Spectroscopy_B.Sc._(PCM)._SEM-V_AMC.pptx

Spectroscopy

1. SPECTROSCOPY UNIT: 1 INSTRUCTOR: Dr. NEHA SAXENA Reference: [1] C. N. Banwell , “Fundamentals of Molecular Spectroscopy”, 3 rd Edition, McGraw – Hill Book Company Europe, London, 1983. [2] Puri , Sharma and Pathania , “Principles of Physical Chemistry”, Vishal Publishing co., 2017-2018, New Delhi, India.

2. What is Spectroscopy? Interaction of matter with energy Electromagnetic radiation: Transmission of energy Dual nature: Particle and wave Electric component and magnetic component perpendicular to each other E = h ν = hc / λ , where E = Energy; h = Plank’s constant; c = speed of light λ = wavelength of the wave Spectrum of Radiation: γ -rays ˃ X-rays ˃ UV ˃ Visible ˃ IR ˃ Microwave ˃ Radio

3. IR & UV: Interaction with Electric vector NMR: Interaction with Magnetic vector Q. (a) What two types of spectra observed in spectroscopy? (b) What factors determines the wavelength of light absorbed? (c) Explain why the absorption spectrum for each molecule is unique? (d) What is quantization of energy? Absorption spectra G.S. E.S. Emission spectra G.S. E.S. Ans : (a) ∆E ∆E (b) ∆E = Difference of lower energy level and a higher one (c) ∆E is unique for each molecules (d) Each energy level have fixed value of energy: Quantization of energy E 1 E 2 E 1 E 2

4. Born Oppenheamer Approximation: E Total = E electronic + E vibrational + E rotational Usefulness of molecular spectra in chemistry: To calculate: Bond length, bond angle, bond strength, bond dissociation energy, to determine shape & size of molecules IR: Functional groups detection UV: Conjugation detection NMR: Carbon skeleton & types of hydrogen Dual Behaviour: EMR shows dual behaviour Given by de-Broglie: λ = h/P = h/(mv) ; P = Momentum of particle; λ = Wavelength; h = Plank’s constant P = Momentum shows particle nature λ = Wavelength shows wave nature

5. Types of various molecular spectra: (1) Electronic Spectra: Electronic transition Region: UV-Visible (2) Vibrational (IR) spectra: Transition between vibrational energy levels Comes under IR region Change in the dipole moment (3) Rotational (Microwave) spectra: Transition between the rotational energy levels Come under microwave region Molecules which posses a Permanent dipole moment, e.g. HCl , CO, H 2 O, NO Homonuclear diatomic molecule: Inactive (4) Raman Spectra: Scattering due to vibration/rotation Visible region

6. Regions of the spectrum

7. Intensity of Spectral lines: Transition Probability: Possibility of changing of one state to another state: Selection Rule (ii) Population of States: Given by Boltzmann distribution Where, N upper = population of upper energy state N lower = population of lower energy state ∆E = Energy difference between two states k = Boltzmann constant and T = Absolute temperature (iii) Path length of sample: Concentration ∆E E 2 N upper N lower

8. UV-Visible Spectrometry: Lambert – Beer’s law A = ε c ℓ A = absorbance; ε = Extinction co-efficient; C = concentration; ℓ= path length l l KMnO 4 solution Light, Visible ℓ l = l absorbed + l transmitted For dilute solution Note: l reflected is neglected l = incident ray intensity I = transmitted ray (observed) intensity About ε : Absorption co-efficient or extinction co-efficient of the absorbing medium. Characteristics of the solute. Depends upon the nature of the solvent, temperature and the wavelength of light employed. Independent of concentration of the solute. Unit: L mol -1 m -1 Molar ε : When C is expressed in mol. dm —3 and path length in cm it is known as molar ε . cuvette

9. A = ε c ℓ A (wavelength) λ λ max A (Concentration) C % T (Concentration) C Transmittance (T) = I/I Absorbance & Transmittance Relationship: A = Log(I /I) = - Log(I/I ) = - Log(T) T = 10 -A = 10 - ε C ℓ Slope = ε ℓ Limitations of L-B law: Radiation must be monochromatic. Applicable for dilute solution only. At high conc., l reflection becomes significant. Temperature must be constant. A (Concentration) C Deviation from L-B law : Limitations Theoretical (Absorbance) (Absorbance) (Absorbance) (Transmittance)

10. METHYL ORANGE (AN AZO DYE) Azo Group UV-Visible (Electronic) Spectra of Polyatomic Molecules ACETONE Atomic Orbitals (AO) Molecular Orbitals (MO) Bonding MO Anti-Bonding MO σ , π σ *, π * MO σ , π σ *, π * E i E f Δ E = E f - E i ENERGY Types of bonds (electrons) present in methyl orange or acetone: σ , π and non-bonding (lone pair).

11. Relative Energy of MO Electronic Transition (Visible region) Allowed Forbidden (Least probable) High ε : Allowed Low ε : Forbidden Chromophor : Functional groups which absorb light in UV-visible region. Examples: C=C, C=O, N=N, NO 2 , etc. Auxochromophor : Functional groups which do not absorb UV-Visible light itself but when attached with a chromophor absorb light in UV-Visible region. Examples: -OH, -NH 2 , -CN, lone pair e-, etc. MO ENERGY σ → σ * >> n → σ * > π → π * > n → π * Highest Energy (Not in visible region) Least Energy (Forbidden) n & π * orbitals are Perpendicular to each other σ σ * E 1 E 2 π * n π E 3 E 4 E 5 ε (wavelength) λ Bathochromic or Red shift: Increase of λ Blue shift: Decrease of λ Hypochromic shift: Decrease in intensity ( ε ) Hyperchromic shift : Increase in intensity ( ε )

12. Transition between vibrational energy levels Comes under IR region Change in the dipole moment POLYATOMIC INFRARED (VIBRATIONAL) SPECTROSCOPY Change in dipole moment during vibration: IR active Degree of freedom for N atoms in a molecule: 3N Translational degree of freedom = 3 Degree of freedom for rotation for linear molecule = 2 Degree of freedom for rotation for non-linear molecule = 3 Total vibrational modes possible for a linear molecule = 3N – 5 Total vibrational modes possible for a non-linear molecule = 3N – 6 Modes of vibration: (i) Stretching: Symmetric & Asymmetric: Change of Bond length (ii) Bending: Change of Bond angle

13. Qualitative Application of IR Skeletal Vibration: Which involves all the atoms to almost the same extent Group Frequency: These characteristics group vibrations involve only the Small portion of the molecule, rest of the molecule is more or less stationary. (iii) Finger Print Region: Except optical isomers no two compound have identical absorption curves. It is independent of the structure of the molecule. Hook’s Law is given by: (wave number) Reduced mass =

14. Example: 1. H 2 O molecule : Non-linear : 3N – 6 = 3 Normal Modes : All are IR active

15. Example: 2. CO 2 molecule : Linear : 3N – 5 = 4 Normal Modes: Symmetric + Antisymmetric + 2 Bending degenerate : 3 are IR active Modes and 1 IR inactive Inactive Active Active: Degenerate