Assessing and characterizing the inductive effect through silicon containing backbones and on silicon reactivity
Smith, Ashlyn Patricia
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This thesis describes a study of the inductive effect in derivatives of bicyclo[1.1.1.]pentane; chosen because it eliminates any possibility of conjugation between the substituent and probe, and keeps the steric effect constant by providing a rigid backbone. The substituent effects, both upon a silicon center and transmitting through silicon atoms, in these systems were studied using Density Functional Theory and the isodesmic reaction approach to Hammett's methods. The elctron density distribution was analyzed using the Quantum Theory of Atoms in Molecules. Although less sensitive to substitution, it was discovered that the effect as measured on a Si‐probe (–Si(OH)3) is the same as that measured using a C‐probe (–COOH). In both cases, the transmission of the “so‐called” inductive effect appears to operate in the same fashion: through the molecule using the atomic dipole moment. The x‐component (axis connecting the substituent and probe) of the substituent dipole was determined to be the controlling property. Despite minor differences in structure, replacing the backbone atoms with silicon appears to have little effect upon the mechanism of transmission, but a general decrease in sensitivity, to the effect of substitution, is apparent. As the atomic dipole moment conforms to the principle of atomic transferability, it is possible to describe the inductive effect in terms of the substituent‐only dipole (μx(RH); determined for the RH system). In fact, we were able to replace the substituent constant, an empirically derived parameter, with μx(RH), a quantum mechanically derived parameter. Linear free energy relationships to describe the inductive effect with μx(RH), as well as an electronegativity term and steric terms to describe the backbone and probe, were developed that essentially recreate the entire substituent effect.