11.1 Infrared Spectroscopy Flashcards
spectroscopy
the study of the absorption and emission of light and other radiation by matter
used to identify unknown compounds; determines the frequencies of electromagnetic radiation absorbed by the molecules
only a small sample quantity is needed
infrared spectroscopy
measures molecular vibrations, which can be seen as bond stretching, bending, or combinations of different vibrational modes
to record the IR spectrum, IR light is passes through a sample, and the absorbance is measured
the functional groups of a molecule are inferred based upon the determined bonds
wavenumber
an analog of frequency used on the IR spectrum
units: cm-1 or 1/wavelength
when IR light is absorbed…
the molecules enter excited vibrational states
4 types of vibrations that can occur
others include twisting and folding
fingerprint region
the 1500 to 400 cm-1 range where more complex vibrational patterns are seen
the fingerprint region of a given compound is unique and, therefore, can be used to distinguish between compounds
what is required for an absorption to be recorded
a change in the bond dipole moment
molecules without a dipole moment do not exhibit absorption (ex. O2 or Br2)
symmetric stretches do not show up in IR spectra because they involve no net change in dipole movement
absorption of OH bond
3300 cm-1 for alcohols
3000 cm-1 for carboxylic acids
broad/wide peak
absorption of the carbonyl bond
1700 cm-1
sharp/deep peak
absorption of NH bond
around 3300 cm-1
sharp peak
for the MCAT, all of the information from an IR spectrum will come from which wavenumber?
between 1400 and 4000 cm-1
anything in the fingerprint region is out of scope for the MCAT
IR spectra are plotted as…
% transmittance (the amount of light that reaches the detector) vs wavenumber
maximum absorbance occurs at the “valleys”
