UV-Visible Spectroscopy PDF

UCB009-CHEMISTRY Molecular Spectroscopy: Brief overview UV-Visible Spectroscopy 2024-2025 ODD Semester UCB009 (Chemistry) An Outline of UV-Visible Spectroscopy: Revisiting Fundamental Concepts Covalent bonds in an organi...

UCB009-CHEMISTRY Molecular Spectroscopy: Brief overview UV-Visible Spectroscopy 2024-2025 ODD Semester UCB009 (Chemistry) An Outline of UV-Visible Spectroscopy: Revisiting Fundamental Concepts Covalent bonds in an organic molecule Energy order of molecular orbitals Principles of UV-Visible Spectroscopy Possible Transitions: Allowed and Forbidden transitions Energy Requirement for Electronic Transitions W h y  −  * transition is not observed conventionally? Absorption maxima or λmax Effect of conjugation on λmax Relative energies of molecular orbitals Examples of extended conjugation Chromophore and Auxochrome Spectral shifts and effects Applications of UV-visible spectroscopy 2024-2025 ODD Semester UCB009 (Chemistry)  Revisiting Fundamental Concepts Atomic spectrum: Line Spectrum Spectrum of Na Molecular spectrum: Band Spectrum 2024-2025 ODD Semester UCB009 (Chemistry)  Energy levels in a molecule: ∆E= ∆Etranslational+ ∆Erotational+∆Evibrational +∆Eelectronic ∆Erotational < ∆Evibrational < ∆Eelectronic Line spectrum appears due to discrete electronic transitions in an atom Band spectrum appears due to the overlap of vibrational and rotational transitions during an electronic transition in a molecule 2024-2025 ODD Semester UCB009 (Chemistry) UV-Visible Spectroscopy – Electronic Transitions 2024-2025 ODD Semester UCB009 (Chemistry) Energy levels in an atom: Revisiting Fundamental Concepts s-orbital p-orbital 2024-2025 ODD Semester UCB009 (Chemistry)  Revisiting Fundamental Concepts Formation of Molecular Orbitals Linear Combination of Atomic Orbitals (LCAO) (σ*-orbital) 1) Mixing of s-atomic orbitals (σ-orbital) 2) Mixing of p-atomic orbitals: Head-on mixing Energy (E) 2024-2025 ODD Semester UCB009 (Chemistry) 3) Mixing of p-atomic orbitals: Side-ways mixing Revisiting Fundamental Concepts antibonding Energy (E) bonding 2024-2025 ODD Semester UCB009 (Chemistry) Covalent bonds in an organic molecule -bond  −bond     (bonding) (anti-bonding) (bonding) (anti-bonding) Compounds containing single bonds, will have σ- and σ*-molecular orbitals. Compounds containing double or triple bonds will have π-and π* molecular orbitals Compounds containing hetero atoms [e.g. N, O, S, P, X (=Cl, Br, F, I)] have non-bonding electrons  Exercise 1: Find all possible molecular orbitals in the following molecules 1. Methane 2. Ethylene 3. Acetylene 4. Water 5. Pyridine 6. Benzaldehyde 7. Benzoic acid 8. Benzene Hints to solve the given exercise: 1. Draw the chemical structure of the molecules 2. Show all the bonds explicitly 3. Based on the type of bond present, identify the molecular orbitals 4. If there is any heteroatom in the molecule, identify the additional molecular orbital Energy order of molecular orbitals: Concept of HOMO and LUMO: HOMO (Highest occupied molecular orbital) is the highest energy molecular orbital filled by electrons LUMO (Lowest unoccupied molecular orbital) is the lowest energy molecular orbital that is empty LUMO Energy HOMO 2024-2025 ODD Semester UCB009 (Chemistry) Exercise 2: Arrange the molecular orbitals available in the following molecules in the order of increasing energy and identify the HOMO and LUMO 1. Acetone 2. Aniline 3. Acetylene 4. Water 5. Pyridine 6. Benzaldehyde 7. Benzoic acid Hints to solve the given exercise: 1. Draw the chemical structure of the molecules 2. Show all the bonds explicitly 3. Based on the type of bond present, identify the molecular orbitals 4. If there is any heteroatom in the molecule, identify the additional molecular orbital 5. Arrange the MO in the increasing energy order (Lowest to highest) 6. Identify the highest occupied, which will be HOMO, and the lowest unoccupied, which will be LUMO  Principles of UV-Visible Spectroscopy When a molecule is irradiated with UV-visible light, electronic transitions occur from bonding orbitals to antibonding orbitals. The UV radiation region extends from 10 nm to 400 nm, and the visible radiation region extends from 400 nm to 800 nm. Near UV Region: 200 nm to 400 nm, Far UV Region: below 200 nm Far UV spectroscopy is studied under vacuum conditions. The common solvent used for preparing the sample to be analyzed is either ethyl alcohol or hexane. Note: The transition will occur from Bonding Molecular Orbital (BMO) to Antibonding Molecular Orbital (ABMO) HOMO (Highest occupied molecular orbital) to LUMO (Lowest unoccupied molecular orbital) 2024-2025 ODD Semester Lower energy state (filled) to higher energy state (empty) UCB009 (Chemistry)  Possible Transitions: Allowed and Forbidden transitions ✓  Symmetry   Symmetry   Symmetry Forbidden Allowed transitions Forbidden transitions but Observed transitions * (anti-bonding) n→* →* * (anti-bonding)  n→*   →* Energy →*  n (non-bonding) ✓ →* ✓  (bonding)  (bonding)  Exercise 3: In the following molecules, find i) All possible electronic transitions ii) Allowed transitions iii) Forbidden transition 1. Methane 2. Ethylene 3. Acetylene 4. Water 5. Pyridine 6. Benzaldehyde 7. Benzoic acid 8. Benzene Hints to solve the given exercise: 1. Draw the chemical structure of the molecules 2. Show all the bonds explicitly 3. Based on the type of bond present, identify the molecular orbitals 4. If there is any heteroatom in the molecule, identify the additional molecular orbital 5. Write down all possible transitions. 6. Identify the allowed and forbidden transitions. 2024-2025 ODD Semester UCB009 (Chemistry)  Energy Requirement for Electronic Transitions -* > n- * ~ - * > n-* ~150 nm ~170-190 nm 280 nm -* saturated hydrocarbons e.g. ethane saturated compounds containing hetero atom(s) having n-  unshared pairs of electrons. e.g. saturated halides, alcohols, ethers, aldehydes, amines, etc compounds having double or triple bond and aromatics e.g. -* butadiene, benzene, etc. unsaturated compounds containing hetero atom having unshared n-* pair of electrons e.g. carboxylic acids, aldehydes, ketones, etc. 2024-2025 ODD Semester UCB009 (Chemistry)  Why  -* transition is not observed conventionally? UV V I B G Y O R  - * Far UV ~150 nm 100 200 380 760 nm nm nm nm Vacuum ultraviolet region σ- σ* requires photons of 150 nm. Conventional UV-visible spectrophotometer works in the range of 200 nm to 760 nm. This instrument cannot be used below 200 nm as oxygen in air strongly absorbs in far UV region. Thus, σ- σ* transitions cannot be observed using a conventional UV- visible spectrophotometer (This transition can be observed only under a vacuum UV-visible spectrophotometer) 2024-2025 ODD Semester UCB009 (Chemistry) Exercise 4 Why σ to σ* transition is not observed by a conventional UV-Vis spectrophotometer? Ans. The energy gap between σ – σ* is high and comes below 200 nm wavelength. The general UV-Vis spectrophotometer provides radiations from 200 nm to 800 nm only not below 200 nm. That’s why this transition from σ to σ* is not observed (even though it is allowed) by conventional UV-Vis spectrophotometer. 2024-2025 ODD Semester UCB009 (Chemistry) Absorption maxima or λmax max The wavelength at which a substance shows maximum absorbance is called absorption maximum or λmax. 2024-2025 ODD Semester UCB009 (Chemistry)  Effect of conjugation on max What is conjugation? When two double bonds are separated by just one single bond, the two double bonds are said to be conjugated. CH2 CH3 H 2C H 3C ? Non-conjugated Conjugated Allenes (Fun Question) When double bonds are conjugated in a compound, λmax is shifted to a longer wavelength. CH2 CH3 H 2C H 3C 1,5 - hexadiene : λmax = 178 nm 2,4 - hexadiene : λmax = 227 nm Benzene Styrene λmax = 255 nm λmax = 274 nm 2024-2025 ODD Semester UCB009 (Chemistry)  Effect of conjugation on λmax O O C C H 2C CH 2 H 2C H 3C CH 3 CH 3 Ethylene Acetone Crotonaldehyde λmax = 171 nm λmax = 279 nm λmax = 290 nm Conjugation of C=C and carbonyl group shifts the λmax of both groups to longer wavelengths Effect of Conjugation: A comparison of the π-molecular orbital energy levels in the ethylene and 1,3-butadiene π4* LUMO LUMO π* π* π3* Energy π2 π HOMO π HOMO π1 Ethylene 1,3 butadiene Ethylene 2024-2025 ODD Semester UCB009 (Chemistry) Relative energies of molecular orbitals 2024-2025 ODD Semester Kemp, W., Organic Spectroscopy, Palgrave Publ. UCB009 (Chemistry) Effects of conjugation: Points to remember Higher the conjugation= lower the gap between HOMO and LUMO= lower the energy required= Higher the wavelength λmax since, E = hc/ Compounds having 8 or > 8 double bonds in conjugation will appear colored to the human eye Examples of extended conjugation -carotene, max = 455 nm lycopene, max = 474 nm O H N N H O indigo λmax for lycopene is at 474 nm in the near blue region of the spectrum – this is absorbed, the complement is now red! 2024-2025 ODD Semester UCB009 is Λmax for indigo is at 602 nm in the orange region of the spectrum – this is absorbed, and the compliment (Chemistry) now indigo! Exercise 5: Arrange the following molecules in order of increasing λmax. (i) (a) C6H6 (b) CH2=CH2-CH2-CH2=CH2 (c) C6H5CHO (d) C6H5CH=CH-CH=CH2 Ans. Following is the order: (i) CH2=CH2-CH2-CH2=CH2 < C6H6 < C6H5CHO < C6H5CH=CH-CH=CH2 (ii) b>a 2024-2025 ODD Semester UCB009 (Chemistry)  Chromophore and Auxochrome Chromophore The functional groups containing multiple bonds capable of absorbing radiations in the UV/visible region due to -* and n-* transitions e.g. C=C, C≡C, N=O, N=N, NO2, C=O, C=N, C≡N, C=C,C=S, CONH2, -COOH, etc Auxochrome The functional group with non-bonding electrons, that does not absorb radiation in UV/visible region (200-800 nm), when attached to a chromophore increases the wavelength and intensity of absorption e.g. -OH, -OR, -NH2 , -NHR, -NR2, -SH etc. Chromophore New Chromophore Chromophore + Auxochrome λmax = 255 nm 270 nm New Chromophore (having increased max) λmax = 255 nm 280 nm 2024-2025 ODD Semester UCB009 (Chemistry) Spectral shifts and effects: 2024-2025 ODD Semester UCB009 (Chemistry) Bathochromic Shift (Red Shift) When absorption maximum (max) is shifted towards longer wavelength, it is known as bathochromic shift or red shift. The effect is due to presence of an auxochrome or by the change of solvent. For example, an auxochrome group like –OH, -OCH3 causes absorption of compound at longer wavelength 2024-2025 ODD Semester UCB009 (Chemistry) Why does p-nitrophenol show bathochromic shift (red shift) in the alkaline medium? p-nitrophenol, in an alkaline medium, shows a red shift because the negative charge on the oxygen delocalizes more effectively, leading to a decrease in the HOMO- LUMO gap. Conjugation increases due to negative charge λmax = 255 nm λmax = 265 nm 2024-2025 ODD Semester UCB009 (Chemistry)  Hypsochromic Shift (Blue Shift): When absorption maximum (max) is shifted towards a shorter wavelength, it is known as a Hypsochromic shift or blue shift. The effect is due to the removal of conjugation or by the change of solvent. Why does aniline show Hypsochromic shift (blue shift) in the acidic medium? Aniline in an acidic medium shows a blue shift because the lone pair of electrons on the nitrogen is shared with the hydrogen ion, so it is not available for delocalization, leading to an increase in the HOMO-LUMO gap. Conjugation decreases due to protonation λmax = 280 nm λmax = 265 nm 2024-2025 ODD Semester UCB009 (Chemistry)  Hyperchromic effect When the intensity of absorption maximum (max) of a compound increases, it is known as hyperchromic effect. Hypochromic effect When the intensity of absorption maximum (max) of a compound decreases, it is known as a hypochromic effect. 2024-2025 ODD Semester Sharma YR, Elementary Organic Spectroscopy, S. Chand & Co. UCB009 (Chemistry) Applications of UV-Visible spectroscopy The extent of conjugation in polyenes can be estimated Distinction between conjugated and non-conjugated compounds Shift in the absorption, due to the addition of unsaturation in a compound, towards a longer wavelength can be detected. Presence or absence of a chromophore can be detected.. It is used for characterizing aromatic compounds and conjugated olefins. 2024-2025 ODD Semester UCB009 (Chemistry)