Computational studies of reactions involving 1st, 2nd, and 3rd row elements and the performance of theory

Islam, Mohammad Shahidul (2007) Computational studies of reactions involving 1st, 2nd, and 3rd row elements and the performance of theory. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

This thesis has involved detailed computational studies of the mechanisms of several chemical reactions involving first, second and third row elements. Geometries of the relevant molecules were optimized at the HF, MP2 and B3LYP levels using the 6-31G(d), 6-31+G(d), 6-31G(d,p) and aug-cc-pVDZ basis sets. Gaussian-n theories such as G3MP2, G3MP2B3 and G3B3 were also used, as they are expected to adequately reproduce experimental data. The complete reaction pathways for all the mechanisms have been verified using intrinsic reaction coordinate (IRC) analysis. -- The reactions of SiH₃ X (X = H, Cl, Br, I) with HCN were investigated and three different mechanisms were obtained. One of the mechanisms involves HX elimination by a one-step pathway producing SiH₃ CN. The second mechanism consists of H₂ elimination, producing SiH₂ XCN via a one-step pathway or three multiple-step pathways. The third mechanism involves dissociation of SiH₃ X to various products, which can then react with HCN. -- We have found for the first time that the mechanism of the addition of bromine to alkenes involves reaction with two bromine molecules in non-polar aprotic solvents, while in polar protic solvents the mechanism involves reaction with a single bromine molecule mediated by a solvent molecule. For both cases, the calculated activation energies were found to be in excellent agreement with experiment. We proposed a kinetic expression that accounts for the difference between bromination of alkenes in protic and aprotic solvents. We also found that bromination of adamantylideneadamantane should occur spontaneously in the gas phase as well as in some solvents with no reaction barrier. -- We have found that for third row elements the BC6-31G basis set which is widely used as a 6-31G basis set in most of the commercial quantum chemistry packages does not meet the definition of the standard 6-31G basis set. A comparative study of the performance of the standard 6-31G and Binning-Curtiss (BC6-31G) basis sets for third row elements, Ga, Ge, As, Se, and Br, was carried out. Frequencies and thermodynamic values obtained by using the standard 6-31G basis set are better than those obtained using the BC6-31G basis set when compared to experiment and G3MF2. We recommend that the standard 6-31G basis set be used for calculations involving 3rd row elements. -- The kinetic isotope effects (KIEs), a major experimental tool to determine the transition state (TS) structure, have been used to characterize the transition state structure of SN2 reactions. Chlorine leaving group k³⁵/k³⁷ , nucleophile carbon k¹¹/k¹⁴ and secondary α-deuterium [(kH/kD)α] kinetic isotope effects (KIEs) have been calculated for the SN2 reactions between para-substituted benzyl chlorides and cyanide ion and compared to the experimental results to determine whether these isotope effects can be used to determine the substituent effect on the structure of the transition state. It was found that both leaving group and nucleophile KIE vary with the TS structure. However, a correct and measurable substituent effect on leaving group KIEs will only be found for a very reactant-like or for a very product-like TS. The substituent effect on nucleophile KIEs will only be found when the Nu-Cα bond formation in the TS is well advanced i.e., in a product-like SN2 TS. -- Nucleophile carbon k¹¹/k¹⁴ and secondary a-deuterium [(kH/kD)α] kinetic isotope effects (KIEs) were also calculated for the SN2 reactions between tetrabutylammonium cyanide and ethyl iodide, bromide, chloride and tosylate and compared to the experimental results to determine whether these isotope effects can be used to determine the structure of the SN2 transition states. The results showed that the nucleophile carbon k¹¹/k¹⁴ KIEs can be used to determine the transition state structure in different reactions and the results suggest that changing to a poorer leaving group leads to a tighter transition state. The magnitude of the experimental secondary α-deuterium KIE is related to the nucleophile - leaving group distance in the SN2 transition state (RTS ) for reactions with a halogen leaving group. However, the calculated and experimental α-deuterium KIEs show opposite trends with leaving group ability. -- In conclusion, the results of my doctoral research have greatly increased our knowledge of the mechanisms, transition state structures and the thermodynamic properties of reactions involving 1st, 2nd and 3rd row elements.

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