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

WhatsApp Image 2021-05-23 at 10.31.52

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