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Spectroscopy - Coggle Diagram
Spectroscopy
Fundemental of Molecular Spectroscopy
Uses
Identification of unknown molecules
Detection of known molecules
Determination of molecular structure
Measurement of cencentration
Electromagnetic Radiation
Form of energy that is produced by oscillating electric and magnetic disturbance, or by the movement of electrically charged particles traveling through a vacuum or matter
Electric and magnetic field are at right angle with each other(perpendicular)
General Properties
Travel through empty space(vacuum)
Speed of light always constant
Characteristics of wave
Frequency
The number of waves that pass a point ina given perios of time
Number of waves+Number og cycle
f=1/t
Period
Amount of time a wave takes to travel one wavelength(s)
Amplitude
Distance from the maximum vertical diplacement of the wave to the middle of wave
Larger the amplitude,higher the energy
Tells intensity or brigtness of wave
Wavelength
Distance of one full cycle of oscillation
Larger wavelength, lower energy(Radio), lower frequency
Tells type of light
Study of absorption and emmision of electromagnetic radiation
Quantisation of Energy
A absorbed energy(endothermic)+
Emitted energy(exothermic)
Planck's Equation
Electromagnetic spectrum
Rotational(Microwave spectroscopy)
Rigid Rotor(Do not distort under the stress of rotation)
Symmetric Tops
Prolate
Oblate
Spherical Tops
No dipole moment, no rotational spectrum is observable
Linear Molecules
Homonuclear
No change on electric dipole moment
Heteronuclear
Have different electronegativity
Asymmetric Tops
Rigid Diatomic Molecule
Selectrion rule, Delta J=
Rotational Energy Levels
Schrodinger Equation
Rotational costant,B
Rotational Transition
Centrifugal Distortion
Non-rigid Diatomic Molecule
Schrodinger Equation become
If force field is inharmonic
Linear Polyatomic Molecule
Only one value for moment of inertia can be determined from a spectrum
B value much smaller and spectra lines more closely spaced
Energy Levels and Transition
Internal energy is quantized
Absorption of photon
Upward transition to a higher energy level
Increase in internal energy
Exitation
Emmision of photon
Downward transition to a lower energy level
Decays in internal energy
Spontaneous emmision
Reverse of absorption
Stimulated emmision
Trigged by incoming photon of the same frequency
Factors Affecting The Width of Spectral Lines
Doppler Broadening
Mechannism
Energy levels not sharp: Emitted photons have a range of frequencies
Atom in gas are moving relative to the observer: Observeed photons do not have same frequency as the emitted photons (Doppler effect)
Motion in gas causes absorption and a=emission frequencies to show Doppler shift, shift to both high and low frequency, hence the spectra line is broadened
Heisenberg Uncertainty Principle
Electrons in excited state remain there for average time before decaying to ground state
If system existsts in energy state with limited time, the energy state will be unceertain
Consider excited state with energy E above the ground state
Collision broadening
Liquid & Gas
Continual motion and collide frequently
Equally vibrational and rotational spectra are broadened
Solid
Motion of particle more limited in extent and less random in direction(sharp spectra)
Intensity of Spectra Lines
Population
Number of atoms or molecules initially in the state from which the transition occurs
If 2 levels from whoch transitions to a third are equally probable, obviously the most intense spectral line will arise from the level which initially has the greater population
Boltzmann Distribution
Amount of Material Present
The concentration or path length of the sample
Intensity of light abroed is quantified by Beer-Lambert Law
Transmittance=T=I=Io
Molae absorption coefficient
Transition Probility
The likelihood of a system in one state changing to another state
Selection Rule
Gross
Molecules must have permanent electric dipole moment
Specific
Changes in quantum number
Symmetry
Symmetry Operation
Inversion Centre, i
Inversion of all atoms on given centre
Inverts the molecules by moving all atoms through a point at the centre, to a position at eequal distance on the opposite side
n-Fold Rotation, Cn
Rotation by(360/n) at axis of rotation
C3
Rotation by 120
C4
Rotation by 90
C2
Rotation by 180
Plane Symmetry,
Mirror reflection in plane
Dihedral Plane
Plane parallel to principlr axis+bisecting angle between atom
Horizontal Plane
Plane perpindicular to principle axis
Vertical Plane
Plane parallel to principle axis
Improper Rotation, Sn
Rotation by (360/n) + Reflection in
plane perpendicular to rotation axis
S1
Identical to mirror plane
S2
Identical to inversion
Sn
Odd
Require both Cn and mirror plane
S3
Even
may or may not have Cn and mirror plane
Benzene has no S2 because identical to i
Identity, E
Do nothing(Object unchanged)
Point Group Determination
Indentify exact position of all symmetry elements
Determine x,y and z axis using Right Hand Rule
x-axis
If the molecule is planar+z-axis is in plane
x is vertical to plane
If molecule is plane+z-axis is vertical to plane
x-axis is inn plane(with y-axis)+ pass
through the most number of atoms
y-axis
Determine according to Right Hand Rule
z-axis
If only one axis of rotation
it is z-axis
If has many axis of rotation with highest order
the axis pass through the most number of atom is z-axis
If has many axis of rotaion
highest order is z-axis
Identify the correct structure of molecule
Determine the point group
Decision Tree