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Chapter 15

CHEM 267: Chapter 15 Textbook Notes

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University of Waterloo
CHEM 267
Monica Barra

Chem 267 Chapter 15 Notes Winter 2012 15.2 IR Spectroscopy Vibrational Excitation  IR radiation causes vibrational excitation of the bonds in a molecule  Different kinds of vibrational excitation; bonds can: o Stretch (like a spring) o Bend Identification of Functional Groups with IR Spectroscopy  For every bond in a molecule, the energy gap between vibrational states is dependent on the nature of the bond (ex. the energy gap for a C-H bond is much larger than for a C-O bond)  Each type of bond will absorb a characteristic frequency o By irradiating the compound with al frequencies of IR radition, you can determine which types of bonds are present in a molecule by detecting which frequencies were absorbed IR Spectrometer  A sample is irradiated with frequencies of IR radiation, and the frequencies that pass through (those that are NOT absorbed by the sample) are detected The General Shape of an IR Absorbance Spectrum  IR spectrometer measure the % transmittance as a function of frequency o Plotted on absorption spectrum o all the signals (absorption bands) point down  the location of each signal is reported in terms of wavenumber (̃) o wavenumber (̃) = frequency of light divided by the speed of light (c); in cm  wavenumber is proportional to frequency; larger wavenumber = higher energy  left side of plot (@ 4000) = higher energy radiation; right side (@ 400) = lower energy radiation  every signal in an IR spectrum has 3 characteristics: wavenumber, intensity, and shape 15.3 Signal Characteristics: Wavenumber Hooke’s Law  for every bond, the wavenumber associated with bond stretching is dependent on 2 factors: 1. bond strength 2. masses of the atoms sharing the bond  the reduced mass allows us to treat the 2 atoms as one system  smaller atoms give bonds that vibrate at higher frequencies (∴higher wavenumber)  stronger bonds will vibrate at higher frequencies (∴higher wavenumber)  all single bonds are found on the right of the spectrum (< 1500) EXCEPT for X-H bonds which appear on the left (b/c H has the smallest mass) Effect of Hybridization States on Wavenumber of Absorption  The wavenumber of absorption for a C-H bond is dependent on the hybridization state of the C  The CspH bond also has the shortest bond length, and is therefore the strongest bond Effect of Resonance on Wavenumber of Absorption  Unsaturated compound = compound containing double or triple bonds (not all C-C single bonds)  the one additional resonance stru
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