Infrared (IR) Spectroscopy

Intensity: Peaks are described as strong, medium, or weak based on how much radiation is absorbed. Stronger absorptions usually correspond to bonds with larger dipole changes, such as $C=O$.

Width: Peaks can be broad or sharp. Broad peaks are characteristic of bonds involved in hydrogen bonding, such as $O-H$ groups in alcohols or carboxylic acids.

Position: The specific wavenumber where a peak occurs indicates the type of bond. For example, double bonds ($C=C$, $C=O$) typically appear between $1600-1850\ cm^{-1}$, while single bonds to hydrogen ($C-H$, $O-H$, $N-H$) appear above $2700\ cm^{-1}$.

Carbonyl Distinction: The $C=O$ peak is often the most diagnostic feature in a spectrum due to its high intensity and consistent position around $1700\ cm^{-1}$.

Hydroxyl Distinction: An alcohol $O-H$ peak is a distinct 'U' shape, whereas a carboxylic acid $O-H$ is much broader and often overlaps with the $C-H$ stretching region.

Step 1: Identify the Carbonyl: Look for a strong, sharp peak between $1630-1820\ cm^{-1}$. If present, the molecule contains a $C=O$ group (e.g., ketone, aldehyde, ester, or acid).

Step 2: Check for Hydroxyl Groups: Look for a broad peak above $3000\ cm^{-1}$. If it is very broad and centered lower (near $3000\ cm^{-1}$), it suggests a carboxylic acid; if it is smoother and higher, it suggests an alcohol.

Step 3: Analyze the C-H Region: Most organic molecules show peaks just below $3000\ cm^{-1}$ for $sp^3$ $C-H$ bonds. Peaks just above $3000\ cm^{-1}$ suggest $sp^2$ $C-H$ bonds (alkenes or arenes).

Step 4: Fingerprint Region: The area below $1500\ cm^{-1}$ contains complex patterns unique to specific molecules. While difficult to interpret manually, it can be compared to a database for definitive identification.

The 'No-Peak' Rule: Sometimes the absence of a peak is as informative as its presence. For example, if a molecule has the formula $C_3H_6O$ but no peak at $1700\ cm^{-1}$, it cannot be propanone or propanal.

Watch the Overlap: Students often mistake the broad $O-H$ of a carboxylic acid for a messy $C-H$ region. Always check if the 'baseline' of the $C-H$ peaks seems to be pulled down by a larger, broader absorption.

Verify with Molecular Formula: Always use the provided molecular formula to narrow down possibilities. If there is only one oxygen atom, you cannot have a carboxylic acid or an ester.

Units Check: Ensure you are reading the x-axis correctly; IR spectra are usually plotted with the wavenumber decreasing from left to right ($4000$ to $400$).