UV - VISIBLE SPECTROSCOPY

Transcription

1 Course: M. Phil (Chemistry) Unit: I UV - VISIBLE SPECTROSCOPY Syllabus: Electronic transition Chromophores and Auxochromes Factors influencing position and intensity of absorption bands Effect of solvent on spectra Absorption spectra of Dienes, Polyene, Unsaturated carbonyl compounds Woodward Fieser rules MPC102 PHYSICAL METHODS IN CHEMISTRY Dr. K. SIVAKUMAR Department of Chemistry SCSVMV University chemshiva@gmail.com 1 ν Electromagnetic Waves - Terminologies Electromagnetic wave parameters: Wavelength (λ): Wavelength is the distance between the consecutive peaks or crests Wavelength is expressed in nanometers (nm) 1nm = 10-9 meters = 1/ meters 1A = meters = 1/ meters 2 1

2 ν Electromagnetic Waves - Terminologies Electromagnetic wave parameters: Frequency (ν): Frequency is the number of waves passing through any point per second. Frequency is expressed in Hertz (Hz) 3 ν Electromagnetic Waves - Terminologies Electromagnetic wave parameters: Wave number ( ν ): Wave number is the number of waves per cm. Wavelength, Wave number and Frequency are interrelated as, Where, λ is wave length 1 ν = λ = ν c ν is wave number ν is frequency c is velocity of light in vacuum. i.e., 3 x 10 8 m/s 4 2

3 Electromagnetic Spectral regions nm 10-4 to to to to to to to 10 9 EM waves γ-rays X-rays UV Visible IR Microwave Radio 5 Electromagnetic Spectrum E = hν h Planck s constant 6 3

4 The Electromagnetic wave lengths & Some examples 7 Electromagnetic radiation sources EM radiation Spectral method Radiation source Gamma rays Gamma spec. gamma-emitting nuclides X-rays X-ray spec. Synchrotron Radiation Source (SRS), Betatron (cyclotron) Ultraviolet UV spec. Hydrogen discharge lamp Visible Visible spec. tungsten filament lamp Infrared IR spec. rare-earth oxides rod Microwave ESR spec. klystron valve Radio wave NMR spec. magnet of stable field strength 8 4

5 Electromagnetic Spectrum Type of radiation and Energy change involved 9 Electromagnetic Spectrum Type of radiation and Energy change involved 10 5

6 Electromagnetic Spectrum Type of radiation and Energy change involved 11 Effect of electromagnetic radiations on chemical substances The absorption spectrum of an atom often contains sharp and clear lines. Absorption spectrum of an atom; Hydrogen Energy levels in atom; Hydrogen 12 6

7 Effect of electromagnetic radiations on chemical substances But, the absorption spectrum of a molecule is highly complicated with closely packed lines This is due to the fact that molecules have large number of energy levels and certain amount of energy is required for transition between these energy levels. Energy levels in molecule Absorption spectrum of a molecule; Eg: H 2 O 13 Effect of electromagnetic radiations on chemical substances The radiation energies absorbed by molecules may produce Rotational, Vibrational and Electronic transitions. 14 7

8 Effect of electromagnetic radiations on chemical substances Rotational transition Microwave and far IR radiations bring about changes in the rotational energies of the molecule Example: Rotating HCl molecule 15 Effect of electromagnetic radiations on chemical substances Vibrational transition Infrared radiations bring about changes in the vibration modes (stretching, contracting and bending) of covalent bonds in a molecule Examples: Example: Vibrating HCl molecule 16 8

9 Effect of electromagnetic radiations on chemical substances Electronic transition UV and Visible radiations bring about changes in the electronic transition of a molecule Example: Cl 2 in ground and excited states 17 Effect of electromagnetic radiations on chemical substances Cl 2 in Ground state 18 9

10 Effect of electromagnetic radiations on chemical substances Cl 2 in Excited state 19 The Ultraviolet region [10 800nm] The Ultraviolet region may be divided as follows, 1. Far (or Vacuum) Ultraviolet region [ nm] 2. Near (or Quartz) Ultraviolet region [ nm] 3. Visible region [ nm] 20 10

11 The Ultraviolet region Far (or Vacuum) Ultraviolet region [10 200nm] Electromagnetic spectral region from nm can be studied in evacuated system and this regions is termed as vacuum UV The atmosphere absorbs the hazardous high energy UV <200nm from sunlight Excitation (and maximum separation) of σ - electrons occurs in nm Near (or Quartz) Ultraviolet region [ nm] Electromagnetic spectral region from nm normally termed as Ultraviolet region The atmosphere is transparent in this region and quartz optics may be used to scan from nm Excitation of p and d orbital electrons, π - electrons and π - conjugation (joining together) systems occurs in nm Example for π conjugation Benzene 21 The Visible region Visible region [ nm] Electromagnetic spectral region from nm is termed as visible region The atmosphere absorbs the hazardous high energy UV <200nm from sunlight Excitation of π-conjugation occurs in visible region; nm Conjugation of double bonds lowers the energy required for the transition and absorption will move to longer wavelength (i.e., to low energy) 22 11

12 VISIBLE region in Electromagnetic Spectrum Violet : nm Indigo : nm Blue : nm Green : nm Yellow : nm Orange : nm Red : nm 23 UV - VISIBLE SPECTROSCOPY In UV - Visible Spectroscopy, the sample is irradiated with the broad spectrum of the UV - Visible radiation If a particular electronic transition matches the energy of a certain band of UV - Visible, it will be absorbed The remaining UV - Visible light passes through the sample and is observed From this residual radiation a spectrum is obtained with gaps at these discrete energies this is called an absorption spectrum 24 12

13 Lambert s law fraction of the monochromatic light absorbed by a homogeneous medium is independent of the intensity of the incident light and each successive unit layer absorbs an equal fraction of the light incident on it Lambert Beer s law fraction of the incident light absorbed is proportional to the number of the absorbing molecules in the light-path and will increase with increasing concentration or sample thickness. Beer 25 Beer Lambert law / Beer Lambert Bouguer law / Lambert Beer law log (I 0 /I) = ε c l = A Where, I 0 - the intensity of incident light I - the intensity of transmitted light ε - molar absorptivity / molar extinction coefficient in cm 2 mol -1 or L mol -1 cm -1. c - concentration in mol L -1 l - path length in cm A - absorbance (unitless) Molar absorptivity 26 13

14 Absorption intensity ε ε max Intensity of absorption is directly proportional to the transition probability A fully allowed transition will have ε max > A low transition probability will have ε max < 1000 λ max wavelength of light corresponding to maximum absorption is designated as λ max and can be read directly from the horizontal axis of the spectrum Absorbance (A) is the vertical axis of the spectrum A = log (I 0 /I) I 0 - intensity of the incident light; I - intensity of transmitted light ε max = Generalizations Regarding λ max If spectrum of compound shows, Absorption band of very low intensity (ε max = ) in the nm region, and no other absorptions above 200 nm, Then, the compound contains a simple, nonconjugated chromophore containing n electrons. The weak band is due to n π* transitions. If the spectrum of a compound exhibits many bands, some of which appear even in the visible region, the compound is likely to contain long-chain conjugated or polycyclic aromatic chromophore. If the compound is colored, there may be at least 4 to 5 conjugated chromophores and auxochromes. Exceptions: some nitro-, azo-, diazo-, and nitroso-compounds will absorb visible light

15 Generalizations Regarding ε max If ε max = 10,000-20,000; generally a simple α, β-unsaturated ketone or diene If ε max = 1,000-10,000 normally an aromatic system Substitution on the aromatic nucleus by a functional group which extends the length of the chromophore may give bands with ε max > 10,000 along with some which still have ε max < 10,000. Bands with ε max < 100 represent n π* transitions. molar absorptivities vary by orders of magnitude: values of are termed high intensity absorptions values of are termed low intensity absorptions values of 0 to 10 3 are the absorptions of forbidden transitions 29 Beer Lambert law / Beer Lambert Bouguer law / Lambert Beer law Bouguer Actually investigated the range of absorption Vs thickness of medium Lambert Extended the concepts developed by Bouguer Beer Applied Lambert s concept to solutions of different concentrations? Bernard Beer released the results of Lambert s concept just prior to those of Bernard 30 15

16 Electronic Energy Levels Absorption of UV - Visible radiation by an organic molecule leads to electronic excitation among various energy levels within the molecule. Electron transitions generally occur in between a occupied bonding or lone pair orbital and an unoccupied non-bonding or antibonding orbital. The energy difference between various energy levels, in most organic molecules, varies from 30 to 150 kcal/mole 31 σ Bonding and anti-bonding formation from s atomic orbitals (Eg: H 2 molecule) Bonding between two hydrogen atoms One molecular orbital with 2 electrons 2 atomic orbitals of 2 hydrogen atoms According to Molecular Orbital Theory One antibonding orbital without electrons and two nuclei 2 atomic orbitals of 2 hydrogen atoms One bonding orbital with 2 electrons 32 16

17 σ Bonding and anti-bonding formation from s atomic orbitals (Eg: H 2 molecule) According to Molecular Orbital Theory Higher energy than original atomic orbitals and bonding orbital - Because of repulsion 2 atomic orbitals of 2 hydrogen atoms Lower energy than original atomic orbitals Bonding orbitals are lower in energy than its original (atoms) atomic orbitals. Because, energy is released when the bonding orbital is formed, i.e., hydrogen molecule is more energetically stable than the original atoms. However, an anti-bonding orbital is less energetically stable than the original atoms. A bonding orbital is stable because of the attractions between the nuclei and the electrons. In an anti-bonding orbital there are no equivalent attractions - instead of attraction you get repulsions. There is very little chance of finding the electrons between the two nuclei - and in fact half-way between the nuclei there is zero chance of finding electrons. There is nothing to stop the two nuclei from repelling each other apart. So in the hydrogen case, both of the electrons go into the bonding orbital, because that produces the greatest stability 33 - more stable than having separate atoms, and a lot more stable than having the electrons in the anti-bonding orbital. σ Bonding and anti-bonding formation from p atomic orbitals 34 17

18 π Bonding and anti-bonding formation from p atomic orbitals 35 Electronic Energy Levels σ (anti-bonding) π (anti-bonding) Energy n (non-bonding) π (bonding) σ (bonding) σ - orbitals are the lowest energy occupied molecular orbitals σ* - orbitals are the highest energy unoccupied molecular orbitals π - orbitals are of somewhat higher energy occupied molecular orbitals π* - orbitals are lower in energy (unoccupied molecular orbitals) than σ* n - orbitals; Unshared pairs (electrons) lie at the energy of the original atomic orbital. Most often n - orbitals energy is higher than σ and π. since no bond is formed, there is no benefit in energy 36 18

19 Electronic Energy Levels Graphically, σ π Unoccupied levels Energy Atomic orbital n Atomic orbital π Occupied levels σ Molecular orbitals 37 Electronic Transitions The valence electrons in organic molecules are involved in bonding as σ - bonds, π - bonds or present in the non-bonding form (lone pair) Due to the absorption of UV - Visible radiation by an organic molecule different electronic transitions within the molecule occurs depending upon the nature of bonding. The wavelength of UV - Visible radiation causing an electronic transition depends on the energy of bonding and antibonding orbitals. The lowest energy transition is typically that of an electron in the Highest Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO) σ π Unoccupied levels Energy Atomic orbital n Atomic orbital π σ Molecular orbitals Occupied levels 38 19

20 Types of Electronic Transitions Transition between bonding molecular orbitals and anti-bonding molecular orbitals They are of three types: σ σ* π π* σ π* 39 Types of Electronic Transitions Transition between bonding molecular orbitals and anti-bonding molecular orbitals σ σ* (bonding σ to anti-bonding σ) σ σ* transition requires large energies in far UV region in nm range. Molar absorptivity: Low ε max = σ (anti-bonding) π (anti-bonding) n (non-bonding) π (bonding) Examples: Alkanes - 150nm σ (bonding) Methane Cyclohexane Propane 40 20

21 Types of Electronic Transitions Transition between bonding molecular orbitals and anti-bonding molecular orbitals σ σ* (bonding σ to anti-bonding σ) + C _ + C _ σ* C-C _ C + C _ σ C-C 41 Types of Electronic Transitions Transition between bonding molecular orbitals and anti-bonding molecular orbitals π π* (bonding π to anti-bonding π) π π* occur in nm range. Molar absorptivity: High ε max = Examples: Unsaturated compounds double or triple bonds aromatic rings Carbonyl groups azo groups Conjugated π electrons Carbonyl Azo σ (anti-bonding) π (anti-bonding) n (non-bonding) π (bonding) σ (bonding) ε max is high because the π and π* orbitals are in same plane and consequently the probability of jump of an electron from π π* orbital is very high

22 Types of Electronic Transitions Transition between bonding molecular orbitals and anti-bonding molecular orbitals π π* (bonding π to anti-bonding π) _ + C C + _ π* C + _ C π 43 Types of Electronic Transitions Transition between bonding molecular orbitals and anti-bonding molecular orbitals σ π* (bonding σ to anti-bonding π) σ π* occur only in <150 nm range. Molar absorptivity: Low Examples: Carbonyl compounds σ (anti-bonding) π (anti-bonding) n (non-bonding) π (bonding) σ (bonding) σ σ* and σ π* transitions: high-energy, accessible in vacuum UV (λmax <150 nm). Not usually observed in molecular UV-Vis

23 Types of Electronic Transitions Transition between non-bonding atomic orbitals and anti-bonding molecular orbitals They are of two types: n π* n σ* σ (anti-bonding) π (anti-bonding) n π* (non-bonding n to anti-bonding π) n π* occur in nm range. Molar absorptivity: Low ε max = n (non-bonding) π (bonding) σ (bonding) Examples: Compounds with double bonds involving unshared pair(s) of electrons Aldehydes, Ketones C=O, C=S, N=O etc., 45 Types of Electronic Transitions Transition between non-bonding atomic orbitals and anti-bonding molecular orbitals n π* (non-bonding n to anti-bonding π) _ + C O + _ π* C _ O + n(p y ) C + _ C π 46 23

24 Types of Electronic Transitions Spectra of aldehydes or ketones exhibit two bands; A High intense band at nm due to π π* A low intense band at 300nm due to n π* transition n π* transition is always less intense because. The electrons in the n-orbitals are situated perpendicular to the plane of π bond and hence to the plane of π* orbital. n to π Consequently, the probability of jump of an electron from n π* orbital is very low and in fact zero according to symmetry selection rules. But, vibrations of atoms bring about a partial overlap between the perpendicular planes and so n π* transition does occur, but only to a limited extent. 47 Types of Electronic Transitions Transition between non-bonding atomic orbitals and anti-bonding molecular orbitals n σ* (non-bonding n to anti-bonding σ) Excitation of an electron in an unshared pair on Nitrogen, oxygen, sulphur or halogens to an antibonding σ orbital is called n σ* transitions. n σ* occur in nm range. Molar absorptivity: Low εmax = Example: Methanol λmax = 183nm (ε = 500) 1-Iodobutane λmax = 257nm (ε = 486) Trimethylamine λmax = 227nm (ε = 900) σ (anti-bonding) π (anti-bonding) n (non-bonding) π (bonding) σ (bonding) 48 24

25 Types of Electronic Transitions Transition between non-bonding atomic orbitals and anti-bonding molecular orbitals n σ* (non-bonding n to anti-bonding σ) + C _ + N _ σ* C-N C _ N + n _ C + N _ σ C-N 49 Types of Electronic Transitions σ σ* (bonding σ to anti-bonding σ) π π* (bonding π to anti-bonding π) σ π* (bonding σ to anti-bonding π) n π* (non-bonding n to anti-bonding π) n σ* (non-bonding n to anti-bonding σ) σ (anti-bonding) π (anti-bonding) n (non-bonding) π (bonding) σ (bonding) Energy required for various transitions obey the order: σ σ* > n σ* > π π*> n π* 50 25

26 Types of Electronic Transitions From the molecular orbital diagram it is clear that, In all compounds other than alkanes there are several possible electronic transitions that can occur with different energies. σ π σ σ σ π alkanes 150 nm carbonyls 170 nm Energy n π n π σ unsaturated compounds 180 nm O, N, S, halogens 190 nm If conjugated π n π carbonyls 300 nm σ 51 Selection Rules Not all transitions that are possible in UV region are not generally observed. For an electron to transition, certain quantum mechanical constraints apply these are called selection rules. The selection rules are, Rule - 1:The transitions which involve an change in the spin quantum number of an electron during the transition are not allowed to take place or these are forbidden. Rule - 2: singlet triplet transitions are forbidden Multiplicity of states (2S+1); Where, S is total spin quantum number. Singlet state: have electron spin paired Triplet state: have two spins parallel Here, For excited singlet state: S=0; therefore, 2S+1=1 - transition allowed For excited triplet state: S=1; therefore, 2S+1=3 - transition forbidden 52 26

27 Selection Rules Rule - 3: Symmetry of electronic states; n π* transition in formaldehyde is forbidden by local symmetry. i.e., Energy is always a function of molecular geometry. In formaldehyde (H 2 C=O), In n π* excited state an electron arrives at the antibonding π orbital, while the electron pair in the bonding π orbital is still present. Due to the third antibonding π electron, the C=O bond becomes weaker and longer. In the π π* excited configuration, the situation is somewhat worse because there is only one π electron in the bonding orbital, while the other π electron is anti-bonding (i.e. π*). Consequently, the excited state bond lengths will be longer than a genuine C=O double bond but shorter than a σ -type single C-O bond. In other words, these excited states will have their energy minima somewhere in between that of H 2 C=O and H 3 C-OH. To further complicate matters, forbidden transitions are sometimes observed (albeit at low intensity) due to other factors. 53 Franck and Condon Principle Electronic transitions will take place only when the inter-nuclear distances are not significantly different in the two states and where the nuclei have little or no velocity. Thus, the forbidden transitions may arise when the inter-nuclear distances are significantly different in the two states and where the nuclei have significant velocity. Franck Condon principle is the approximation that an electronic transition is most likely to occur without changes in the positions of the nuclei in the molecular entity and its environment

28 Origin and General appearance of UV bands Electronic spectra is a graphical output of transitions between electronic energy levels. We know that, electronic transitions are accompanied by changes in both vibrational and rotational states. The wavelength of absorption depends on the energy difference between bonding/antibonding and non-bonding orbitals concerned. When gaseous sample is irradiated with UV - Visible light and the spectrum is recorded, a spectrum with number of closely spaced fine structure line is obtained. When the electronic spectrum of a solution is recorded, a absorption band is obtained in which closely spaced fine lines are merging together due to the solvent-solute interaction. Usually electronic absorption spectrums are broader bands than IR or NMR bands. 55 Designation of UV bands The absorption bands in the UV - Visible spectrum may be designated either by using electronic transitions [σ σ*, σ π*, π π*, n π*, n σ*] or the letter designation as given below. R bands (German, radikalartig) The bands due to n π* transitions of single chromophoric groups are referred to as the R - Bands. Example: Carbonyl group, Nitro group Shows low molar absorptivity (ε max <100) and hypsochromic shift with an increase in solvent polarity. K bands (German, konjugierte) The bands due to π π* transitions in molecules containing conjugated π systems are referred to as the K Bands. Example: Butadiene, mesityl oxide They show high molar absorptivity (ε max <10,000)

29 Designation of UV bands B and E - bands The B and E bands are characteristic of the spectra of aromatic or heteroaromatic molecules. Examples: All benzenoid compounds exhibit E and B bands representing π π* transitions. In benzene, E 1 and E 2 bands occur near 180nm and 200nm respectively and their molar absorptivity varies between (ε max = 2000 to ε max = 14000). The B-band occurs in the region from 250nm to 255nm as a broad band containing multiple fine structure and represents a symmetry-forbidden transition which has finite but low probability due to forbidden transitions in high symmetrical benzene molecule. The vibrational fine structure appears only in the B-band and disappears frequently in the more polar solvents. 57 Chromophores The coloured substances owe their colour to the presence of one or more unsaturated groups responsible for electronic absorption. These groups are called chromophores. Examples: C = C, C=C, C = N, C=N, C = O, N = N, etc.. Chromophores absorb intensely at the short wavelength But, some of them (e.g, carbonyl) have less intense bands at higher wavelength due to the participation of n electrons. Methyl orange 58 29

30 Chromophores: examples Chromophore Example Excitation λ max, nm ε Solvent C=C Ethene Π > Π* ,000 hexane C C 1-Hexyne Π > Π* ,000 hexane C=O Ethanal n > Π* Π > Π* ,000 hexane hexane N=O Nitromethane n > Π* Π > Π* ,000 ethanol ethanol C-X; X=Br X=I Methyl bromide Methyl Iodide n > σ* n > σ* hexane hexane 59 Auxochromes An auxochromes is an auxillary group which interact with chromophore and deepens colour; its presence causes a shift in the UV or visible absorption maximum to a longer wavelength Examples: NH 2, NHR and NR 2, hydroxy and alkoxy groups. Property of an auxochromic group: Provides additional opportunity for charge delocalization and thus provides smaller energy increments for transition to excited states. The auxochromic groups have atleast one pair of non-bonding electrons (lone pair) that can interact with the π electrons and stabilizes the π * state 60 30

31 Auxochromes: examples Auxochrome Unsubstitued chromophore λ max (nm) Substituted chromophore λ max (nm) -CH 3 H 2 C=CH-CH = CH H 2 C=CH-CH=CHCH OR H 3 C-CH=CH-COOH 204 H 3 C-C(OCH 3 ) = CHCOOH 234 -C1 H 2 C=CH H 2 C = CHCl Bathochromic shift (Red shift) - λ max to longer wavelength Shift of an absorption maximum to longer wavelength is called bathochromic shift. Occurs due to change of medium (π π* transitions undergo bathochromic shift with an increase in the polarity of the solvent) OR when an auxochrome is attached to a carbon-carbon double bond Example: Ethylene 1-butene / isobutene : λ max = 175nm : λ max = 188 nm The bathochromic shift is progressive as the number of alkyl groups increases

32 Hypsochromic shift (Blue shift) - λ max to Shorter wavelength Shift of absorption maximum to shorter wavelength is known as hypsochromic shift. Occurs due to change of medium (n π* transitions undergo hypsochromic shift with an increase in the polarity of solvent) OR when an auxochrome is attached to double bonds where n electrons (eg: C=O) are available Example: Acetone λmax = 279nm in hexane λmax = 264.5nm in water This blue shift results from hydrogen bonding which lowers the energy of the n orbital. 63 Hyperchromic effect - increased (ε max ) absorption intensity It is the effect leading to increased absorption intensity Example: intensities of primary and secondary bands of phenol are increased in phenolate Compound Primary band Secondary band λ max (nm) ε max λ max (nm) ε max Phenol C 6 H 5 OH Phenolate anion C 6 H 5 O

33 Hypochromic effect - decreased (ε max ) absorption intensity It is the effect leading to decreased absorption intensity Example: intensities of primary and secondary bands of benzoic acid are decreased in benzoate Compound Primary band Secondary band λ max (nm) ε max λ max (nm) ε max Benzoic acid C 6 H 5 COOH Benzoate C 6 H 5 COO Effect of substituents on λ max and ε max Graphically, Shift to increased ε max Shift to shorter λ max Shift to Longer λ max Shift to decreased ε max 66 33

34 Isosbestic point A point common to all curves produced in the spectra of a compound taken at various ph values is called isosbestic point. If one absorbing species, X, is converted to another absorbing species, Y, in a chemical reaction, then the characteristic behaviour shown in the figure below is observed. If the spectra of pure X and pure Y cross each other at any wavelength, then every spectrum recorded during this chemical reaction will cross at the same point, called an isosbestic point. The observation of an isosbestic point during a chemical reaction is good evidence that only two principal species are present. Example: Absorption spectrum of M methyl red as a function of ph between ph 4.5 and 7.1 The aniline-anilinium or phenol-phenolate conversion as a function of ph can demonstrate the presence of the two species in equilibrium by the appearance of an isosbestic point in the UV spectrum. 67 UV Spectroscopy (Electronic Spectra) - Terminologies Beer-Lambert Law Absorbance Molar absorptivity Extinction coefficicent concentration Path length λ max ε max Band HOMO LUMO Chromophore Auxochrome Bathochromic shift Hypsochromic shift Hyperchromic effect Hypochromic effect Isosbestic point A = ε.c.l A, a measure of the amount of radiation that is absorbed ε, absorbance of a sample of molar concentration in 1 cm cell. An alternative term for the molar absorptivity. c, concentration in moles / litre l, the length of the sample cell in cm. The wavelength at maximum absorbance The molar absorbance at λ max Term to describe a uv-vis absorption which are typically broad. Highest Occupied Molecular Orbital Lowest Unoccupied Molecular Orbital Structural unit responsible for the absorption. A group which extends the conjugation of a chromophore by sharing of nonbonding electrons The shift of absorption to a longer wavelength. shift of absorption to a shorter wavelength An increase in absorption intensity A decrease in absorption intensity point common to all curves produced in the spectra of a compound taken at various ph 68 34

35 Instrumentation log(i 0 /I) = A UV-VIS sources monochromator/ beam splitter optics I 0 sample I 0 I 2 reference I 1 detector I λ, nm 69 Instrumentation Radiation source, monochromator and detector Two sources are required to scan the entire UV-VIS band: Deuterium lamp covers the UV Tungsten lamp covers The lamps illuminate the entire band of UV or visible light; the monochromator (grating or prism) gradually changes the small bands of radiation sent to the beam splitter The beam splitter sends a separate band to a cell containing the sample solution and a reference solution The detector (Photomultiplier, photoelectric cells) measures the difference between the transmitted light through the sample (I) vs. the incident light (I 0 ) and sends this information to the recorder 70 35

36 Sample Handling Virtually all UV spectra are recorded solution-phase Only quartz is transparent in the full nm range; plastic and glass are only suitable for visible spectra nm Concentration: 0.1 to 100mg 10-5 to 10-2 molar concentration may safely be used Percentage of light transmitted: 20% to 65% At high concentrations, amount of light transmitted is low, increasing the possibility of error A typical sample cell (commonly called a cuvet): Cells can be made of plastic, glass or quartz (standard cells are typically 1 cm in path length) 71 Solvents Solvents must be transparent in the region to be observed solvents must preserve the fine structure solvents should dissolve the compound Non-polar solvent does not form H-bond with the solute (and the spectrum is similar to the spectrum of compound at gaseous state) Polar solvent forms H-bonding leading to solute-solvent complex and the fine structure may disappear. The wavelength from where a solvent is no longer transparent is termed as cutoff Common solvents and cutoffs: nm acetonitrile 190 chloroform 240 cyclohexane 195 1,4-dioxane % ethanol 205 n-hexane 201 methanol 205 isooctane 195 water

37 Factors affecting the position of UV bands 1. Non-conjugated alkenes A π π* transition can occur in simple non-conjugated alkene like ethene and other alkenes with isolated double bonds below 200 nm. π π 73 Factors affecting the position of UV bands 1. Non-conjugated alkenes Alkyl substitution of parent alkene moves the absorption to longer wavelengths. From λ max di-, tri & tetra substituted double bonds in systems can be identified acyclic and alicyclic 74 37

38 Factors affecting the position of UV bands 1. Non-conjugated alkenes This bathochromic effect of alkyl substitution is due to the extension of the chromophore, in the sense that there is a small interaction, due to hyperconjugation, between the σ electrons of the alkyl group and the chromophoric group. C C H C H H Methyl groups also cause a bathochromic shift, even though they are devoid of p-or n-electrons HYPERCONJUGATION This effect is thought to be through what is termed HYPERCONJUGATION or sigma bond resonance This effect is progressive as the number of alkyl groups increases. The intensity of alkene absorption is essentially independent of solvent because of the non-polar nature of the alkene bond. 75 Factors affecting the position of UV bands 2. Conjugated Dienes A conjugated system requires lower energy for the π π* transition than an unconjugated system. Example: Ethylene and Butadiene In conjugated butadiene (λ max =217nm; ε max = 21000) π and π* orbitals have energies much closer together than those in ethylene, resulting in a lower excitation energy Ethylene has only two orbitals; one ground state π bonding orbital and one excited state π* antibonding orbital. The energy difference ( Ε) between them is about 176 kcal/mole

39 Factors affecting the position of UV bands [i.e., From MOT, two atomic p orbitals, from two sp2 hybrid carbons combine to form two MOs π and π* in ethylene,] π 2 p p π 1 π 77 Factors affecting the position of UV bands - 2. Conjugated Dienes In butadiene, 4 p orbitals are mixing and 4 MOs of an energetically symmetrical distribution compared to ethylene. Therefore, the following π and π* for ethylene and butadiene will be obtained. π 2 π 4 π 3 π 1 π π 2 π 1 Ethylene Butadiene 78 39

40 Factors affecting the position of UV bands - 2. Conjugated Dienes Butadiene, however, with four π electrons has four available π orbitals, two bonding (π 1 and π 2 ) and two antibonding (π* 3 and π* 4 ) orbitals. The π 1 bonding orbital encompasses all the four π electrons over the four carbon atoms of the butadiene system and is somewhat more stable than a single π bonding orbital in ethylene. The π 2 orbital is also bonding orbital, but is of higher energy than the π 1 orbital. The two π* orbitals (π* 3 and π* 4 ) are respectively, more stable ((π* 3 ) and less stable (π* 4 ) than the π* orbital of ethylene. Energy absorption, with the appearance of an absorption band, can thus occur by a π 2 (bonding) (π* 3 (antibonding transition. HOMO to LUMO), the energy difference of which (136 kcal/mole) is less than that of the simple π π* transition of ethylene (176 kcal/mole) giving a λ max = 217 nm; (i.e., at a longer wavelength). It is to be expected that the greater the number of bonding π orbitals, the lower will be the energy between the highest bonding π orbital and the lowest excited π* orbital. The obvious extension of this in terms of λ max is that the greater the number of conjugated double bonds, the longer the wavelength of absorption. 79 Factors affecting the position of UV bands - 2. Conjugated Dienes π 2 π 4 π 3 Ε = 176 kcal/mole π 1 π 136 kcal/mole π 2 π 1 E for the HOMO LUMO transition is REDUCED 80 40

41 Factors affecting the position of UV bands - 2. Conjugated Dienes Extending this effect out to longer conjugated systems the energy gap becomes progressively smaller: For example Energy Lower energy = Longer wavelengths ethylene butadiene hexatriene octatetraene 81 Factors affecting the position of UV bands - 2. Conjugated Dienes - Types Acyclic dienes: 1,3-Butadiene with the structural formula Homo-annular conjugated dienes: Both conjugated double bonds are in same ring Hetero-annular dienes: Conjugated double bonds are not present in same ring 82 41

42 Factors affecting the position of UV bands - 2. Conjugated Dienes - Types Exocyclic and Endocyclic double bond: Endocyclic double bond Exocyclic double bond 83 Factors affecting the position of UV bands - 2. Conjugated Dienes - Types 1. Acyclic diene or Heteroannular diene Most acyclic dienes have transoid conformation; i.e. trans disposition of double bonds about a single bond. Base λ max =217 nm (ε max = ). s-trans Base λ max =217 nm ε max = Heteroannular diene, is a conjugated system in which the two double bonds are confined to two different rings. A B Base λ max = 214 nm (ε max = ). Base λ max =214 nm ε max =

43 Factors affecting the position of UV bands - 2. Conjugated Dienes - Types 2. Homoannular diene In homoannular diene, the two conjugated double bonds are confined to a single ring. i.e., the cyclic dienes are forced into an s-cis (cisoid) conformation. Base λ max = 253 nm (ε max = ). Base λ max =253 nm ε max = Homoannular dienes contained in other ring sizes possess different base absorption values. s-cis Example: Cyclopentadiene; λ max =228nm Cycloheptadiene; λ max = 241nm 85 Factors affecting the position of UV bands - 2. Conjugated Dienes When two or more C=C units are conjugated, The energy difference E between the highest bonding π orbital (HOMO) and the lowest excited π* orbital (LUMO) becomes small and results in a shift of λ max to longer wavelength i.e., Bathochromic shift. This concept helps to distinguish between the two isomeric diens, 1,5-hexadiene and 2, 4- hexadiene, from the relative positions of λmax. H 2 C=CH-CH 2 -CH 2 -CH=CH 2 CH 3 -CH=CH-CH=CH-CH 3 1,5-Hexadiene 2,4-Hexadiene (non-conjugated diene) (conjugated diene) λ max = 178 nm λ max = 227 nm 86 43

44 Factors affecting the position of UV bands - 2. Conjugated Dienes 87 Factors affecting the position of UV bands - 2. Conjugated Dienes - Types As the number of double bonds in conjugation increases, E for the excitation of an electron continues to become small and consequently there will be a continuous increase in the value of λ max Example: λ max = nm Longer wavelengths = Lower energy 88 44

45 Factors affecting the position of UV bands - 2. Conjugation with hetero atoms Conjugation with a heteroatom [N, O, S, X] moves the (π π*) absorption of ethylene to longer wavelengths Example: CH 2 =CH-OCH 3 (λmax=190nm) - ε max ~10000 CH 2 =CH-NMe 2 (λ max =230nm) - ε max ~10000 Methyl vinyl sulphide absorbs at 228 nm (ε max =8000) Here we create 3 MOs this interaction is not as strong as that of a conjugated π-system π Ψ 2 Ψ 3 A Energy π n A Ψ 1 89 Factors affecting the position of UV bands 3. Effect of Geometrical isomerism - Steric effect In compounds where geometrical isomerism is possible. Example: trans - stilbene absorbs at longer wavelength [λ max =295 nm] (low energy) cis - stilbene absorbs at shorter wavelength [λ max =280 nm] (high energy) due to the steric effects. Coplanarity is needed for the most effective overlap of the π - orbitals and increased ease of the π π* transition. The cis-stilbene is forced into a nonplanar conformation due to steric effects

46 Factors affecting the position of UV bands 4. Effect of steric hindrance on coplanarity (steric inhibition of resonance) UV spectroscopy is very sensitive to distortion of the chromophore and consequently the steric repulsions which oppose the coplanarity of conjugated π-electron systems can easily be detected by comparing its UV spectrum with that of a model compound. Distortion of the chromophore may lead to RED or BLUE shifts depending upon the nature of the distortion. Example-1: Distortion leading to RED shift The strained molecule Verbenene exhibits λ max =245.5nm whereas the usual calculation shows at λ max =229 nm. Verbenene Actual; λ max =245.5nm Calculated; λ max =229nm 91 Factors affecting the position of UV bands 4. Effect of steric hindrance on coplanarity (steric inhibition of resonance). Example-2: Distortion leading to BLUE shift The diene shown here might be expected to have a maximum at 273nm. But, distortion of the chromophore, presumably out of planarity with consequent loss of conjugation, causes the maximum to be as low as 220nm with a similar loss in intensity (ε max =5500). Actual; Calculated; λ max =220nm λ max =273nm 92 46

47 Factors affecting the position of UV bands 4. Effect of steric hindrance on coplanarity (steric inhibition of resonance).. Example-3: trans-azobenzene and the sterically restricted cis-azobenzene H H Absorption of Azobenzene (in ethanol) Example π π * transition n π * transition λ max ε max λ max ε max trans-isomer cis-isomer Such differences between cis and trans isomers are of some diagnostic value 93 Factors affecting the position of UV bands 5. Effect of Solvents The position and intensity of an absorption band is greatly affected by the polarity of the solvent used for running the spectrum. Such solvent shifts are due to the differences in the relative capabilities of the solvents to solvate the ground and excited states of a molecule. Non-polar compounds like Conjugated dienes and aromatic hydrocarbons exhibit very little solvent shift, 94 47

48 Factors affecting the position of UV bands 5. Effect of Solvents The following pattern of shifts are generally observed for changes to solvents of increased polarity: α, β-un saturated carbonyl compounds display two different types of shifts. (i) n π* Band moves to shorter wavelength (blue shift). (ii) π π* Band moves to longer wavelength (red shift) 95 Factors affecting the position of UV bands 5. Effect of Solvents α, β-un saturated carbonyl compounds - For increased solvent polarity n π* Band moves to shorter wavelength (blue shift). In n π * transition the ground state is more polar than excited state. The hydrogen bonding with solvent molecules takes place to a lesser extent with the carbonyl group in the excited state. Example: λ max = 279nm in hexane λ max = 264nm in water π* B D AB < CD n A Non-polar solvent C Polar solvent Shorter wavelength 96 48

49 Factors affecting the position of UV bands 5. Effect of Solvents α, β-un saturated carbonyl compounds - For increased solvent polarity (ii) π π* Band moves to longer wavelength (Red shift). In π π * the dipole interactions with the solvent molecules lower the energy of the excited state more than that of the ground state. Thus, the value of λ max in ethanol will be greater than that observed in hexane. i.e., π* orbitals are more stabilized by hydrogen bonding with polar solvents like water and alcohol. Thus small energy will be required for such a transition and absorption shows a red shift. Example: π* B D AB > CD π A Non-polar solvent C Polar solvent Longer wavelength 97 Factors affecting the position of UV bands 5. Effect of Solvents α, β-un saturated carbonyl compounds - For increased solvent polarity (iii) In general, a) If the group (carbonyl) is more polar in the ground state than in the excited state, then increasing polarity of the solvent stabilizes the non-bonding electron in the ground state due to hydrogen bonding. Thus, absorption is shifted to shorter wave length. b) If the group (carbonyl) is more polar in the excited state, the absorption is shifted to longer wavelength with increase in polarity of the solvent which helps in stabilizing the non-bonding electrons in the excited state

50 Factors affecting the position of UV bands 6. Conformation and geometry in polyene systems The position of absorption depends upon the length of the conjugated system. Longer the conjugated system, higher will be the absorption maximum and larger will be the value of the extinction coefficient. If in a structure, the π electron system is prevented from achieving coplanarity, In long-chain conjugated polyenes, steric hindrance to coplanarity can arise when cis-bonds are present. This is illustrated by the naturally occurring bixin (`all trans methyl carotenoid) and its isomer with a central cis-double bonds. In the latter the long wavelength band is weakened and a diagnostically useful `cis-band` probably due to partial chromophore, appears at shorter wavelength. 99 Absorption spectra of Unsaturated carbonyl compounds. Enones unsaturated systems incorporating N or O can undergo n π* transitions in addition to π π* π π* transitions; λmax~188 nm; ε max = 900 n π* transitions; λ max ~285 nm; ε max = 15 Low intensity is due to the fact this transition is forbidden by the selection rules it is the most often observed and studied transition for carbonyls Similar to alkenes and alkynes, non-substituted carbonyls undergo the π π* transition in the vacuum UV (λ max =188 nm; ε max =900) Both this transitions are also sensitive to substituents on the carbonyl

51 Absorption spectra of Unsaturated carbonyl compounds. Enones π n π Remember, the π π* transition is allowed and gives a high ε, but lies outside the routine range of UV observation The n π* transition is forbidden and gives a very low e, but can routinely be observed 101 Absorption spectra of Unsaturated carbonyl compounds. Enones Carbonyls n π* transitions (~285 nm); π π* (188 nm) π O n C O It has been determined from spectral studies, that carbonyl oxygen more approximates sp rather than sp 2! π O σ CO transitions omitted for clarity

52 Absorption spectra of Unsaturated carbonyl compounds. Enones For auxochromic substitution on the carbonyl, pronounced hypsochromic (blue) shifts are observed for the n π* transition (λ max ): O H O CH 3 O Cl O NH nm 279 nm 235 nm 214 nm Blue shift λ max This is explained by the inductive withdrawal of electrons by O, N or halogen from the carbonyl carbon this causes the n-electrons on the carbonyl oxygen to be held more firmly It is important to note this is different from the auxochromic effect on π π* which extends conjugation and causes a bathochromic shift O O O OH 204 nm 204 nm In most cases, this bathochromic shift is not enough to bring the π π* transition into the observed range 103 Absorption spectra of Unsaturated carbonyl compounds. Enones Conversely, if the C=O system is conjugated both the n π* and π π* bands are Bathochromically (Red) shifted Here, several effects must be noted: the effect is more pronounced for π π* if the conjugated chain is long enough, the much higher intensity π π* band will overlap and drown out the n π* band the shift of the n π* transition is not as predictable For these reasons, empirical Woodward-Fieser rules for conjugated enones are for the higher intensity, allowed π π* transition

53 Absorption spectra of Unsaturated carbonyl compounds. Enones Conjugation effects are apparent; from the MO diagram for a conjugated enone: Ψ 4 π Ψ 3 π n n π Ψ 2 Ψ 1 O π O 105 Absorption spectra of Alkanes - Miscellaneous Alkanes only posses σ-bonds and no lone pairs of electrons, so only the high energy σ σ* transition is observed in the far UV This transition is destructive to the molecule, causing cleavage of the σ-bond σ C C σ C C

54 Absorption spectra of Aliphatic compounds - Miscellaneous Alcohols, ethers, amines and sulfur compounds in the cases of simple, aliphatic examples of these compounds the n σ* is the most often observed transition; like the alkane σ σ* it is most often at shorter λ than 200 nm Note how this transition occurs from the HOMO to the LUMO σ CN C N C N n N sp 3 C N anitbonding orbital σ CN C N 107 Woodward Fieser rules It is used for calculating λ max Calculated λ max differs from observed values by 5-6%. Effect of substituent groups can be reliably quantified by use Woodward Fieser Rule Separate values for conjugated dienes and trines and α- β-unsaturated ketnones are available Robert B. Woodward Nobel Prize in Chemistry :

55 Woodward Fieser rules Woodward-Fieser Rules Woodward and the Fiesers performed extensive studies of terpene and steroidal alkenes and noted similar substituents and structural features would predictably lead to an empirical prediction of the wavelength for the lowest energy π π* electronic transition This work was distilled by Scott in 1964 into an extensive treatise on the Woodward-Fieser rules in combination with comprehensive tables and examples (A.I. Scott, Interpretation of the Ultraviolet Spectra of Natural Products, Pergamon, NY, 1964) A more modern interpretation was compiled by Rao in 1975 (C.N.R. Rao, Ultraviolet and Visible Spectroscopy, 3 rd Ed., Butterworths, London, 1975) 109 Woodward Fieser rules for Dienes The rules begin with a base value for λ max of the chromophore being observed: For acyclic butadiene = 217 nm or 214 nm The incremental contribution of substituents is added to this base value from the group tables: Group Increment Extended conjugation +30 Each exo-cyclic C=C +5 Alkyl +5 -OCOCH OR +6 -SR +30 -Cl, -Br +5 -NR

56 Woodward Fieser rules for Dienes Examples -1 & 2 Isoprene - acyclic butadiene = one alkyl subs. Calculated value Observed value 217 nm + 5 nm 222 nm 220 nm Allylidenecyclohexane - acyclic butadiene = 217 nm one exocyclic C=C + 5 nm 2 alkyl subs. +10 nm Calculated value 232 nm Observed value 237 nm 111 Woodward Fieser rules for Dienes Problem - 1 acyclic butadiene = 217 nm Group Increment Extended conjugation +30 Each exo-cyclic C=C +5 Alkyl +5 -OCOCH OR +6 -SR +30 -Cl, -Br +5 -NR Solution: acyclic butadiene extended conjugation Calculated value = 217 nm = +30 nm = 247 nm

57 Woodward Fieser rules for Dienes Example Woodward Fieser rules for Cyclic Dienes Heteroannular (transoid) Homoannular (cisoid) Base λ max = 214 Base λ max = 253 The increment table is the same as for acyclic butadienes with a couple additions: Group Increment Additional homoannular +39 Where both types of diene are present, the one with the longer λ becomes the base Group Increment Extended conjugation +30 Each exo-cyclic C=C +5 Alkyl +5 -OCOCH OR +6 -SR +30 -Cl, -Br +5 -NR

58 Woodward Fieser rules for Cyclic Dienes Example-4 1,2,3,7,8,8a-hexahydro-8a-methylnaphthalene Heteroannular diene = 214 nm 3 alkyl subs. (3 x 5) = +15 nm 1 exo C=C = + 5 nm Calculated value 234 nm Observed value 235 nm 115 Woodward Fieser rules for Dienes Problem - 2 Heteroannular diene = 214 nm Group Increment Extended conjugation +30 Each exo-cyclic C=C +5 Alkyl +5 -OCOCH OR +6 -SR +30 -Cl, -Br +5 -NR Solution: Heteroannular diene Ring residues / = 214 nm Alkyl substitution 3 x 5 = + 15 nm Exocyclic C=C bond 1 x 5 Calculated value Observed value = + 5 nm = 234 nm = 247 nm

59 Woodward Fieser rules for Cyclic Dienes Example Woodward Fieser rules for Cyclic Dienes Example-6 abietic acid C O OH heteroannular diene = 214 nm 4 alkyl subs. (4 x 5) +20 nm 1 exo C=C + 5 nm 239 nm

60 Woodward Fieser rules for Cyclic Dienes Example-7 levopimaric acid homoannular diene = 253 nm C O OH 4 alkyl subs. (4 x 5) +20 nm 1 exo C=C + 5 nm 278 nm 119 Woodward Fieser rules for Dienes Problem - 3 Homoannular diene = 253 nm Group Additional homoannular Increment +39 Extended conjugation +30 Each exo-cyclic C=C +5 Alkyl +5 -OCOCH OR +6 -SR +30 -Cl, -Br +5 -NR Solution: Homoannular diene = 253 nm Extended conjugation 1 x 30 = +30 nm Alkyl substitution 2 x 5 = + 10 nm Calculated value = 293 nm

61 Woodward Fieser rules for Cyclic Dienes Example Woodward Fieser rules for Dienes Examples 9,10 &

62 Woodward Fieser rules for Cyclic Dienes PRECAUTIONS Be careful with your assignments three common errors: R This compound has three exocyclic double bonds; the indicated bond is exocyclic to two rings This is not a heteroannular diene; you would use the base value for an acyclic diene Likewise, this is not a homooannular diene; you would use the base value for an acyclic diene 123 Woodward Fieser rules for Enones β β α C C C O δ δ C γ C β α C C C O Group 6-membered ring or acyclic enone 5-membered ring parent enone Acyclic dienone Increment Base 215 nm Base 202 nm Base 245 nm Double bond extending conjugation 30 Alkyl group or ring residue α, β, γ and higher 10, 12, 18 -OH α, β, γ and higher 35, 30, 18 -OR α, β, γ, δ 35, 30, 17, 31 -O(C=O)R α, β, δ 6 -Cl α, β 15, 12 -Br α, β 25, 30 -NR 2 β 95 Exocyclic double bond 5 Homocyclic diene component

63 Woodward Fieser rules for Enones Aldehydes, esters and carboxylic acids have different base values than ketones Unsaturated system Base Value Aldehyde 208 With α or β alkyl groups 220 With α,β or β,β alkyl groups 230 With α,β,β alkyl groups 242 Acid or ester With α or β alkyl groups 208 With α,β or β,β alkyl groups 217 Group value exocyclic α,β double bond +5 Group value endocyclic α,β bond in 5 or 7 membered ring Woodward Fieser rules for Enones Unlike conjugated alkenes, solvent does have an effect on λ max These effects are also described by the Woodward-Fieser rules Solvent correction Increment Water +8 Ethanol, methanol 0 Chloroform -1 Dioxane -5 Ether -7 Hydrocarbon