LON-CAPA NMR1

Group Electronegativities and NMR Spectroscopy

Introduction

Nuclear magnetic resonance (NMR) spectroscopy provides a simple means of measuring group electronegativities. In proton nuclear magnetic resonance spectroscopy ( ¹H-NMR), each unique hydrogen nucleus produces a signal. The intensity of the signal (peak height) is plotted against a parameter known as the chemical shift. The symbol for the chemical shift is d; the units are parts per million (ppm). For hydrogen nuclei, typical d values range from 0-10 ppm. For the purposes of this discussion, the primary factor that affects the chemical shift of a particular proton is the electron density distribution in the bond between the proton and its bonding partner. That electron density distribution is determined by the electronegativity of the atom or group that is attached to the bonding partner. Figure 1 demonstrates the changes in chemical shift that accompany changing the group, G, that is attached to the carbon atom of a series of compounds represented by the general formula H₃C-G.

Figure 1

Some NMR Basics

The logic required to understand this information goes as follows:

The three H atoms of a methyl group are identical. Therefore they produce one signal.
The chemical shift of the signal depends upon the electron density around each H.
The electron density around each H depends on the electronegativity of the group G that is attached to the carbon that is bonded to the H atoms.
If G is more electronegative than C, it will pull electron density away from C. This will, in turn, reduce the electron density around the H atoms.
Reducing the electron density around the H atoms causes the chemical shift to move to higher d values.

Exercise 1 What would you expect the chemical shift of the methyl group in bromomethane, CH₃Br, to be? less than 2.2 between 2.2 and 3.1 between 3.1 and 4.3 greater than 4.3

Exercise 2 The chemical shift of the protons in ethane, CH₃CH₃, is 0.9 ppm. Is a methyl group more or less electronegative than a halogen atom? more less

Exercise 3 The chemical shifts of methane and ethane are -0.2 and 0.9 ppm, respectively. Which is more electronegative, a hydrogen atom or a methyl group. (Hint: Think of methane, CH₄, as CH₃-H and ethane as CH₃-CH₃.) H CH₃