Which of the following in NMR spectrum tells us how many protons of each kind are there in a molecule ?

Question

Accepted Answer

In NMR (Nuclear Magnetic Resonance) spectroscopy, the intensities of signals are crucial for determining the number of protons of each kind in a molecule. Here's a detailed explanation supporting why Option 3 is the correct answer:

1. **Intensities of signals (Option 3):** The intensity of an NMR signal is directly proportional to the number of nuclei (protons) contributing to that signal. This means that the area under each peak in the NMR spectrum corresponds to the number of protons that generate that particular signal. By integrating the area under the peaks, chemists can determine the relative number of protons of each type in the molecule. This is why the intensities of signals are essential for quantifying the number of protons.

2. **Number of signals (Option 1):** The number of signals in an NMR spectrum indicates the number of different types of proton environments in a molecule, not the quantity of protons. Each unique chemical environment for protons will produce a separate signal, but this does not provide information about how many protons are in each environment.

3. **Positions of signals (Option 2):** The positions of signals, also known as chemical shifts, provide information about the electronic environment surrounding the protons. Chemical shifts help identify the types of functional groups present but do not indicate the number of protons.

4. **Splitting of signals (Option 4):** The splitting of signals, or multiplicity, arises from spin-spin coupling between neighboring protons. This provides information about the number of adjacent protons but does not directly indicate the total number of protons in a particular environment.

In summary, while the number, position, and splitting of signals provide valuable structural information about a molecule, it is the intensities of signals that specifically tell us how many protons of each kind are present. Therefore, Option 3 is the correct answer.