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Bonds constantly vibrate,
Bonds constantly vibrate,
stretching and/or bending at a frequency within the IR range.
stretching and/or bending at a frequency within the IR range.
The electromagnetic spectrum, with a special focus on the infrared (IR) range:
The electromagnetic spectrum, with a special focus on the infrared (IR) range:
Wavenumber, the inverse of wavelength, is the unit-of-choice for IR spectroscopy;
(wavenumber, cm-1) ṽ = 1 / λ (wavelength, cm)
How does the frequency of vibration correlate to wavenumber?
How does the frequency of vibration correlate to wavenumber?
Bonds are constantly moving!
Bond stretching = expand & contract
The frequency of the expand - contract cycle can be expressed in wave form
High energy = contracted
Low energy = expanded
The distance between 2 troughs or 2 crests of the wave = wavelength (cm)
Wavenumber is the inverse of wavelength
Sample placed in IR spectrometer
IR radiation is emitted by the instrument, specific wavenumbers are absorbed by the molecule, depending on types of bonds.
When the wavenumber of the instrument matches the wavenumber of the bond vibration, the bond absorbs the radiation, causing the wave to amplify & bond to vibrate more vigorously without changing its frequency.
The instrument detects the transmitted (not absorbed) wavenumbers, resulting in the IR spectrum.
Factors Effecting Bond Length
Factors Effecting Bond Length
Atomic Radius - smaller atoms make smaller bonds
See trends on the left
Hybridization / Type of Bond
(longest) single > double > triple (shortest)
(longest) sp3 > sp2 > sp (shortest)
Conjugation - more resonance creates more single bond character = longer bond
Look for alternating double-single-double... bonds
When the functional group is conjugated, or participates in resonance, the overall bond length is increased and the stretching frequency decreases.
Saturated vs. Conjugated Ketone
Saturated vs. Conjugated Ketone
How does conjugated effect the stretching frequency (wavenumber)?
Sample spectrum of an alcohol & main regions of IR spectrum
Sample spectrum of an alcohol & main regions of IR spectrum
The sample absorbs wavenumbers (x-axis) specific to the types of bonds & functional groups in the compound.
The remaining IR wavenumbers are transmitted, y-axis = percent transmittance, %T
background.mp4Before you start!
Before you start!
Collect a background spectrum:
The IR spectrum is usually displayed as percent transmittance (%T), which is how much of the original IR intensity is left after passing through the sample.
In order to calculate this, we first need to know the IR intensity with no sample in the pathlength, so we run a background.
Discuss:
If no sample is in the spectrometer during the background scan, what is around to absorb IR light? (hint: there are IR peaks near 3800 and 1600 cm-1)
Let's start analyzing the compounds!
Let's start analyzing the compounds!
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