Mass spectrometry is a powerful analytical technique used to determine molecular mass and structural information for organic compounds. In IB Chemistry (Topic 11 and Topic 21), the molecular ion peak is one of the most important features students must recognize. This article explains what the molecular ion peak is, how it forms, and how to use it to identify molecules quickly and accurately.
What Is a Molecular Ion?
When a molecule enters a mass spectrometer, it is bombarded with high-energy electrons.
This often knocks out one electron from the molecule without breaking any bonds.
This process forms the molecular ion, also called the parent ion:
M → M⁺· + e⁻
- M⁺· is the molecular ion
- It is positively charged
- It has the same mass as the original molecule (minus one electron, which is negligible)
Because the molecular ion retains the full structure and mass of the original compound, it provides crucial information about molecular identity.
What Is the Molecular Ion Peak?
The molecular ion peak (M⁺) is the peak on the mass spectrum that corresponds to the molecular mass of the original molecule.
It is usually the highest m/z peak that still represents the entire molecule rather than fragments.
The molecular ion peak allows you to:
- Determine the relative molecular mass (Mr)
- Identify the molecular formula
- Narrow down possible structures
For many molecules, it is the most valuable peak in the entire spectrum.
How the Molecular Ion Peak Forms
During electron-impact ionization:
- Molecules are vaporized
- High-energy electrons bombard the vapor
- An electron is knocked off the molecule
- The molecule becomes a radical cation (M⁺·)
- The mass analyzer detects this ion and displays it as the molecular ion peak
Because the ion carries a +1 charge, its m/z value equals its mass number.
Fragmentation and Other Peaks
The molecular ion is often unstable and quickly breaks into fragments.
These fragments create additional peaks in the mass spectrum.
Types of peaks:
1. Molecular Ion Peak (M⁺)
- Corresponds to full molecule
- Highest meaningful m/z (except isotopes)
2. Fragment Peaks
- Represent smaller charged pieces
- Help identify structural features
3. Base Peak
- Tallest peak in the spectrum
- Most stable and abundant fragment
4. Isotopic Peaks (M+1, M+2)
- Caused by isotopes like ¹³C, ²H, ³⁷Cl, ⁸¹Br
- Provide info about atom types in the molecule
Understanding the relationship between these peaks is essential for structural analysis.
Why the Molecular Ion Peak Matters
The M⁺ peak gives:
1. Molecular Mass
You can instantly determine Mr by reading the m/z value.
2. Molecular Formula Clues
By combining Mr with elemental analysis or isotopic patterns, you can deduce the formula.
3. Structural Information
Some structures fragment in characteristic ways.
Seeing (or not seeing) a molecular ion can confirm structural hypotheses.
When the Molecular Ion Peak May Be Weak or Missing
Some molecules do not show a strong M⁺ peak because the molecular ion is extremely unstable.
This happens with:
- Alcohols
- Alkanes
- Highly branched molecules
- Compounds that fragment easily
In such cases, the base peak or isotopic pattern becomes more important.
IB Chemistry Examples
1. Propanone
Molecular ion peak at m/z = 58, confirming Mr = 58.
2. Chlorinated Compounds
Show M⁺ and M+2 peaks in a 3:1 ratio (due to ³⁵Cl and ³⁷Cl).
3. Brominated Compounds
Show M⁺ and M+2 peaks in a 1:1 ratio (due to ⁷⁹Br and ⁸¹Br).
These characteristic patterns help identify halogens in unknown samples.
FAQs
Why is the molecular ion a radical cation?
It forms when one electron is removed, leaving the molecule positively charged with an unpaired electron.
Why are m/z values whole numbers?
Mass spectrometers usually detect integer mass-to-charge ratios because most ions have a +1 charge and isotopes have integer masses.
Does every molecule show a molecular ion peak?
No. Some molecular ions fragment too quickly. However, many do show a clear M⁺ peak that is easy to identify.
Conclusion
The molecular ion peak represents the original molecule with one electron removed and provides the molecular mass directly. Recognizing this peak is essential for interpreting mass spectra and identifying unknown compounds in IB Chemistry. Once you can pinpoint the M⁺ peak, analyzing isotopes, fragments, and structural clues becomes much easier.
