UV-Visible Instrumental Methods of Analysis - Spectroscopy PDF

Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) First Year BTech 1  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College...

Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) First Year BTech 1  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Module 4 Instrumental methods of analysis First Year BTech 2  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) 4.1 Optical methods of analysis: Origin of spectra, interaction of radiation with matter, fundamental laws of spectroscopy and selection rules, validity of Beer-Lambert’s law. 4.2 UV Spectroscopy: Types of electronic transitions. 4.3 Infrared spectroscopy: Fundamental and non-fundamental molecular vibrations; IR absorption positions of O, N and S containing functional groups; Effect of H-bonding, conjugation, resonance and ring size on IR absorptions. 4.4 Emission spectroscopy: Flame photometry, fluorescence and phosphorescence Jablonski diagram. 4.5 Electroanalytical techniques: Potentiometry, conductometry, amperometry. 4.6 Electrochemical sensor: Electrochemical gas sensors for SOx and NOx. Biosensors. First Year BTech 3  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) 4.1 Optical methods of analysis: Origin of spectra, interaction of radiation with matter, fundamental laws of spectroscopy selection rules, validity of Beer-Lambert’s law. 4.2 UV Spectroscopy: Types of electronic transitions. First Year BTech 4  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Learning Objectives: Explain fundamental laws of spectroscopy Explain selection rules for electronic transition State the Beer Lambert’s law for absorption spectroscopy and it’s use in UV-Visible spectroscopy First Year BTech 5  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Introduction to Spectroscopy Definition: Spectroscopy is the study of the interaction between electromagnetic radiation and matter. Purpose: Helps in qualitative and quantitative analysis of molecular structures. Applications: Used across various fields of science and technology such as medicine, security, research for analysis purposes. First Year BTech 6  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) 𝒉𝒄 𝜟𝑬 = 𝒉𝝑 = = 𝒉ഥ 𝒗 𝝀 h - plank’s constant E h𝜗 h𝜗 Absorption spectrum Emission Spectrum9 First Year BTech  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Electromagnetic Spectra: First Year BTech 10  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Regions of electromagnetic spectrum: First Year BTech 11  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Origin of Spectrum Absorption spectrum Emission Spectrum Spectra can be classified into two categories Atomic Spectra: transition of electrons between atomic energy level Molecular spectra due to transition of electron between molecular energy level First Year BTech 12  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Types of energy present in the molecules: 1. Translational energy E(trans)is concerned with overall movement of molecules along the three axis it is significant only in gases and to lesser extent for liquids 2. Rotational energy E(rot) involves the spinning of molecule about the axis passing through the centre of gravity. 3. Vibrational energy E(vib) is associated with vibrations within the molecule such as stretching or the bending of bonds. 4. Electronic energy E(elec) involves change in the distribution of Electrons by the promotion of electrons to higher levels on absorption of energy If E is the energy of the molecule then E= E(trans) + E(rot) +E(vib) +E(elec) First Year BTech 13  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) : https://www.researchgate.net/figure/Schematic-representation-of-two-electronic-states-ground- and-excited-and-their_fig2_338665929 First Year BTech 14  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) : First Year BTech 15  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Radiation Microwave Infrared Ultraviolet and visible absorbed: Effect on the Change in the Change in the Change in the electronic molecule rotational energy rotational and energy levels within the levels of the vibrational energy molecules. molecules levels of the Vibrational and rotational molecules transitions are also brought about but their resolution is not possible in solids and liquids Information Calculation of force Detection of In quantitative and qualitative obtained constant bound functional groups in analysis length bond angle the compound and etc calculation of force constant bond length etc in qualitative analysis. First Year BTech 16  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Selection Rules: In quantum mechanics , certain electron transitions are more likely to take place than others. Transitions having high probability of taking place are known as “allowed transition” Transitions which are less likely to take place are known as “forbidden transition” Selection Rules explain which transitions are allowed transitions. Rule No.1 Δs=0 Spin angular momentum of an electron does not change during absorption or emission of light. The change in the spin quantum number during transition is zero. Thus the singlet to triplet transitions are forbidden transitions First Year BTech 17  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Selection Rules: In quantum mechanics , certain electron transitions are more likely to take place than others. Transitions having high probability of taking place are known as “allowed transition” Transitions which are less likely to take place are known as “forbidden transition” Selection Rules explain which transitions are allowed transitions. Rule No. 2 The transition between orbitals of different symmetry do not occur. For example n to pi * transition is symmetry forbidden First Year BTech 18  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Representation of Absorption Spectra UV-visible absorption spectra of the compound (6) and C-8 in dichloromethane (c = 1 × 10 −5 M). First Year BTech 19  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) 4.2 UV – Visible Spectroscopy: Types of electronic transitions. First Year BTech 20  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) UV Spectroscopy: Wavelength range 200nm to 400nm Visible Spectroscopy: Wavelength range 400nm to 700nm First Year BTech 21  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Electronic Transitions Electronic transition is the movement of electrons between different energy levels within a molecule due to excitation. Absorption of radiation in UV and visible region of electromagnetic spectrum results in transition between electronic levels. The energy change corresponds to 125 to 650 kJmol-1. Simultaneously change in vibration and rotational energies take place along with electronic transitions. The most probable transition is from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) Ground State: Lowest energy state.(HOMO) Excited State: Higher energy state.(LUMO) Observation: Energy changes during transitions provide the data about the molecule First Year BTech 22  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Characteristics of UV and visible spectra 1. 𝝀𝒎𝒂𝒙 𝒗𝒂𝒍𝒖𝒆 is the wavelength at which maximum absorption occurs. 𝝀𝒎𝒂𝒙 𝒗𝒂𝒍𝒖𝒆 for benzene and acetone is 255nm and 279 nm respectively 2. ε-value is molar absorptivity (or molar extinction coefficient). For given concentration of compound is indicated by ε-value and is related to height of absorption band. Origin of absorption bands in UV and visible spectra: According to MOT due to interaction of atomic orbitals leads to formation of “bonding” and “antibonding” molecular orbitals. (i) Sigma type(𝝈) (ii) Pi type(𝝅) (𝟏) 𝝈 and 𝝅 𝐚𝐫𝐞 𝐛𝐨𝐧𝐝𝐢𝐧𝐠 𝐌𝐎 (𝟐 𝝈 * and 𝝅 ∗ 𝐚𝐫𝐞 𝐚𝐧𝐭𝐢 𝐛𝐨𝐧𝐝𝐢𝐧𝐠 𝐌𝐎 First Year BTech 23  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Types of Electronic Transitions The three kinds of electrons responsible for electronic transitions are: 1. Sigma (σ) electrons in saturated molecules 2. Pi (π) electrons in unsaturated molecules 3. Nonbonding (n) electrons in non bonded elements 𝞂* anti bonding 𝞹* anti bonding ENERGY n- 𝞂 * n- 𝞹* n 𝑏𝑜𝑛𝑑𝑖𝑛𝑔 𝞹 - 𝞹* 𝞹-𝞂* 𝞹 bonding 𝞂 - 𝞂 *𝞂 bonding First Year BTech 24  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) When a molecule absorbs UV-Visible light, there is the possibility of four types of electronic transitions. 1. σ to σ* transition: A high amount of energy is required for this type of transition. It occurs in saturated hydrocarbons like methane, propane, etc. 2. n to σ* transition: Compounds having lone pair of electrons undergo this transition. For example alcohols, ethers, aldehydes and ketones, etc. 3. n to π* transition: Less amount of energy is required for this transition as shown by compounds having a double or triple bond. For example; saturated aldehydes, etc. 4. π to π* transition: Such transition occurs in the unsaturated center of molecules and also in aromatics compounds. It requires slightly greater energy than n to π. For example Ethene, ethyne, other alkene and alkynes. First Year BTech 25  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) "n" electrons (or the nonbonding electrons) are the ones located on the hetero atoms like O,N,S,Cl,F,Br etc Thus, the n to π* transition corresponds to the excitation of an electron from one of the unshared pair to the π* orbital. Molecules having n or 𝜋 electrons are responsible for absorption are known as chromophores. C=C, -C=C-, -C=N, -N=N-, -C=O First Year BTech 26  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) n to π* transition and π to σ* transition are symmetrically forbidden transitions Molecules having n or π electrons are responsible for absorption are known as chromophores. C=C, -C=C-, -C=N, -N=N-, -C=O First Year BTech 27  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Chromophores The primary molecule responsible for color in a compound. Examples of chromophores include carbon-carbon double bonds, carbon-nitrogen double bonds, carbonyl, azo, nitro, methine, and quinoid rings. Groups of atoms attached to a chromophore that enhance the color of the chromogen are called as Auxochromes. They do not produce color on their own,but instead modify the chromophore's ability to absorb light. Examples of auxochromes include hydroxyl, amino, aldehyde, and methyl mercaptan groups. First Year BTech 28  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) First Year BTech 29  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) First Year BTech 30  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Bathochromic shift It is an effect by which the absorption maximum is shifted towards a longer wavelength due to the presence of an auxochrome or by a change of solvent. Bathochromic shifts are also called redshifts. n to π* transition of carbonyl compounds experiences bathochromic shifts when polarity of the solvent is decreased Hypsochromic shifts This is the effect by which the absorption maximum is shifted towards a shorter wavelength. It is also called the blue shift. It may be caused by the removal of conjugation and changing the solvent’s polarity. for example Aniline shows maximum absorption at 280nm, in acidic solution it shows 203nm First Year BTech 31  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) 3.Hyperchromic effect: Hyperchromic shift is an effect by the virtue of which intensity of absorption maximum increases. The introduction of an auxochrome in the compounds generally results in the hyperchromic effect. 4. Hypochromic effect: Hypochromic effect is defined as the effect by virtue of the intensity of absorption maximum decreases. The hypochromic effect occurs due to the distortion of the geometry of the molecules with the introduction of the new group. First Year BTech 32  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Factors influencing absorption and wavelength change Effect of conjugation According to MOT, as the number of pi electrons increases, so does delocalization. Because of this rise in delocalization, the molecules in the sample become stabilized and attain a lower energy state. This decrease in energy causes a shift in wavelength towards a higher wavelength, which is known as redshift. Due to double bond conjugation, 1,3-butadiene absorbs at a higher wavelength, 217 nm, while 1,2-butadiene absorbs at 210 nm. First Year BTech 33  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) First Year BTech 34  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Presence of aromatic ring The aromatic ring, particularly when two or more rings are conjugated (polycyclic compounds), absorbs a greater wavelength in the visible range, altering the absorption spectra. For example, Naphthalene absorbs at 268nm while anthracene absorbs at 311nm. First Year BTech 35  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Solvent polarity Polarity has a significant impact on the position and strength of absorption bands. The increase is related to the n- -π* and π – -π* transitions. In the presence of a polar hydrolytic solvent (such as water), hydrogen bonds form with the auxochrome’s lone pair of electrons. As a result, the auxochrome’s energy decreases to an amount equivalent to the bond formation energy, and the energy difference between HOMO and LUMO increases, resulting in a hypsochromic shift for the n- -π* transition. First Year BTech 36  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Beer Lambert’s law Beer Lambert’s law is the principle on which an instrument named a spectrophotometer operates. A source emits light of different intensities, which passes through the monochromator that converts distinct colors into a single color. The light is incident on the cuvette containing the sample, and it gets transmitted further. A detector is placed which measures the light intensity and converts it into an electrical signal. The measurement of absorbed light increases with an increase in the concentration of the sample, according to Beer’s law. However, it also increases with path length, which is stated byLambert’s law. The combination of both gives rise to Beer Lambert’s law, and its equation is as follows: A= ∈ c l = log (Io/I) First Year BTech 37  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Beer-Lambert Law Principle: Describes the relationship between absorbance, concentration, and path length The combination of both gives rise to Beer Lambert’s law, and its equation is as follows: A= ∈ c l = log (Io/I) Here, A = Absorbance ε= Molar absorption coefficient c = Molar concentration of the sample l = optical path length Io = Intensity of incident light I = Intensity of emitted light First Year BTech 38  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) The molar extinction coefficient ∈ is dependent on the nature of the absorbing solute as well as on the wavelength of the incident light used. ∈ is expressed in L/mol/cm. Transmittance (T) is measured as the ratio of light passing through a substance. It can be calculated as I/Io. A = log (Io/I)= -logT First Year BTech 39  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Numerical on Beer-Lambert Law Q 1. 2.5× 104 M of a substance in a 1 cm length cell at λ_max 245nm have absorbance 1.17. Calculate εmax for this transition. Q2. In a spectrophotometric cell of 2 cm path length The solution of a substance shows the absorption value of 1.0. If the molar absorptivity of the compound is 2 × 104 L mol -1 cm -1 Calculate the concentration of that substance in solution. First Year BTech 40  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Numerical on Beer-Lambert Law Q 3. A solution of chemical ‘A’ with concentration 0.14 mol/L has an absorbance of 0.42. Another solution of ‘A’ under the same condition have absorbance of 0.36. What is the concentration of this solution of ‘A’? Q4. A solution of thickness 2 cm transmit 40% of its incident light. Calculate the concentration of the solution given that εmax = 6000 L/mol/cm First Year BTech 41  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Practical Applications of UV-Visible Spectroscopy Qualitative Analysis: Identifying functional groups and molecular structures. Quantitative Analysis: Determining concentrations of compounds. Example Uses: Monitoring chemical reactions, quality control in pharmaceuticals. First Year BTech 42  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Conclusion Summary: UV-Visible spectroscopy is a versatile tool for analyzing electronic transitions in molecules. Importance: Provides valuable insights into molecular structures and concentrations. First Year BTech 43  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) Types of Spectroscopy Absorption Spectroscopy: Description: Measures the absorption of electromagnetic radiation by a sample. Examples: UV/Visible, Infrared (IR), Nuclear Magnetic Resonance (NMR). Emission Spectroscopy: Description: Measures the emitted light from a sample as it returns to a lower energy state. Examples: Atomic Emission (AE), Flame Photometry, Fluorimetry. Scattering Spectroscopy: Description: Analyzes the scattered light to obtain information about molecular vibrations and rotations. Examples: Raman Spectroscopy. First Year BTech 44  Mahavir Education Trust’s Shah & Anchor Kutchhi Engineering College Chembur, Mumbai-400088 (Affiliated to University of Mumbai) References List: https://www.bartleby.com/subject/science/chemistry/concepts /electronic-transitions-and-spectroscopy Books referred: Engineering Chemistry: Jain & Jain Elementary Spectroscopy : Y.R. Sharma First Year BTech 45