Dr. D’Meo AP Chemistry
Spectroscopy
There is a profound relationship between light and matter.
Recall that the electromagnetic spectrum ranges from radio waves (low frequency, low energy and long
wavelength) to gamma waves (high frequency, high energy, short wavelengths.
The different forms of spectroscopy allow us to elucidate the identity and structure of substances.
Your eye and brain together constitute a natural spectroscope! Together they can sort out the various
wavelengths of light emerging form an object. Sensitive to different wavelengths, the brain sees
different colors. So the brain distinguishes photons of different energies, BUT only over a short range:
400nm – 700 nm is the visible range of the spectrum.
We need stronger, more discerning eyes to look deeply into matter, but even the most sophisticated
spectroscopic instruments follow the overall plan of your brain plus eye:
1. Light goes in
2. Light does work on the matter
3. Light comes out
Case 1: Radio waves
These are the longest wavelengths with low frequencies. A typical radio wave might be 3 meters long.
Question: What happens when a radio wave bombards a particle?
Answer: a) To the electrons: nothing! Why? Because the orbitals are separated by more energy than a
radio wave can deliver.
b) But! Recall that each proton and neutron has a spin associated with it, a special type of
angular momentum. Radio waves can deliver just the right amount of energy to flip the nuclear spin.
Each time there is a flip, we observe a NUCLEAR MAGNETIC RESONANCE, or NMR. This flip in the spin of
nuclear particles allows us to look deeply into molecules.
Question: What information can we obtain from NMR?
Answer: The structure of molecules, including the bonding pattern, and cis- versus trans- isomers
A typical nmr spectrum might look like the following diagram, which represents ethanol:
Case 2: Microwaves
Microwaves have more energy than radiowaves. They are typically a few mm to a few cm in length.
Question: What happens when a microwave bombards a particle?
Answer: Two things!
1.
Microwaves are strong enough to flip the spin of an electron (must have unpaired electrons
present)
2. Microwaves can induce change in rotational motion. Rotation, like all other forms of internal
motion, is quantized: only certain motions are allowed. A molecule is by no means a spinning
top, allowed to rotate about any axis at any speed. There are rotational quantum numbers and
rotational energy levels. Microwaves can excite a molecule from one rotational level to another.
In a microwave oven, water molecules acquire energy by jumping from one rotational level to
another: the water molecules rotate faster, and your food gets hotter!
3. A rotational spectrum allows us to elucidate the structure of a molecule. The rotational
spectrum of ethanol is shown below.
Case 3: Infrared Waves
Infrared waves have more energy than microwaves. Their wavelengths range from about 0.001 mm to 1
mm.
Question: What happens when infrared waves bombard particles?
Answer: The molecules vibrate faster and more energetically than before The bonds, stretch and
compress, bend and wag! Recall that all molecules vibrate, even without bombarding them with infrared
radiation. Realize that they typically vibrate in the infrared region of the spectrum. The oscillating light
(infrared waves) are in tune with the oscillating matter, and therefore, this external driving force which
is synchronized with the internal motion, acts like a series of well-timed rhythmical pushes delivered to
the molecule. This effect is called RESONANCE: to be IN-RHYTHM is to transfer energy most efficiently.
NMR, EPR (electron paramagnetic resonance), rotational transitions, vibrational transitions, electronic
transitions—each derives from the phenomenon of resonance.
The IR spectrum of ethanol is shown below:
Interestingly, it seems that a chemical bond, for example C—H or O—H, vibrates at nearly the same
frequency regardless of the molecule in which it resides. The table below lists some vibrational
frequencies for common functional groups.
Functional Group Type of Vibration Characteristic Absorptions (cm-1)
Intensity
Alcohol
O-H
(stretch, H-bonded)
3200-3600
strong, broad
O-H
(stretch, free)
3500-3700
strong, sharp
C-O
(stretch)
1050-1150
strong
C-H
stretch
2850-3000
strong
-C-H
bending
1350-1480
variable
Alkene
=C-H
stretch
3010-3100
medium
=C-H
bending
675-1000
strong
Alkane
C=C
stretch
1620-1680
variable
C-F
stretch
1000-1400
strong
C-Cl
stretch
600-800
strong
C-Br
stretch
500-600
strong
C-I
stretch
500
strong
Alkyl Halide
Case 4: Visible Radiation and above
In this situation there is enough energy to excite the most weakly bound electrons, the valence
electrons. Electrons are promote to higher energy levels and either drop down to lower levels or are
removed (photoionization). Values of successive ionization energies give rise to the shell model.
Mass Spectroscopy
This type of spectroscopy is used to identify substances. A mass spectrometer does the following:
1.
2.
3.
4.
Takes a collection of molecules and ionizes them
Ions are accelerated as a beam
The beam is sent through a magnetic field
Ions arrive at a detector and the mass of the substance can be determined
Look at the following diagram:
Let’s look at the mass spectrum of chlorine in this problem:
Answers:
(a) Mass spectrometry
(b) Chlorine exists as two isotopes
(c) II:
37
Cl+
IV: 35 37 ( 35Cl 37Cl)+ −
Follow-up Activities:
1. PSI Chemistry Activity on Mass Spectroscopy
2. Take a look at the following youtube video on PES
https://www.youtube.com/watch?v=rz0gTxnpqiM