Chapter 14 - IR & UV Spectra
Infrared Spectroscopy
- probes different molecular vibrations
- absorption occurs when the frequency of the IR wave
matches a vibrational frequency of the molecule
- molecules typically have many possible vibrations
- bond stretching vibrations are most characteristic
(detect different kinds of A-B bonds)
- bond bending involves several atoms at once
more complex vibrations (rock, scissor, wag, twist)
- one exception: a completely symmetrical bond vibration does
not absorb IR
e.g., the C=C double bond stretch in ethylene is IR-silent
IR Spectra
- x-axis:
wavelength range typically 2 - 15 µm
wavenumber also used (second x-axis)
wavenumber = reciprocal wavelength (in cm-1)
- y-axis:
usually transmittance (0 - 100%)
(valleys correspond to absorptions)
"peak" position noted by wavenumber
Regions of an IR Spectrum
- Bonds to H (4000 - 2500 cm-1)
- Triple Bonds (2500 - 2000 cm-1)
- Double Bonds (2000 - 1500 cm-1)
- "Fingerprint" (1500 - 600 cm-1)
- Tables 14.1 and 14.2 will be provided for quizzes and exams
Interpreting an Unknown IR Spectrum
- work from left to right (decreasing wavenumber)
- look for characteristic bond stretches
- try to confirm functional groups with additional absorptions
- if possible, compare to a known spectrum
Ultraviolet/Visible Spectroscopy
- x-axis:
typically 200 - 800 nm
- y-axis:
typically absorbance in % or log units
(peaks correspond to absorptions)
peak positions noted by lmax
Electronic Absorption Spectra
- uv/vis absorptions correspond to electron transitions
from one molecular orbital to another
- generally pi bonds are clearly seen in the uv region
- absorptions are called pi->pi* transitions
(from bonding to antibonding orbitals)
- one pi bond (ethylene) 165 nm (l max)
- conjugated dienes and polyenes absorb at longer wavelengths
(lower energies, or more closely spaced orbitals)
- 1,3-butadiene 217 nm
- benzene 254 nm
- beta-carotene 483 nm (absorbs violet - looks yellow)
- Table 14.5 will be provided for quizzes and exams
Photochemistry
- what do molecules do once an electron is promoted to a higher
energy orbital?
- many possibilities:
- emit light (fluorescence)
- lose energy as heat
- rearrange
- decompose
- transfer an electron to (or from) another molecule