MOLECULAR
SPECTROSCOPY
CHY 6040
Text Book:
Colin N Banwell, Elaine M. McCash, Fundamentals of
Molecular Spectroscopy, Tata McGraw – Hill Publishing Co.
Ltd., 5th Edition, 2013.
What is spectroscopy?
Studying the properties of matter through its
interaction with different frequency components of
the electromagnetic spectrum
With light, you aren’t looking directly at the molecule—the
matter—but its “ghost.”
You observe the light’s interaction with different degrees of
freedom of the molecule.
Each type of spectroscopy—different light frequency—gives a
different picture → the spectrum.
Goals:
•
Understand how light interacts with matter and how
you can use this to quantitatively understand your
sample.
•
Understand spectroscopy the way you understand other
common tools of measurement like the watch or the
ruler.
•
Spectroscopy is a set of tools that you can put together
in different ways to understand systems → solve
chemical problems.
The immediate questions that we want
to address are:
1. What does light do to sample?
2. How do you produce a spectrum?
3. What EXACTLY is a spectrum a measurement of ?
Molecular Spectroscopy
Study of interaction of electromagnetic waves and matter
• Nature of electromagnetic radiation
• Sort of interactions
• Experimental methods
ELECTROMAGNETIC RADIATION
 Also known as radiant heat or radiant energy
 One of the ways by which energy travels through space
 Consists of perpendicular electric and magnetic fields that are
also perpendicular to direction of propagation
Examples
heat energy in microwaves
light from the sun
X-ray
radio waves
ELECTROMAGNETIC RADIATION
Wavelength (m)
Gamma X rays Ultrrays
violet
Visible
10-11
103
Infrared Microwaves
Radio frequency
FM Shortwave AM
Frequency (s-1)
1020
Visible Light: VIBGYOR
Violet, Indigo, Blue, Green, Yellow, Orange, Red
400 – 750 nm
• White light is a blend of all visible wavelengths
• Can be separated using a prism
104
ELECTROMAGNETIC RADIATION
λ1
node
amplitude
ν1 = 4 cycles/second
λ2
ν2 = 8 cycles/second
peak
λ3
ν3 = 16 cycles/second
trough
one second
ELECTROMAGNETIC RADIATION
Wavelength (λ)
• Distance for a wave to go through a complete cycle
• (distance between two consecutive peaks or troughs in a wave)
Frequency (ν)
• The number of waves (cycles) passing a given point
• in space per second
Cycle
• Crest-to-crest or trough-to-trough
Speed (c)
• All waves travel at the speed of light in vacuum (3.00 x 108 m/s)
ELECTROMAGNETIC RADIATION
Plane Polarized Light
• Light wave propagating along only one axis (confined to one
plane)
Monochromatic Light
• Light of only one wavelength
Polychromatic Light
• Consists of more than one wavelength (white light)
Visible light
• The small portion of electromagnetic radiation to which the
human eye responds
ELECTROMAGNETIC RADIATION
Inverse relationship between wavelength and frequency
λ α 1/ν
c=λν
λ = wavelength (m)
ν = frequency (cycles/second = 1/s = s-1 = hertz = Hz)
c = speed of light (3.00 x 108 m/s)
ELECTROMAGNETIC RADIATION
• Light appears to behave as waves and also considered
as stream of particles (the dual nature of light)
• Is sinusoidal in shape
• Light is quantized
Photons
Particles of light
ELECTROMAGNETIC RADIATION
hc
Energy of one photon (E photon )  hν 
 hc~ν
λ
~ν  1  wavenumber (m 1 )
λ
h = Planck’s constant (6.626 x 10-34 J.s)
ν = frequency of the radiation
λ = wavelength of the radiation
E is proportional to ν and inversely proportional to λ
INTERACTIONS WITH MATTER
• Takes place in many ways
• Takes place over a wide range of radiant energies
• Is not visible to the human eye
• Light is absorbed or emitted
• Follows well-ordered rules
• Can be measured with suitable instruments
INTERACTIONS WITH MATTER
Molecules
 Many types of motion are involved
- Rotation
- Vibration
- Translation (move from place to place)
 These motions are affected when molecules interact with
radiant energy
 Molecules vibrate with greater energy amplitude when
they absorb radiant energy
INTERACTIONS WITH MATTER
Molecules
• Bonding electrons move to higher energy levels when
molecules interact with visible or UV light
• Changes in motion or electron energy levels result in
changes in energy of molecules
Transition
• Change in energy of molecules
(vibrational transitions, rotational transitions, electronic
transitions)
INTERACTIONS WITH MATTER
Atoms or Ions
• Move between energy levels or in space but cannot rotate or
vibrate
• The type of interactions of materials with radiant energy are
affected by
- Physical state
- Composition (chemical nature)
- Arrangement of atoms or molecules
INTERACTIONS WITH MATTER
• Light striking a sample of matter may be
- Absorbed by the sample
- Transmitted through the sample
- Reflected off the surface of the sample
- Scattered by the sample
• Samples can also emit light after absorption (luminescence)
• Species (atoms, ions, or molecules) can exist in certain
discrete states with specific energies
INTERACTIONS WITH MATTER
Transmission
• Light passes through matter without interaction
Absorption
• Matter absorbs light energy and moves to a higher energy state
Emission
• Matter releases energy and moves to a lower energy state
Luminescence
• Emission following excitation of molecules or atoms by
absorption of electromagnetic radiation
INTERACTIONS WITH MATTER
Ground State: The lowest energy state
Excited state: higher energy state (usually short-lived)
Energy
Excited
state
Absorption
Emission
Ground
state
INTERACTIONS WITH MATTER
 Change in state requires the absorption or emission of energy
Change in energy ( E)  hν 
hc
λ
Matter can only absorb specific wavelengths or frequencies
These correspond to the exact differences in energy between
the two states involved
Absorption: Energy of species increases (ΔE is positive)
Emission: Energy of species decreases (ΔE is negative)
INTERACTIONS WITH MATTER
 Frequencies and the extent of absorption or emission of
species are unique
 Specific atoms or molecules absorb or emit specific
frequencies
 This is the basis of identification of species by spectroscopy
Relative energy of transition in a molecule
Rotational < vibrational < electronic
The are many associated rotational and vibrational sublevels
for any electronic state (absorption occurs in closely spaced
range of wavelenghts)
INTERACTIONS WITH MATTER
Absorption Spectrum
A graph of intensity of light absorbed versus frequency or wavelength
Emission spectrum is obtained when molecules emit energy by
returning to the ground state after excitation
Excitation may include
- Absorption of radiant energy
- Transfer of energy due to collisions between atoms or molecules
- Addition of thermal energy
- Addition of energy from electrical charges
Spectroscopy is a general methodology
It can be adapted in many ways to extract the information you
need (energies of electronic, vibrational, rotational states,
structure and symmetry of molecules, dynamic information).
ATOMS AND ATOMIC SPECTROSCOPY
MOLECULES AND MOLECULAR SPECTROSCOPY
ATOMS AND ATOMIC SPECTROSCOPY
The electronic state of atoms are quantized
Elements have unique atomic numbers (numbers of protons and
electrons)
Electrons in orbitals are associated with various energy levels
An atom absorbs energy of specific magnitude and a valence
electron moves to the excited state
The electron returns spontaneously to the ground state and emits
energy
ATOMS AND ATOMIC SPECTROSCOPY
Emitted energy is equivalent to the absorbed energy (ΔE)
Each atom has a unique set of permitted electronic energy levels
(due to unique electronic structure)
The wavelength of light absorbed or emitted are characteristic of a
specific element
The absorption wavelength range is narrow due to the absence of
rotational and vibrational energies
The wavelength range falls within the ultraviolet and visible
regions of the spectrum (UV-VIS)
ATOMS AND ATOMIC SPECTROSCOPY
Wavelengths of absorption or emission are used for qualitative
identification of elements in a sample
The intensity of light absorbed or emitted at a given wavelength is
used for the quantitative analysis
Atomic Spectroscopy Methods
 Absortion spectroscopy
 Emission spectroscopy
 Fluorescence spectroscopy
 X-ray spectroscopy (makes use of core electrons)
MOLECULES AND MOLECULAR SPECTROSCOPY
Molecular Processes Occurring in Each Region
103
Gamma
UltrX
rays
rays
violet
1020
Visible
10-11
Infrared Microwaves
Radio frequency
FM Shortwave AM
rotation
vibration
Electronic
excitation
Bond breaking
and ionization
104
The entire electromagnetic spectrum is used by chemists
Frequency, n in Hz
~1019
~1017
~1015
~1013
~1010
~105
0.01 cm
100 m
~10-4
~10-6
Wavelength, l
~.0001 nm
~0.01 nm
10 nm
1000 nm
Energy (kcal/mol)
> 300
g-rays
nuclear
excitation
(PET)
X-rays
core
electron
excitation
(X-ray
cryst.)
300-30
300-30
UV
electronic
excitation
(p to p*)
IR
molecular
vibration
Visible
Microwave
Radio
molecular
rotation
Nuclear Magnetic
Resonance NMR
(MRI)
MOLECULES AND MOLECULAR SPECTROSCOPY
Energy states are quantized
Rotational Transitions
Molecules rotate in space and rotational energy is associated
Absorption of the correct energy causes transition to a higher
energy rotational state
Molecules rotate faster in a higher energy rotational state
Rotational spectra are usually complex
MOLECULES AND MOLECULAR SPECTROSCOPY
Rotational Transitions
Rotational energy of a molecule depends on shape, angular velocity,
and weight distribution
Shape and weight distribution change with bond angle
Molecules with more than two atoms have many possible shapes
Change in shape is therefore restricted to diatomic molecules
Associated energies are in the radio and microwave regions
MOLECULES AND MOLECULAR SPECTROSCOPY
Vibrational Transitions
Atoms in a molecule can vibrate toward or away from each
other at different angles to each other
Each vibration has characteristic energy associated with it
Vibrational energy is associated with absorption in the infrared
(IR region)
Increase in rotational energy usually accompanies increase in
vibrational energy
MOLECULES AND MOLECULAR SPECTROSCOPY
Vibrational Transitions
IR absorption corresponds to changes in both rotational and
vibrational energies in molecules
IR absorption spectroscopy is used to deduce the structure
of molecules
Used for both qualitative and quantitative analysis
MOLECULES AND MOLECULAR SPECTROSCOPY
Electronic Transitions
Molecular orbitals are formed when atomic orbitals combine to
form molecules
Absorption of the correct radiant energy causes an outer electron to
move to an excited state
Excited electron spontaneously returns to the ground state (relax)
emitting UV or visible energy
Excitation in molecules causes changes in the rotational and
vibrational energies
MOLECULES AND MOLECULAR SPECTROSCOPY
Electronic Transitions
The total energy is the sum of all rotational, vibrational, and
electronic energy changes
Associated with wide range of wavelengths
(called absorption band)
UV-VIS absorption bands are simpler than IR spectra
MOLECULES AND MOLECULAR SPECTROSCOPY
Molecular Spectroscopy Methods
Molecular absorption spectroscopy
Molecular emission spectroscopy
Nuclear Magnetic Resonance (NMR)
UV-VIS
IR
MS
Molecular Fluorescence Spectroscopy
Regions of spectrum
Change of orientation
ESR
Microwave
Change of configuration Infrared
Change of electron
distribution
Change of nuclear
configuration
Visible and UV
X-rays
Gamma rays
Increasing energy
Change of spin
NMR
10 m
100 cm
1 cm
100 µm
1 µm
10 nm
100 pm
What variables do we need to characterize a molecule?
Nuclear and electronic configurations:
What is the structure of the molecule?
What are the bond lengths?
How strong or stiff are the bonds?
What is the symmetry?
Where is the electron density?
Molecular Behavior:
How much do the nuclei move (vibration/rotation)?
How do the structural variables change with time?
Dynamics
Different spectroscopies will tell us about different
variables
Rotational spectroscopy: will tell us where re is.
Vibrational spectroscopy: will tell us how stiff the bond is and
about the curvature of potential.
Electronic spectroscopy: will tell us about where electronic
states lie
→ potential energy curves, barriers, dissociation energies
Molecular coordinates
For any free particle (nucleus or electron):
Kinetic and potential energy associated with the motion of
a particle
Translation along x, y, z → n particles
→ 3n degrees of freedom (d.o.f).
In a molecule the positions of these particles is not independent
To solve problems in molecular structure and motion, chose a molecular frame
of reference:
• Coordinates along symmetry axes
• Origin (often) at the center of mass
To simplify problem, treat nuclear and electronic
motion separately
 Electrons are much lighter than nuclei. Therefore, we expect
that the electrons will occupy a fixed distribution in space
about a nuclear configuration.
 Separate different types of motion based on time-scale or
energy of motion.
 The basis for drawing potential energy curves is the BornOppenheimer Approximation:
Motion of electron is much faster than nuclei
Doesn’t depend on nuclear kinetic energy
Electrons: The spatial extent motion is described by atomic or molecular orbitals.
Nuclei: Three types of motion for nuclei with respect to the C.O.M. of the molecule
Consider a diatomic (with 6 d.o.f).
Displacement of atoms relative
to one another / C.O.M. fixed
Motion about C.O.M. /
C.O.M. fixed
move C.O.M.
How are these motions related to the structure of the molecule?
A classical description allows you to say a lot about
spectroscopy,
but it doesn’t explain experiments very well!
We need quantum mechanics!
Classically:
Position and motion of particles with arbitrary accuracy
No restriction on energy of system or spatial configuration
Quantum:
Energy levels are quantized
Fixed energy and spatial configuration
Probabilistic description of particle position and motion
We represent the state of the system through quantum
numbers—usually integers.
The energy is related to these numbers.
“Ground State” : configuration of electrons, nuclei, … (a set of
quantum numbers) that represent the lowest energy state of
the system.
Electronic States:
atomic + molecular orbitals:
which represent discrete energy states that electrons can occupy
through discrete quantum numbers, i.e. for atom: n, l, m, ms
n: principle quantum number/Size & energy
l: orb. ang. mom./Shapes
m: degeneracy/orientation
ms: spin
Electronic states have definite energy and electron density
distribution (spatial dimension).
There is no way for an electron to occupy an intermediate energy
between quantized values.
The energy to move an electron from the ground (lowest energy)
state to another state: Typically 20,000-100,000 cm-1 = 10-50
kJ/mol. This corresponds to UV and visible light, λ~100-500nm
For polyatomic molecules, we can use linear combinations of
atomic orbitals.
Quantization of nuclear degrees of freedom:
The vibration, rotation, and translation of the 3n nuclear d.o.f. of
a molecule are also quantized.
Vibrations:
Classically
vibra onal mo on → moves energy
levels to higher displacement
Quantum mechanics
there are discrete vibrational energy levels
corresponding to specific vibrational states
(wave functions)
The simplest quantum mechanical model for
Vibration:
Quantum mechanical harmonic oscillator
Rotation:
Rotational angular momentum is quantized
– Rigid Rotor/non-Rigid Rotor
Translation: (motion of center of mass)
Energy levels given by particle-in-a-box
Quantum mechanical energies are summed over all degrees
of freedom:
 specify energy through a set of quantum numbers, and
characteristic vib./rot./etc. constants