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Ultraviolet spectroscopy (UV)

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Mona Ismail
Mona Ismail

UV-Vis spectrophotometer measure absorption of UV radiation (200-400 nm) and visible light (400-800 nm) by molecule.

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Ultraviolet spectroscopy (UV)
Ultraviolet spectroscopy (UV) : UV-Vis spectrophotometer measure absorption of UV radiation (200-400 nm) and visible light (400-800 nm) by molecule. Promotion of electrons to higher energy levels through irradiation of the molecule with ultraviolet light. Provides mostly information about the presence of conjugated π systems and the presence of double and triple bonds.
It is called electronic spectroscopy, because the energy absorbed (ΔE) is used to induce electron transition i.e. one electron is promoted from one molecular orbital to higher energy one. UV-Visible spectrometry commonly used because of its simplicity, versatility, speed, accuracy and cost-effectiveness.
Ultraviolet spectroscopy (UV)
Electronic spectrophotometer consist of the following components:- 1-Radiation source: There are 2 different sources of radiation; A- tungsten lamp for scanning in the visible region. B- hydrogen discharge tube for scanning in the UV region. 2- Monochromator: which resolve polychromatic light into a spectrum of monochromatic (i.e. of single wavelength) light of the entire range.
3- Sample and reference cells: the radiation source is split into 2 beams; The reference beam which passes through reference cell containing solvent only. Sample beam which passes through sample solution. 4- Detector and recorder: record absorbance against wavelength (λ) in nm. This double beam technique is used to cancel out solvent absorption between sample and reference beams.
UV-Vis spectrophotometer
Solvent and Solution:- Solvent must be transparent to UV-Vis radiation.  Common solvents are; 1- Polar solvents: Methanol, ethanol and water. 2- Non polar solvents: Hexane, cyclohexane. 3- Intermediate polarity: Ether and dioxane. The UV cell (cuvette) is made of quartz (glass absorb UV light) and has length of 1 cm.
The energy of a photon absorbed or emitted during a transition from one molecular energy level to another is given by the equation; therefore, E= hc/λ The shorter the wavelength, the greater the energy of the photon and vice versa.
The energy supplied by the light will promote electrons from their ground state orbitals to higher energy, excited state orbitals or antibonding orbitals. Potentially, three types of ground state orbitals may be involved: i) σ (bonding) molecular as in These contain normal bonding pairs of electrons. The lowest energy level (it is the most stable).
ii) π (bonding) molecular orbital as in These contain normal bonding pairs of electrons. They are of higher energy than sigma electrons. iii) n (non-bonding) atomic orbital as in Atomic orbitals of hetero atoms (N, O, or S) which do not participate in bonding, they usually occupy the highest level of ground state.
In addition, two types of anti-bonding orbitals may be involved in the transition: i) σ* (sigma star) orbital ii) π* (pi star) orbital The following electronic transitions can occur by the absorption of ultraviolet and visible light: σ to σ* n to σ* n to π* π to π*
Ultraviolet spectroscopy (UV)
In the exited state  σ electrons occupy an anti-bonding energy level denotes as σ* and the transition is termed σ to σ* transition.  π electrons occupy an anti-bonding energy level denotes as π* and the transition is termed π to π* transition.  n electrons occupy either π* or σ*.
Ultraviolet spectroscopy (UV)
Both σ to σ* and n to σ* transitions require a great deal of energy and therefore occur in the far ultraviolet region (short λ) or weakly in the region 180-240nm. Consequently, saturated groups do not exhibit strong absorption in the ordinary ultraviolet region. Transitions of the n to π* and π to π* type occur in molecules with unsaturated centers; they require less energy and occur at longer wavelengths than transitions to σ* anti-bonding orbitals.
Table illustrates the type of transition and the resulting maximum wavelength.
Some terms related to UV-Vis spectrophotometer: 1- Bathochromic shifts: The shift of absorption to a longer wavelength due to substitution or solvent effect (red shift) 2- Hypsochromic shifts: The shift of absorption to a shorter wavelength due to substitution or solvent effect (blue shift) 3- Hyperchromic effect: It is increase in absorption intensity. 4- Hypochromic effect: It is simply a decrease in the absorption intensity.
A Wavelength λmax Hypsochromic shift Bathochromic shift Hyperchromic effect Hypochromic shift
5- Chromophore: It is unsaturated group responsible for electronic absorption e.g. C = C, C = O, NO2. 6- Auxochrome: It is a saturated group, which when attached to a chromophore alter both of wavelength λ and the intensity of the maximum absorption. These groups are generally characterized by having unshared electron pairs e.g. -OH, -NH2, -Cl.
Factors affecting on Wavelength λ :- 1- Conjugation:- E.↓λ↑more conjugated compounds 2- Substitution:- E.↓λ↑more substituted compounds  Alkyl group substitution in benzene(λmax =256 nm)ring cause slight bathochromic shift e.g. toluene has λmax at 261 nm. Auxochrome e.g. –OH, -NH2, -OR … the interaction of non-bounding electrons with π electrons of benzene increase conjugation & cause bathochromic shift e.g. aniline & phenol.
3- Solvent:- when the UV-Vis spectrum is recorded in different solvents, The position and intensity of an absorption band may shifted. For change to solvents of increasing polarity the pattern of shifts will be as follows; 1- Conjugated dienes & aromatic hydrocarbons display very little solvent effects. 2- α,β- unsaturated carbonyl compounds show 2 different shifts; a- The π π* band moves to longer wavelength (bathochromic shift) red shift b- The n π* band moves to shorter Wavelength (hypsochromic shift) blue shift
Calculation of λmax The first extensive set of rules was created by R. B. Woodward and used extensively in the structure elucidation of natural products. Each type of diene or triene system is having a certain fixed value at which absorption takes place; this constitutes the Base value or Parent value. The contribution made by various alkyl substituents or ring residue, double bond extending conjugation and polar groups such as – Cl, -Br etc are added to the basic value to obtain λmax for a particular compound
CONJUGATED DIENE CORRELATIONS: 1-Homoannular Diene:- Cyclic diene having conjugated double bonds in same ring. 2-Heteroannular Diene:- Cyclic diene having conjugated double bonds in different rings. 3-Endocyclic double bond:- Double bond present in a ring.
4- Exocyclic double bond: - Double bond in which one of the doubly bonded atoms is a part of a ring system. Here Ring A has one exocyclic and endocyclic double bond. Ring B has only one endocyclic double bond.
i) Base value for homoannular diene = 253 nm ii) Base value for heteroannular diene = 214 nm iii) Alkyl substituent or Ring residue attached to the parent diene = 5 nm iv) Double bond extending conjugation = 30 nm v) Exocyclic double bonds = 5 nm vi) Polar groups: 1- OAc = 0 nm 2- OAlkyl = 6 nm 3- Cl, Br = 5 nm
e.g. Base value = 214 nm Ring residue = 3 x 5 = 15 nm Exocyclic double bond = 1 x 5 = 5 nm λmax = 214 + 15 +5 = 234 nm
α, β UNSATURATED CARBONYL COMPOUNDS OR KETONES: 1. Base value: a) Acyclic α, β unsaturated ketones = 214 nm b) 6 membered cyclic α, β unsaturated ketones = 215 nm c) 5 membered cyclic α, β unsaturated ketones = 202 nm d) α, β unsaturated aldehydes = 210 nm e) α, β unsaturated carboxylic acids & esters = 195 nm
2. Alkyl substituent or Ring residue in α position = 10 nm 3. Alkyl substituent or Ring residue in β position = 12 nm 4. Alkyl substituent or Ring residue in γ and higher positions = 18 nm 5. Double bond extending conjugation = 30 nm 6. Exocyclic double bonds = 5 nm 7. Homodiene compound = 39 nm
8. Polar groups: a) –OH in α position = 35 nm –OH in β position = 30 nm –OH in δ position = 50 nm b) –OAc in α, β, γ, δ positions = 6 nm c) –OMe in α position = 35 nm –OMe in β position = 30 nm –OMe in γ position = 17 nm –OMe in δ position = 31 nm d) –Cl in α position = 15 nm –Cl in β position = 12 nm e) –Br in α position = 25 nm –Br in β position = 30 nm f) –NR2 in β position = 95 nm
Base value = 215 nm Double bond extending conjugation =1 x 30 = 30 nm Exocyclic double bond = 5 nm β- Substituents = 1 x 12 = 12 nm δ- Substituents = 1 x 18 = 18 nm λmax = 280 nm α β ɤ ᵹ
Ultraviolet spectroscopy (UV)

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Ultraviolet spectroscopy (UV)

  • 2. Ultraviolet spectroscopy (UV) : UV-Vis spectrophotometer measure absorption of UV radiation (200-400 nm) and visible light (400-800 nm) by molecule. Promotion of electrons to higher energy levels through irradiation of the molecule with ultraviolet light. Provides mostly information about the presence of conjugated π systems and the presence of double and triple bonds.
  • 3. It is called electronic spectroscopy, because the energy absorbed (ΔE) is used to induce electron transition i.e. one electron is promoted from one molecular orbital to higher energy one. UV-Visible spectrometry commonly used because of its simplicity, versatility, speed, accuracy and cost-effectiveness.
  • 5. Electronic spectrophotometer consist of the following components:- 1-Radiation source: There are 2 different sources of radiation; A- tungsten lamp for scanning in the visible region. B- hydrogen discharge tube for scanning in the UV region. 2- Monochromator: which resolve polychromatic light into a spectrum of monochromatic (i.e. of single wavelength) light of the entire range.
  • 6. 3- Sample and reference cells: the radiation source is split into 2 beams; The reference beam which passes through reference cell containing solvent only. Sample beam which passes through sample solution. 4- Detector and recorder: record absorbance against wavelength (λ) in nm. This double beam technique is used to cancel out solvent absorption between sample and reference beams.
  • 8. Solvent and Solution:- Solvent must be transparent to UV-Vis radiation.  Common solvents are; 1- Polar solvents: Methanol, ethanol and water. 2- Non polar solvents: Hexane, cyclohexane. 3- Intermediate polarity: Ether and dioxane. The UV cell (cuvette) is made of quartz (glass absorb UV light) and has length of 1 cm.
  • 9. The energy of a photon absorbed or emitted during a transition from one molecular energy level to another is given by the equation; therefore, E= hc/λ The shorter the wavelength, the greater the energy of the photon and vice versa.
  • 10. The energy supplied by the light will promote electrons from their ground state orbitals to higher energy, excited state orbitals or antibonding orbitals. Potentially, three types of ground state orbitals may be involved: i) σ (bonding) molecular as in These contain normal bonding pairs of electrons. The lowest energy level (it is the most stable).
  • 11. ii) π (bonding) molecular orbital as in These contain normal bonding pairs of electrons. They are of higher energy than sigma electrons. iii) n (non-bonding) atomic orbital as in Atomic orbitals of hetero atoms (N, O, or S) which do not participate in bonding, they usually occupy the highest level of ground state.
  • 12. In addition, two types of anti-bonding orbitals may be involved in the transition: i) σ* (sigma star) orbital ii) π* (pi star) orbital The following electronic transitions can occur by the absorption of ultraviolet and visible light: σ to σ* n to σ* n to π* π to π*
  • 14. In the exited state  σ electrons occupy an anti-bonding energy level denotes as σ* and the transition is termed σ to σ* transition.  π electrons occupy an anti-bonding energy level denotes as π* and the transition is termed π to π* transition.  n electrons occupy either π* or σ*.
  • 16. Both σ to σ* and n to σ* transitions require a great deal of energy and therefore occur in the far ultraviolet region (short λ) or weakly in the region 180-240nm. Consequently, saturated groups do not exhibit strong absorption in the ordinary ultraviolet region. Transitions of the n to π* and π to π* type occur in molecules with unsaturated centers; they require less energy and occur at longer wavelengths than transitions to σ* anti-bonding orbitals.
  • 17. Table illustrates the type of transition and the resulting maximum wavelength.
  • 18. Some terms related to UV-Vis spectrophotometer: 1- Bathochromic shifts: The shift of absorption to a longer wavelength due to substitution or solvent effect (red shift) 2- Hypsochromic shifts: The shift of absorption to a shorter wavelength due to substitution or solvent effect (blue shift) 3- Hyperchromic effect: It is increase in absorption intensity. 4- Hypochromic effect: It is simply a decrease in the absorption intensity.
  • 20. 5- Chromophore: It is unsaturated group responsible for electronic absorption e.g. C = C, C = O, NO2. 6- Auxochrome: It is a saturated group, which when attached to a chromophore alter both of wavelength λ and the intensity of the maximum absorption. These groups are generally characterized by having unshared electron pairs e.g. -OH, -NH2, -Cl.
  • 21. Factors affecting on Wavelength λ :- 1- Conjugation:- E.↓λ↑more conjugated compounds 2- Substitution:- E.↓λ↑more substituted compounds  Alkyl group substitution in benzene(λmax =256 nm)ring cause slight bathochromic shift e.g. toluene has λmax at 261 nm. Auxochrome e.g. –OH, -NH2, -OR … the interaction of non-bounding electrons with π electrons of benzene increase conjugation & cause bathochromic shift e.g. aniline & phenol.
  • 22. 3- Solvent:- when the UV-Vis spectrum is recorded in different solvents, The position and intensity of an absorption band may shifted. For change to solvents of increasing polarity the pattern of shifts will be as follows; 1- Conjugated dienes & aromatic hydrocarbons display very little solvent effects. 2- α,β- unsaturated carbonyl compounds show 2 different shifts; a- The π π* band moves to longer wavelength (bathochromic shift) red shift b- The n π* band moves to shorter Wavelength (hypsochromic shift) blue shift
  • 23. Calculation of λmax The first extensive set of rules was created by R. B. Woodward and used extensively in the structure elucidation of natural products. Each type of diene or triene system is having a certain fixed value at which absorption takes place; this constitutes the Base value or Parent value. The contribution made by various alkyl substituents or ring residue, double bond extending conjugation and polar groups such as – Cl, -Br etc are added to the basic value to obtain λmax for a particular compound
  • 24. CONJUGATED DIENE CORRELATIONS: 1-Homoannular Diene:- Cyclic diene having conjugated double bonds in same ring. 2-Heteroannular Diene:- Cyclic diene having conjugated double bonds in different rings. 3-Endocyclic double bond:- Double bond present in a ring.
  • 25. 4- Exocyclic double bond: - Double bond in which one of the doubly bonded atoms is a part of a ring system. Here Ring A has one exocyclic and endocyclic double bond. Ring B has only one endocyclic double bond.
  • 26. i) Base value for homoannular diene = 253 nm ii) Base value for heteroannular diene = 214 nm iii) Alkyl substituent or Ring residue attached to the parent diene = 5 nm iv) Double bond extending conjugation = 30 nm v) Exocyclic double bonds = 5 nm vi) Polar groups: 1- OAc = 0 nm 2- OAlkyl = 6 nm 3- Cl, Br = 5 nm
  • 27. e.g. Base value = 214 nm Ring residue = 3 x 5 = 15 nm Exocyclic double bond = 1 x 5 = 5 nm λmax = 214 + 15 +5 = 234 nm
  • 28. α, β UNSATURATED CARBONYL COMPOUNDS OR KETONES: 1. Base value: a) Acyclic α, β unsaturated ketones = 214 nm b) 6 membered cyclic α, β unsaturated ketones = 215 nm c) 5 membered cyclic α, β unsaturated ketones = 202 nm d) α, β unsaturated aldehydes = 210 nm e) α, β unsaturated carboxylic acids & esters = 195 nm
  • 29. 2. Alkyl substituent or Ring residue in α position = 10 nm 3. Alkyl substituent or Ring residue in β position = 12 nm 4. Alkyl substituent or Ring residue in γ and higher positions = 18 nm 5. Double bond extending conjugation = 30 nm 6. Exocyclic double bonds = 5 nm 7. Homodiene compound = 39 nm
  • 30. 8. Polar groups: a) –OH in α position = 35 nm –OH in β position = 30 nm –OH in δ position = 50 nm b) –OAc in α, β, γ, δ positions = 6 nm c) –OMe in α position = 35 nm –OMe in β position = 30 nm –OMe in γ position = 17 nm –OMe in δ position = 31 nm d) –Cl in α position = 15 nm –Cl in β position = 12 nm e) –Br in α position = 25 nm –Br in β position = 30 nm f) –NR2 in β position = 95 nm
  • 31. Base value = 215 nm Double bond extending conjugation =1 x 30 = 30 nm Exocyclic double bond = 5 nm β- Substituents = 1 x 12 = 12 nm δ- Substituents = 1 x 18 = 18 nm λmax = 280 nm α β ɤ ᵹ