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From MyMCAT

Contents 1 Introduction 2 What can H NMR spectrum determine? 3 Analyzing an H NMR Plot 3.1 Different Hydrogens 4 Different Numbers of Hydrogens

Introduction

In a manner similar to how IR Spectroscopy was able to identify functional groups by characteristic peaks at specific wavelengths, proton NMR uses the application of nuclear magnetic resonance to identify hydrogen using a characteristic radio frequency selective to hydrogen nuclei. By analyzing hydrogen peaks one can then determine the structure of the molecule as a whole.

What can H NMR spectrum determine?

A Proton NMR spectrum of an organic molecule can provide information concerning:

• the number of different types of hydrogens present in a molecule
• the relative amounts of the different types of hydrogens present
• the electronic environment of the different types of hydrogens (relation to functional groups)
• the number of hydrogen "neighbors" a hydrogen has (the number hydrogens close to to other hydrogens in a molecule)

Always keep this in mind when reading H NMR plots and try to determine what each peak signifies in relation to these concepts.

Analyzing an H NMR Plot

Different Hydrogens

Methane has four hydrogens attached to a carbon center. Can you selectively identify one hydrogen from any other? No, they all "look" exactly the same. Methanol, on the other hand, also has four hydrogens, but this time three are attached to the carbon, but the forth is attached to an oxygen (which is then attached to the carbon). Can you identify the hydrogens now? Well the three carbons all "look" exactly the same again, but the forth is distinctly different!

Proton NMR is only capable of distinguishing different types of hydrogens, thus any time there is a group of hydrogens which all share the same characteristic bonds and neighbours it is impossible to tell them apart.

Consider ethyl acetic ester below.

[[Image:HNMRspec_ethyl_acetic_ester.jpg|center]

How many different groups of hydrogens are present? To the left of the ketone group we have three hydrogens, this is one set of hydrogens which are all the same. To the right, we have a carbon with two hydrogens followed by a carbon with three hydrogens, both of these then can be considered different hydrogen sets. So we have a total of eight hydrogens in three distinct groups. A Proton NMR spectrum for this compound then would have three different peaks (signifying the three distinct sets of hydrogens). Below is the spectra for ethyl acetic ester.

[[Image:HNMRspec_ethyl_acetic_ester_spectra.jpg|center]

Different Numbers of Hydrogens

Lets return to 2-butanone's groups of hydrogens. Two of the groups (A and C) had three hydrogens while the last (B) only had two hydrogens. Proton NMR is also capable of detecting this. While it will never directly tell you how many hydrogens belong to each group, it does give a comparative ratio of them based on how much area is under each peak in the spectrum. Specifically, the area under the NMR resonance peak is proportional to the number of hydrogens which that resonance represents. Thus, in 2-butanone's spectrum, we see that the three peaks are in a 3:2:3 ratio.