(a) Consider entries 1 through 4 of Table 13.1. How does the chemical shift of a proton vary with the electronegativity of the neighboring halogen?

(b) Compare entries 2, 5, and 6 of Table 13.1. How does chemical shift vary with the number of neighboring halogens?

(c) Compare entries 6 and 7. How is the chemical shift of a proton affected by its distance from an electronegative group?

(d) Explain why (CH₃)₄Si absorbs at lower chemical shift than the other molecules in the table. Can you think of a molecule with protons that would have a smaller chemical shift than TMS (that is, a negative d value)?

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TABLE 13.1 Effect of Electronegativity Entry number Compound 1 2 3 4 5 6 7 8 9 * By definition. CH-F CH₂CI CH₂Br CH₂I CH₂Cl₂2 CHCI CH₂CCI3 (CH3)4C (CH3)4Si on Proton Chemical Shift Chemical shift, 8 4.26 3.05 2.68 2.16 5.30 7.27 2.70 0.86 0.00* Electronegativity (Table 1.1) F: 3.98 Cl: 3.16 Br: 2.96 I: 2.66 C: 2.55 Si: 1.90