Infrared spectroscopy is a technique used to identify chemical bonds by measuring which infrared frequencies a sample absorbs. It matters because different bonds vibrate at characteristic frequencies, so an IR spectrum acts like a molecular clue sheet. Chemists use IR spectra to recognize functional groups, compare unknowns with known compounds, and monitor reactions.
The graph is usually plotted as percent transmittance versus wavenumber, with wavenumber decreasing from 4000 to 400 cm^-1 from left to right.
Key Facts
- IR absorption occurs when infrared radiation matches a molecular vibration that changes the molecule's dipole moment.
- Wavenumber is proportional to energy: E = hcν~, where ν~ is wavenumber in cm^-1.
- Frequency and wavenumber are related by ν = cν~, using consistent units.
- Stronger bonds vibrate at higher wavenumber, and heavier atoms vibrate at lower wavenumber.
- The functional group region is about 4000 to 1500 cm^-1 and contains many diagnostic bond stretches.
- The fingerprint region is about 1500 to 400 cm^-1 and is useful for matching an unknown spectrum to a known compound.
Vocabulary
- Infrared spectroscopy
- A method that measures absorption of infrared light to identify molecular vibrations and functional groups.
- Wavenumber
- The number of wave cycles per centimeter, commonly used as the x-axis unit in IR spectra.
- Transmittance
- The percentage of infrared light that passes through a sample without being absorbed.
- Functional group region
- The part of an IR spectrum from about 4000 to 1500 cm^-1 where many important bond stretches appear.
- Fingerprint region
- The complex part of an IR spectrum from about 1500 to 400 cm^-1 that helps distinguish one compound from another.
Common Mistakes to Avoid
- Reading the IR x-axis from left to right as increasing wavenumber is wrong because standard IR spectra decrease from about 4000 to 400 cm^-1.
- Assuming every bond vibration gives a strong IR peak is wrong because a vibration must change dipole moment to absorb IR strongly.
- Identifying a compound from one peak alone is unreliable because many functional groups overlap and the full pattern should be considered.
- Confusing transmittance peaks with absorption peaks is wrong because in a transmittance spectrum, strong absorption appears as a downward dip.
Practice Questions
- 1 An IR absorption occurs at 1700 cm^-1. Using E = hcν~, calculate the energy of one photon in joules. Use h = 6.63 x 10^-34 J s and c = 3.00 x 10^10 cm/s.
- 2 A spectrum shows a broad band near 3300 cm^-1 and a strong sharp band near 1710 cm^-1. Which two functional groups are suggested by these absorptions?
- 3 Two compounds have similar peaks above 1500 cm^-1 but very different patterns between 1500 and 400 cm^-1. Explain why the fingerprint region can help tell them apart.