Required Reading:

Introduction to IR Spectroscopy
Technique 25: Infrared Spectroscopy in Pavia lab manual, pp. 880-896.
Useful reference information: Correlation Tables on pp. 897-912 in Pavia manual. This document provides a short one page summary of these correlation tables.
Use this document as a guide to identify the major functional group present within an organic molecule.

THEORY of IR SPECTROSCOPY:

The vibration of a bond can be modeled as a vibrating spring using Hooke's Law, where the two atoms of the bond are treated as 2 masses joined by a spring. Applying Hooke's Law to the vibration of a spring (assuming quantized energies according to quantum mechanical principles), the vibrational frequency of the bond depends on the mass of the atoms and the strength of the bond:

$LaTeX: \nu=\frac{1}{2\pi c}\cdot\sqrt{\frac{f}{m_{red}}}$

where ν is the vibrational frequency (in cm^-1), f is the force constant of the bond (in dyne/cm), m_red is the reduced mass of the two atoms in the bond, & c is the speed of light (in cm/s). This equation provides two important results:

As the strength of the bond increases, the vibrational frequency increases:
As the mass of the atoms in the bond decrease, the vibrational frequency increases:

What to look for in a signal?

LOCATION: The location of a signal on the x-axis, expressed in wavenumbers (cm^-1), provides information on the type of bond. Numerous factors can affect the exact position of the signal, including bond strength, atomic masses, conjugation, angle strain, hydrogen bonding, and more. Correlation tables are commonly used to relate the wavenumber of a signal to the associated bond; see the 5 Zone Analysis below for a strategic approach to use of correlation tables.
INTENSITY: The strength of a signal is commonly described as weak (w), medium (m) or strong (s). These descriptions of signal intensity are relative terms, where stronger signals have lower percent transmittance (%T) and weaker signals have higher %T. Assuming the concentration of the sample is constant, the signal strength is primarily related to two variables:
- Bond Polarity: All else being equal, the more polar the bonds, the stronger the signal; the less polar the bond, the weaker the signal.
- Number of Bonds: The more bonds of a given type, the stronger the signal; the fewer the bonds, the weaker the signal.
SHAPE: The shape of a signal is commonly described as sharp (sh) or broad (b). Hydrogen bonding leads to signal broadening; more extension hydrogen bonding, broader signals. This accounts for the difference in signal shape between a C-H stretch (no hydrogen bonding), an N-H stretch (moderate hydrogen bonding), an O-H stretch (strong hydrogen bonding) and an O-H stretch in a carboxylic acid (extensive hydrogen bonding). See below for an example of each:

5 ZONE ANALYSIS:

Looking at an entire IR spectrum (singular) at once can be overwhelming. With so many signals and peaks, where does one start? There are many different approaches to analyzing IR spectra (plural). Below, I present an approach that dissects IR spectra into 5 ZONES, where each zone represents a unique region where characteristic signals are observed for various functional groups.

ZONE 1: O-H, N-H, & sp/sp² C-H bonds, 3700-3000 cm^-1

Alcohol O-H stretch: 3650-3200 cm^-1, strong and broad
- The O-H stretch of carboxylic acids extends from ZONE 1 through ZONE 2: 3500-2500 cm^-1
  - If a carboxylic acid is present, a strong signal due the C=O stretch should be observed in ZONE 4
Amine & Amide N-H stretch: 3500-3200 cm^-1, medium strength, slight broadening
- Primary amines/amides have 2 signals
- Secondary amines/amides have 1 signal
- Tertiary amines/amides have NO signals
Terminal alkynes C-H stretch: ~3300 cm^-1
Alkene C-H stretch: 3100-3000 cm^-1

ZONE 2: sp³ C-H & Aldehyde C-H bonds, 3000-2500 cm^-1

Alkanes C-H stretch: 3000-2850 cm^-1
Aldehyde C-H stretch: displays as TWO signals, ~2800 & ~2700 cm^-1
- If an aldehyde is present, a carbonyl C=O stretch should also be observed in ZONE 4

ZONE 3: sp Hybridization, 2500-2000 cm^-1

Alkyne C≡C stretch: 2260-2100 cm^-1
Nitrile C≡N stretch: 2260-2200 cm^-1
Carbon dioxide C=O stretch: 2340 & 2360 cm^-1

ZONE 4: Carbonyl C=O bonds, 2000-1600 cm^-1

Anhydride C=O stretch: two signals at ~1810 & 1760 cm^-1
Ester C=O stretch: ~1735 cm^-1
Aldehyde C=O stretch: ~1725 cm^-1
- If an aldehyde is present, two signals due to the C-H stretch should also be observed in ZONE 2
Ketone C=O stretch: ~1715 cm^-1
Carboxylic acid C=O stretch: ~1710 cm^-1
- If a carboxylic acid is present, a broad signal due to the O-H stretch should be observed in ZONES 1 AND 2
Amide C=O stretch: ~1690 cm^-1
IMPORTANT NOTE: each of the ranges above is approximate and can change depending on the bonding environment. In particular, two factors can affect the location of the C=O stretch:
1. Conjugation with the C=O bond shifts the signal to lower wavenumbers by approximately 30 cm^-1
2. Angle strain or conjugation with the C-O single bond leads to a shift to HIGHER wavenumbers by approximately 30 cm^-1

ZONE 5: The Fingerprint Region, <1600 cm^-1

This region is described as the fingerprint region because no two molecules have identical signals in this portion of the spectrum (except for enantiomers). There are often MANY overlapping signals making interpretation difficult. To determine the major functional groups within a molecule, focus primarily on ZONES 1-4, NOT the fingerprint region. That being said, the following signals can provide useful information in ZONE 5:
Alkene/Aromatic C=C stretch: 1400-1650 cm^-1
Nitro group N=O stretch: two signals are observed, ~1560 & 1350 cm^-1
C-O & C-N stretches: 1300-1000 cm^-1
C-X stretches (where X = F, Cl, Br or I): 1000-600 cm^-1
sp² C-H out of plane bends: 1000-600 cm^-1, can provide information on the substitution pattern of alkenes and aromatic rings: