MOLECULAR SPECTROSCOPY
FUNDAMENTAL
SYMMETRY
TYPES
1) ROTATION
2) TRANSLATION
3) REFLECTION
4) GLIDE REFLECETION
ELEMENTS
MIRROR REFLECTION
-reflects all the atom across a plane to a point
INVERSION
-inverts the molecule through a point
PROPER REFLECTION
-respect to a line which molecules rotate
-360/n
IMPROPER ROTATION
-combined operation of rotation followed by reflection
IDENTITY
-leaves any object unchanged
-all molecules have this element
WORKING STEPS
plays important role in the analysis of the structure, bonding and spectroscopy of the molecule
identify and confirm the correct structure of the molecule
identify and locate the exact position of all symmetry elements
determine the x,y,z axis using the right hand rule and the convention
determine the point group
Z= thumb
X=index
Y=middle finger
using decision tree
IMMEDIATE CONSEQUENCES
polarity
chirality
-only cn, cnv, and cs have permanent electric dipole moment lie along the symmetry axis
molecules with center of inversion are achiral and optically active
QUANSTITATION of ENERGY
ATOMIC SPECTROSCOPY ELEMENTS
ELECTROMAGNETIC RADIATION
AMPLITUDE
-measure the magnitude of
oscillation of a particular wave
FREQUENCY
-number of waves that pass a point in given time
PERIOD &VELOCITY
-amount of time a wave takes to travel one wavelength
WAVELENGTH
-distance of one full cycle of the oscillation
form of energy that is produced by oscillating electric and magnetic disturbance, or by the movement of electrically changed particles traveling through a vacuum or matter
energy is quantised
a molecule can exist in variety energy levels and can move from one level to another
AAS
-spectrum of light emitted through the sample is analyzed
AES
-some electrons in sample are excited into higher levels by an external source of energy
FACTORS AFFECTING BROADERNIG
DOPPLER
-energy levels themselves are not infinitely sharp
-atoms and molecules in the gas are moving relative to the observer
HEINSBURG UNCERTSINTY PRINCIPLE
-electrons in excited state remain there for average time before decaying to ground state
COLLISION
-only in liquid and gaseous phase because there is motion
THE INTENSITY OF SPECTRAL LINES
TRANSITION PROBABILITY
-essentially the deduction of selection rules which decide between which levels transition will give spectral lines
POPULATION
-if two levels from transitions to third are equally probable, the most intense spectral line will arise from the level which initially has the greater population
PATH LENGTH OF SAMPLE
-if a sample is absorbing energy from a beam of radiation, the more energy will be absorbed
tells about internal structure of atoms and molecules
MICROWAVE SPECTROSCOPY
region: 3x10^10 – 3x10^12 Hz
RIGID ROTOR
MOMENT OF INERTIA
-mass of each atom multiples by the square of its distance from the rotational axis passing through the center of mass of the molecule
i)moment A= rotation about the bond axis
ii)moment B= rotation end-over-end in the plane of paper
iii)moment C= rotation end-over-end at right angles to the plane
linear molecules
-has dipole moment, microwave active
asymmetric tops
-permanent dipole moment
symmetric tops
-prolate & oblate
-permanent dipole moment, microwave active
spherical tops
-no dipole moment, no rotational spectrum observable
CENTRIFUGAL DISTORTION
atoms of rotating molecules are subjected to centrifugal forces that tend to distort the molecular geometry and change the moment of inertia
EFFECT:
-stretch the bond
-increase the moment of inertia
RESULT
-reduces the rotational constant
-energy levels are slightly closer
LINEAR POLYATOMIC MOLECULES
-B value is much smaller
-spectral lines more closely spaced
molecule must posses dipole moment to exhibit rotational spectrum
moment of inertia for the end-over-end rotation of a polyatomic molecule is greater than diatomic molecule
FARAH HANI EZZATY BINTI MOHD ALIAS
A19SC0072