined and using computational costs an overall "Performance Efficiency Ratio" was evaluated. The B3LYP/cep-31G* procedure had the best Performance Efficiency Ratio of twelve methods compared. The study of four sulfur-nitrogen systems: S2N2, H(SN)XH (X = 2-10), HCNXSY (X + Y = 4), and a series of fused ring dithiazolyls employed this procedure exclusively. The most viable pathway for the formation of S2N2 isomers is by combining two radical SN diatomic molecules. This pathway is believed to be responsible for the most recent isomer observed experimentally. The only isomer consistent with the new experiment has a trans skeleton with two SN fragments linked through sulfur. An experimentally consistent intermediate species in the polymerization process where S2N2 forms (SN)x has been proposed. The H(SN)XH (X = 2-10) system has been used as a model for (SN)X. The parallel conductivity along the polymer chain is governed by the energy difference between the lowest singlet and triplet electronic states and, consistent with a metallic state, has been shown to converge asymptotically to zero as the chain length increases. The perpendicular conductivity is governed by susceptibility to charge transfer and is given by the disproportionation energy, a combination of ionization potential and electron affinity, which is shown to converge asymptotically to a value of 2.55 eV, consistent with observed conductivity measurements. The HCNXS Y(X + Y = 4) systems reveal that to obtain the most optimum disproportionation energy two sulfur atoms and one nitrogen atom should be included in the cyclo-pentyl ring. The 1,2,3-dithiazolyl isomers are found to exhibit properties that are more useful for the development of building blocks for molecular conductors. The one exception is the 1,3,2-cyclo-octa-tetraene-dithiazolyl species which we predict to be the best overall candidate. This molecule has the lowest disproportionation energy of ca. 4.3 eV, a delocalized spin density, and a SOMO-LUMO gap of 0.03 eV.

Sulfur, Nitrogen, Errors, Statistical assessment, Quantum chemistry procedures