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1 MPC102 – PHYSICAL METHODS IN CHEMISTRY Course: M. Phil (Chemistry) Unit: I UV - VISIBLE SPECTROSCOPY Syllabus: Electronic transition Chromophores and.

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Presentation on theme: "1 MPC102 – PHYSICAL METHODS IN CHEMISTRY Course: M. Phil (Chemistry) Unit: I UV - VISIBLE SPECTROSCOPY Syllabus: Electronic transition Chromophores and."— Presentation transcript:

1 1 MPC102 – PHYSICAL METHODS IN CHEMISTRY 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 Dr. K. SIVAKUMAR Department of Chemistry SCSVMV University

2 22 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 = meters = 1/ meters 1A = meters = 1/ meters

3 33 Electromagnetic Waves - Terminologies Electromagnetic wave parameters: Frequency ( ): Frequency is the number of waves passing through any point per second. Frequency is expressed in Hertz (Hz)

4 44 Electromagnetic Waves - Terminologies Electromagnetic wave parameters: Wave number ( ): Wave number is the number of waves per cm. Where, is wave length is wave number is frequency c is velocity of light in vacuum. i.e., 3 x 10 8 m/s Wavelength, Wave number and Frequency are interrelated as,

5 55 UVX-rays IR -rays RadioMicrowave Visible nm EM waves to to to to to to to 10 9 Electromagnetic Spectral regions

6 66 Electromagnetic Spectrum E = h h – Plancks constant

7 77 The Electromagnetic wave lengths & Some examples

8 88 Electromagnetic radiation sources EM radiationSpectral methodRadiation source Gamma raysGamma spec.gamma-emitting nuclides X-raysX-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

9 99 Electromagnetic Spectrum – Type of radiation and Energy change involved

10 10 Electromagnetic Spectrum – Type of radiation and Energy change involved

11 11 Electromagnetic Spectrum – Type of radiation and Energy change involved

12 12 Effect of electromagnetic radiations on chemical substances The absorption spectrum of an atom often contains sharp and clear lines. Energy levels in atom; Hydrogen Absorption spectrum of an atom; Hydrogen

13 13 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 moleculeAbsorption spectrum of a molecule; Eg: H 2 O

14 14 Effect of electromagnetic radiations on chemical substances The radiation energies absorbed by molecules may produce Rotational, Vibrational and Electronic transitions.

15 15 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 moleculeRotating HCl molecule

16 16 Vibrational transition Effect of electromagnetic radiations on chemical substances Infrared radiations bring about changes in the vibration modes (stretching, contracting and bending) of covalent bonds in a molecule Example: Vibrating HCl molecule Examples:

17 17 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

18 18 Effect of electromagnetic radiations on chemical substances Cl 2 in Ground state

19 19 Effect of electromagnetic radiations on chemical substances Cl 2 in Excited state

20 20 The Ultraviolet region [10 – 800nm] The Ultraviolet region may be divided as follows, 1. Far (or Vacuum) Ultraviolet region [10 – 200 nm] 2. Near (or Quartz) Ultraviolet region [200 – 380 nm] 3. Visible region [ nm]

21 21 The Ultraviolet region Far (or Vacuum) Ultraviolet region [10 – 200nm] Electromagnetic spectral region from 100 – 200nm 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 - electrons occurs in 120 – 200nm Near (or Quartz) Ultraviolet region [ nm] Electromagnetic spectral region from 200 – 380nm normally termed as Ultraviolet region The atmosphere is transparent in this region and quartz optics may be used to scan from 200 – 380nm Excitation of p and d orbital electrons, - electrons and - conjugation (joining together) systems occurs in 200 – 380nm Example for conjugation Benzene

22 22 The Visible region Visible region [380 – 800nm] Electromagnetic spectral region from 380 – 800nm is termed as visible region The atmosphere absorbs the hazardous high energy UV <200nm from sunlight Excitation of -conjugation occurs in visible region; 380 – 800nm Conjugation of double bonds lowers the energy required for the transition and absorption will move to longer wavelength (i.e., to low energy)

23 23 VISIBLE region in Electromagnetic Spectrum Violet : nm Indigo : nm Blue : nm Green : nm Yellow : nm Orange : nm Red : nm

24 24 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 UV - VISIBLE SPECTROSCOPY

25 25 Lambert 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 Lamberts law Beers law Beer 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.

26 26 Beer–Lambert law / Beer–Lambert– Bouguer law / Lambert – Beer law log (I 0 /I) = c l = A Where, I 0 - the intensity of incident light I - the intensity of transmitted light - 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

27 27 Absorption intensity wavelength of light corresponding to maximum absorption is designated as 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 max Intensity of absorption is directly proportional to the transition probability A fully allowed transition will have max > A low transition probability will have max < 1000 max max = 20000

28 28 Generalizations Regarding max If spectrum of compound shows, Absorption band of very low intensity ( 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 * 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.

29 29 Generalizations Regarding max If max = 10, ,000; generally a simple, -unsaturated ketone or diene If max = 1, ,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 max > 10,000 along with some which still have max < 10,000. Bands with max < 100 represent n * 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

30 30 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 Lamberts concept to solutions of different concentrations Bernard ? Beer released the results of Lamberts concept just prior to those of Bernard

31 31 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

32 32 Bonding between two hydrogen atoms According to Molecular Orbital Theory One molecular orbital with 2 electrons One bonding orbital with 2 electrons One antibonding orbital without electrons and two nuclei 2 atomic orbitals of 2 hydrogen atoms Bonding and anti-bonding formation from s atomic orbitals (Eg: H 2 molecule)

33 33 According to Molecular Orbital Theory Lower energy than original atomic orbitals Higher energy than original atomic orbitals and bonding orbital - Because of repulsion 2 atomic orbitals of 2 hydrogen atoms 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 - more stable than having separate atoms, and a lot more stable than having the electrons in the anti-bonding orbital. Bonding and anti-bonding formation from s atomic orbitals (Eg: H 2 molecule)

34 34 Bonding and anti-bonding formation from p atomic orbitals

35 35 Bonding and anti-bonding formation from p atomic orbitals

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

37 37 Electronic Energy Levels Energy n Atomic orbital Molecular orbitals Occupied levels Unoccupied levels Graphically,

38 38 Electronic Transitions The valence electrons in organic molecules are involved in bonding as - bonds, - 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) Energy n Atomic orbital Molecular orbitals Occupied levels Unoccupied levels

39 39 Types of Electronic Transitions Transition between bonding molecular orbitals and anti-bonding molecular orbitals They are of three types: *

40 40 Types of Electronic Transitions (bonding) n (non-bonding) (anti-bonding) * (bonding to anti-bonding ) * transition requires large energies in far UV region in nm range. Molar absorptivity: Low max = Examples: Alkanes - 150nm Methane CyclohexanePropane Transition between bonding molecular orbitals and anti-bonding molecular orbitals

41 41 Types of Electronic Transitions * (bonding to anti-bonding ) Transition between bonding molecular orbitals and anti-bonding molecular orbitals

42 42 Types of Electronic Transitions (bonding) n (non-bonding) (anti-bonding) * occur in nm range. Molar absorptivity: High max = * (bonding to anti-bonding ) Transition between bonding molecular orbitals and anti-bonding molecular orbitals max is high because the and * orbitals are in same plane and consequently the probability of jump of an electron from * orbital is very high. Carbonyl Azo Examples: Unsaturated compounds double or triple bonds aromatic rings Carbonyl groups azo groups Conjugated electrons

43 43 Types of Electronic Transitions * (bonding to anti-bonding ) Transition between bonding molecular orbitals and anti-bonding molecular orbitals

44 44 Types of Electronic Transitions (bonding) n (non-bonding) (anti-bonding) * occur only in <150 nm range. Molar absorptivity: Low * (bonding to anti-bonding ) Transition between bonding molecular orbitals and anti-bonding molecular orbitals * and * transitions: high-energy, accessible in vacuum UV ( max <150 nm). Not usually observed in molecular UV-Vis. Examples: Carbonyl compounds

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

46 46 Types of Electronic Transitions Transition between non-bonding atomic orbitals and anti-bonding molecular orbitals n * (non-bonding n to anti-bonding )

47 47 Types of Electronic Transitions Spectra of aldehydes or ketones exhibit two bands; A High intense band at nm due to * A low intense band at 300nm due to n * transition Consequently, the probability of jump of an electron from n * 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 * transition does occur, but only to a limited extent. n * transition is always less intense because……. The electrons in the n-orbitals are situated perpendicular to the plane of bond and hence to the plane of * orbital. n to

48 48 Types of Electronic Transitions (bonding) n (non-bonding) (anti-bonding) Transition between non-bonding atomic orbitals and anti-bonding molecular orbitals Excitation of an electron in an unshared pair on Nitrogen, oxygen, sulphur or halogens to an antibonding orbital is called n * transitions. n * occur in nm range. Molar absorptivity: Low max = n * (non-bonding n to anti-bonding ) Example: Methanol max = 183nm ( = 500) 1-Iodobutane max = 257nm ( = 486) Trimethylamine max = 227nm ( = 900)

49 49 Types of Electronic Transitions Transition between non-bonding atomic orbitals and anti-bonding molecular orbitals n * (non-bonding n to anti-bonding )

50 50 (bonding) n (non-bonding) (anti-bonding) Types of Electronic Transitions * (bonding to anti-bonding ) n * (non-bonding n to anti-bonding ) * (bonding to anti-bonding ) Energy required for various transitions obey the order: * > n * > *> n *

51 51 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. Types of Electronic Transitions Energy n n alkanes carbonyls unsaturated compounds O, N, S, halogens carbonyls 150 nm 170 nm 180 nm 190 nm 300 nm If conjugated

52 52 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. Selection Rules 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

53 53 Rule - 3: Symmetry of electronic states; n * transition in formaldehyde is forbidden by local symmetry. i.e., Energy is always a function of molecular geometry. Selection Rules To further complicate matters, forbidden transitions are sometimes observed (albeit at low intensity) due to other factors. In formaldehyde (H 2 C=O), In n * excited state an electron arrives at the antibonding orbital, while the electron pair in the bonding orbital is still present. Due to the third antibonding electron, the C=O bond becomes weaker and longer. In the * excited configuration, the situation is somewhat worse because there is only one electron in the bonding orbital, while the other electron is anti-bonding (i.e. *). Consequently, the excited state bond lengths will be longer than a genuine C=O double bond but shorter than a -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.

54 54 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 and Condon Principle 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.

55 55 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. Origin and General appearance of UV bands

56 56 The absorption bands in the UV - Visible spectrum may be designated either by using electronic transitions [ *, *, *, n *, n *] or the letter designation as given below. Designation of UV bands R – bands (German, radikalartig) The bands due to n * transitions of single chromophoric groups are referred to as the R - Bands. Example: Carbonyl group, Nitro group Shows low molar absorptivity ( max <100) and hypsochromic shift with an increase in solvent polarity. K – bands (German, konjugierte) The bands due to * transitions in molecules containing conjugated systems are referred to as the K – Bands. Example: Butadiene, mesityl oxide They show high molar absorptivity ( max <10,000).

57 57 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 * transitions. In benzene, E 1 and E 2 bands occur near 180nm and 200nm respectively and their molar absorptivity varies between ( max = 2000 to 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.

58 58 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

59 59 Chromophores: examples ChromophoreExampleExcitationλ max, nmεSolvent C=CEtheneΠ __ > Π*17115,000hexane CC1-HexyneΠ __ > Π*18010,000hexane C=OEthanal n __ > Π* Π __ > Π* ,000hexane N=ONitromethane n __ > Π* Π __ > Π* ,000ethanol C-X; X=Br X=I Methyl bromide Methyl Iodide n __ > σ* hexane

60 60 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 electrons and stabilizes the * state

61 61 Auxochromes: examples AuxochromeUnsubstitued chromophore max (nm) Substituted chromophore max (nm) -CH 3 H 2 C=CH-CH = CH 2 217H 2 C=CH-CH=CHCH ORH 3 C-CH=CH-COOH204H 3 C-C(OCH 3 ) = CHCOOH234 -C1H 2 C=CH 2 175H 2 C = CHCl185

62 62 Bathochromic shift (Red shift) - max to longer wavelength Shift of an absorption maximum to longer wavelength is called bathochromic shift. Occurs due to change of medium ( * 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: max = 175nm 1-butene / isobutene: max = 188 nm The bathochromic shift is progressive as the number of alkyl groups increases.

63 63 Hypsochromic shift (Blue shift) - max to Shorter wavelength Shift of absorption maximum to shorter wavelength is known as hypsochromic shift. Occurs due to change of medium (n * 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 max = 279nm in hexane max = 264.5nm in water This blue shift results from hydrogen bonding which lowers the energy of the n orbital.

64 64 Hyperchromic effect - increased ( 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 CompoundPrimary bandSecondary band max (nm) max max (nm) max PhenolC 6 H 5 OH Phenolate anionC6H5O-C6H5O

65 65 Hypochromic effect - decreased ( 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 CompoundPrimary bandSecondary band max (nm) max max (nm) max Benzoic acidC 6 H 5 COOH BenzoateC 6 H 5 COO

66 66 Effect of substituents on max and max Shift to Longer max Shift to shorter max Shift to decreased max Shift to increased max Graphically,

67 67 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 3.7×10 -4 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.

68 68 UV Spectroscopy (Electronic Spectra) - Terminologies Beer-Lambert Law A =.c.l AbsorbanceA, a measure of the amount of radiation that is absorbed Molar absorptivity, absorbance of a sample of molar concentration in 1 cm cell. Extinction coefficicentAn alternative term for the molar absorptivity. concentrationc, concentration in moles / litre Path lengthl, the length of the sample cell in cm. max The wavelength at maximum absorbance max The molar absorbance at max BandTerm to describe a uv-vis absorption which are typically broad. HOMOHighest Occupied Molecular Orbital LUMOLowest Unoccupied Molecular Orbital ChromophoreStructural unit responsible for the absorption. Auxochrome A group which extends the conjugation of a chromophore by sharing of nonbonding electrons Bathochromic shiftThe shift of absorption to a longer wavelength. Hypsochromic shiftshift of absorption to a shorter wavelength Hyperchromic effectAn increase in absorption intensity Hypochromic effectA decrease in absorption intensity Isosbestic point point common to all curves produced in the spectra of a compound taken at various pH

69 69 Instrumentation sample reference detector I0I0 I0I0 I2I2 I1I1 log(I 0 /I) = A , nm monochromator/ beam splitter optics UV-VIS sources I

70 70 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 Instrumentation…

71 71 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 380 – 800nm Concentration: 0.1 to 100mg to 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) Sample Handling

72 72 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 chloroform240 cyclohexane195 1,4-dioxane215 95% ethanol205 n-hexane201 methanol205 isooctane195 water190 Solvents

73 73 A * transition can occur in simple non-conjugated alkene like ethene and other alkenes with isolated double bonds below 200 nm. Factors affecting the position of UV bands – 1. Non-conjugated alkenes

74 74 Alkyl substitution of parent alkene moves the absorption to longer wavelengths. Factors affecting the position of UV bands – 1. Non-conjugated alkenes… From max di-, tri & tetra substituted double bonds in acyclic and alicyclic systems can be identified

75 75 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 electrons of the alkyl group and the chromophoric group. 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. Methyl groups also cause a bathochromic shift, even though they are devoid of p-or n-Electrons This effect is thought to be through what is termed HYPERCONJUGATION or sigma bond resonance HYPERCONJUGATION

76 76 A conjugated system requires lower energy for the * transition than an unconjugated system. Example: Ethylene and Butadiene Factors affecting the position of UV bands – 2. Conjugated Dienes Ethylene has only two orbitals; one ground state bonding orbital and one excited state * antibonding orbital. The energy difference ( ) between them is about 176 kcal/mole. In conjugated butadiene ( max =217nm; max = 21000) and * orbitals have energies much closer together than those in ethylene, resulting in a lower excitation energy

77 77 [i.e., From MOT, two atomic p orbitals, from two sp2 hybrid carbons combine to form two MOs and * in ethylene,] Factors affecting the position of UV bands p p

78 78 In butadiene, 4 p orbitals are mixing and 4 MOs of an energetically symmetrical distribution compared to ethylene. Therefore, the following and * for ethylene and butadiene will be obtained. Ethylene Butadiene Factors affecting the position of UV bands - 2. Conjugated Dienes

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

80 80 E for the HOMO LUMO transition is REDUCED = 176 kcal/mole 136 kcal/mole Factors affecting the position of UV bands - 2. Conjugated Dienes

81 81 Extending this effect out to longer conjugated systems the energy gap becomes progressively smaller : For example Energy ethylene butadiene hexatriene octatetraene Lower energy = Longer wavelengths Factors affecting the position of UV bands - 2. Conjugated Dienes

82 82 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 Factors affecting the position of UV bands - 2. Conjugated Dienes - Types

83 83 Exocyclic and Endocyclic double bond: Exocyclic double bond Endocyclic double bond Factors affecting the position of UV bands - 2. Conjugated Dienes - Types

84 84 1. Acyclic diene or Heteroannular diene s-trans Heteroannular diene, is a conjugated system in which the two double bonds are confined to two different rings. Base max = 214 nm ( max = ). Most acyclic dienes have transoid conformation; i.e. trans disposition of double bonds about a single bond. Base max =217 nm ( max = ). B A Base max =217 nm max = Base max =214 nm max = Factors affecting the position of UV bands - 2. Conjugated Dienes - Types

85 85 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 max = 253 nm ( max = ). Homoannular dienes contained in other ring sizes possess different base absorption values. Example: Cyclopentadiene; max =228nm Cycloheptadiene; max = 241nm Base max =253 nm max = s-cis Factors affecting the position of UV bands - 2. Conjugated Dienes - Types

86 86 When two or more C=C units are conjugated, The energy difference E between the highest bonding orbital (HOMO) and the lowest excited * orbital (LUMO) becomes small and results in a shift of max to longer wavelength i.e., Bathochromic shift. This concept helps to distinguish between the two isomeric diens, 1,5-hexadiene and 2, 4- hexadoeme, from the relative positions of max. 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) max = 178 nm max = 227 nm Factors affecting the position of UV bands - 2. Conjugated Dienes

87 87 Factors affecting the position of UV bands - 2. Conjugated Dienes

88 88 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 max max = nm Example: Longer wavelengths = Lower energy Factors affecting the position of UV bands - 2. Conjugated Dienes - Types

89 89 Conjugation with a heteroatom [N, O, S, X] moves the ( *) absorption of ethylene to longer wavelengths Example: CH 2 =CH-OCH 3 ( max=190nm) - max ~10000 CH 2 =CH-NMe 2 ( max =230nm) - max ~10000 Methyl vinyl sulphide absorbs at 228 nm ( max =8000) Factors affecting the position of UV bands - 2. Conjugation… with hetero atoms nAnA Energy Here we create 3 MOs – this interaction is not as strong as that of a conjugated -system

90 90 In compounds where geometrical isomerism is possible. Example: trans - stilbene absorbs at longer wavelength [ max =295 nm] (low energy) cis - stilbene absorbs at shorter wavelength [ max =280 nm] (high energy) due to the steric effects. Coplanarity is needed for the most effective overlap of the - orbitals and increased ease of the * transition. The cis-stilbene is forced into a nonplanar conformation due to steric effects. Factors affecting the position of UV bands – 3. Effect of Geometrical isomerism - Steric effect

91 91 UV spectroscopy is very sensitive to distortion of the chromophore and consequently the steric repulsions which oppose the coplanarity of conjugated -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. Factors affecting the position of UV bands – 4. Effect of steric hindrance on coplanarity (steric inhibition of resonance) Example-1: Distortion leading to RED shift The strained molecule Verbenene exhibits max =245.5nm whereas the usual calculation shows at max =229 nm. Verbenene Actual; max =245.5nm Calculated; max =229nm

92 92 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 ( max =5500). 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 Actual; max =220nm Calculated; max =273nm

93 93 Absorption of Azobenzene (in ethanol) Example * transition n * transition max trans-isomer cis-isomer 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 Such differences between cis and trans isomers are of some diagnostic value

94 94 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. Factors affecting the position of UV bands – 5. Effect of Solvents Non-polar compounds like Conjugated dienes and aromatic hydrocarbons exhibit very little solvent shift,

95 95 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:, -Un saturated carbonyl compounds display two different types of shifts. (i)n * Band moves to shorter wavelength (blue shift). (ii) * Band moves to longer wavelength (red shift)

96 96 Factors affecting the position of UV bands – 5. Effect of Solvents…, -Un saturated carbonyl compounds - For increased solvent polarity n * Band moves to shorter wavelength (blue shift). In n * 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: max = 279nm in hexane max = 264nm in water n * A B C D AB < CD Non-polar solvent Polar solvent Shorter wavelength

97 97 Factors affecting the position of UV bands – 5. Effect of Solvents…, -Un saturated carbonyl compounds - For increased solvent polarity (ii) * Band moves to longer wavelength (Red shift). In * 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 max in ethanol will be greater than that observed in hexane. i.e., * 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: * A B C D AB > CD Non-polar solvent Polar solvent Longer wavelength

98 98 Factors affecting the position of UV bands – 5. Effect of Solvents…, -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.

99 99 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 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. Factors affecting the position of UV bands – 6. Conformation and geometry in polyene systems

100 100 unsaturated systems incorporating N or O can undergo n * transitions in addition to * * transitions; max~188 nm; max = 900 n * transitions; max ~285 nm; 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 * transition in the vacuum UV ( max =188 nm; max =900) Both this transitions are also sensitive to substituents on the carbonyl Absorption spectra of Unsaturated carbonyl compounds……. Enones

101 101 n Remember, the * transition is allowed and gives a high, but lies outside the routine range of UV observation The n * transition is forbidden and gives a very low e, but can routinely be observed Absorption spectra of Unsaturated carbonyl compounds……. Enones

102 102 n CO transitions omitted for clarity It has been determined from spectral studies, that carbonyl oxygen more approximates sp rather than sp 2 ! Carbonyls – n * transitions (~285 nm); * (188 nm) Absorption spectra of Unsaturated carbonyl compounds……. Enones

103 103 For auxochromic substitution on the carbonyl, pronounced hypsochromic (blue) shifts are observed for the n * transition ( 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 * which extends conjugation and causes a bathochromic shift In most cases, this bathochromic shift is not enough to bring the * transition into the observed range Absorption spectra of Unsaturated carbonyl compounds……. Enones 293 nm 279 nm 235 nm 214 nm 204 nm max Blue shift

104 104 Conversely, if the C=O system is conjugated both the n * and * bands are Bathochromically (Red) shifted Here, several effects must be noted: the effect is more pronounced for * if the conjugated chain is long enough, the much higher intensity * band will overlap and drown out the n * band the shift of the n * transition is not as predictable For these reasons, empirical Woodward-Fieser rules for conjugated enones are for the higher intensity, allowed * transition Absorption spectra of Unsaturated carbonyl compounds……. Enones

105 105 Absorption spectra of Unsaturated carbonyl compounds……. Enones Conjugation effects are apparent; from the MO diagram for a conjugated enone: n n

106 106 Alkanes – only posses -bonds and no lone pairs of electrons, so only the high energy * transition is observed in the far UV This transition is destructive to the molecule, causing cleavage of the -bond Absorption spectra of Alkanes - Miscellaneous

107 107 Alcohols, ethers, amines and sulfur compounds – in the cases of simple, aliphatic examples of these compounds the n * is the most often observed transition; like the alkane * it is most often at shorter than 200 nm Note how this transition occurs from the HOMO to the LUMO CN n N sp 3 Absorption spectra of Aliphatic compounds - Miscellaneous

108 108 Woodward – Fieser rules Robert B. Woodward Nobel Prize in Chemistry : 1965 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

109 109 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 * 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) Woodward – Fieser rules

110 110 The rules begin with a base value for max of the chromophore being observed: For acyclic butadiene = 217 nm GroupIncrement Extended conjugation+30 Each exo-cyclic C=C+5 Alkyl+5 -OCOCH OR+6 -SR+30 -Cl, -Br+5 -NR Woodward – Fieser rules for Dienes The incremental contribution of substituents is added to this base value from the group tables: or 214 nm

111 111 Isoprene - acyclic butadiene = 217 nm one alkyl subs.+ 5 nm Calculated value222 nm Observed value220 nm Allylidenecyclohexane - acyclic butadiene = 217 nm one exocyclic C=C + 5 nm 2 alkyl subs.+10 nm Calculated value232 nm Observed value237 nm Woodward – Fieser rules for Dienes – Examples -1 & 2

112 112 Woodward – Fieser rules for Dienes – Problem - 1 GroupIncrement Extended conjugation+30 Each exo-cyclic C=C+5 Alkyl+5 -OCOCH OR+6 -SR+30 -Cl, -Br+5 -NR acyclic butadiene = 217 nm Solution: acyclic butadiene=217 nm extended conjugation=+30 nm Calculated value=247 nm

113 113 Woodward – Fieser rules for Dienes – Example-3

114 114 Heteroannular (transoid) Homoannular (cisoid) GroupIncrement Additional homoannular+39 Where both types of diene are present, the one with the longer becomes the base Woodward – Fieser rules for Cyclic Dienes The increment table is the same as for acyclic butadienes with a couple additions: Base max = 214 Base max = 253 GroupIncrement Extended conjugation+30 Each exo-cyclic C=C+5 Alkyl+5 -OCOCH OR+6 -SR+30 -Cl, -Br+5 -NR 2 +60

115 115 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

116 116 Woodward – Fieser rules for Dienes – Problem - 2 GroupIncrement Extended conjugation+30 Each exo-cyclic C=C+5 Alkyl+5 -OCOCH OR+6 -SR+30 -Cl, -Br+5 -NR Heteroannular diene = 214 nm Solution: Heteroannular diene=214 nm Ring residues / Alkyl substitution 3 x 5=+ 15 nm Exocyclic C=C bond 1 x 5=+ 5 nm Calculated value=234 nm Observed value=247 nm

117 117 Woodward – Fieser rules for Cyclic Dienes – Example-5

118 118 Woodward – Fieser rules for Cyclic Dienes – Example-6 heteroannular diene = 214 nm 4 alkyl subs. (4 x 5)+20 nm 1 exo C=C+ 5 nm 239 nm abietic acid

119 119 homoannular diene = 253 nm 4 alkyl subs. (4 x 5)+20 nm 1 exo C=C+ 5 nm 278 nm Woodward – Fieser rules for Cyclic Dienes – Example-7 levopimaric acid

120 120 Woodward – Fieser rules for Dienes – Problem - 3 GroupIncrement Additional homoannular +39 Extended conjugation+30 Each exo-cyclic C=C+5 Alkyl+5 -OCOCH OR+6 -SR+30 -Cl, -Br+5 -NR Homoannular diene = 253 nm Solution: Homoannular diene=253 nm Extended conjugation 1 x 30=+30 nm Alkyl substitution 2 x 5=+ 10 nm Calculated value=293 nm

121 121 Woodward – Fieser rules for Cyclic Dienes – Example-8

122 122 Woodward – Fieser rules for Dienes – Examples – 9,10 & 11

123 123 Be careful with your assignments – three common errors: 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 Woodward – Fieser rules for Cyclic Dienes – PRECAUTIONS

124 124 Woodward – Fieser rules for Enones GroupIncrement 6-membered ring or acyclic enoneBase 215 nm 5-membered ring parent enoneBase 202 nm Acyclic dienoneBase 245 nm Double bond extending conjugation30 Alkyl group or ring residue and higher 10, 12, 18 -OH and higher 35, 30, 18 -OR 35, 30, 17, 31 -O(C=O)R 6 -Cl 15, 12 -Br 25, 30 -NR 2 95 Exocyclic double bond5 Homocyclic diene component39

125 125 Woodward – Fieser rules for Enones Aldehydes, esters and carboxylic acids have different base values than ketones Unsaturated systemBase Value Aldehyde208 With or alkyl groups 220 With or alkyl groups 230 With alkyl groups 242 Acid or ester With or alkyl groups 208 With or alkyl groups 217 Group value – exocyclic double bond +5 Group value – endocyclic bond in 5 or 7 membered ring +5

126 126 Woodward – Fieser rules for Enones Unlike conjugated alkenes, solvent does have an effect on max These effects are also described by the Woodward-Fieser rules Solvent correctionIncrement Water+8 Ethanol, methanol0 Chloroform Dioxane-5 Ether-7 Hydrocarbon-11

127 127 Some examples – keep in mind these are more complex than dienes cyclic enone = 215 nm 2 x - alkyl subs.(2 x 12)+24 nm Calculated value239 nm Experimental value238 nm cyclic enone = 215 nm extended conj.+30 nm -ring residue+12 nm -ring residue+18 nm exocyclic double bond+ 5 nm 280 nm Experimental280 nm Woodward – Fieser rules for Enones – Examples – 12 & 13

128 128 Woodward – Fieser rules for Enones – Problem – 4 GroupPositionIncrement 6-membered ring or acyclic enoneBase 215 nm 5-membered ring parent enoneBase 202 nm Acyclic dienoneBase 245 nm Double bond extending conjugation30 Alkyl group or ring residue and higher 10, 12, 18 -OH and higher 35, 30, 18 -OR 35, 30, 17, 31 -O(C=O)R 6 -Cl 15, 12 -Br 25, 30 -NR 2 95 Exocyclic double bond5 Homocyclic diene component39

129 129 Woodward – Fieser rules for Enones – Solution for Problem – 4 GroupIncrement 6-membered ring or acyclic enoneBase 215 nm 5-membered ring parent enoneBase 202 nm Acyclic dienoneBase 245 nm Double bond extending conjugation30 Alkyl group or ring residue and higher 10, 12, 18 -OH and higher 35, 30, 18 -OR 35, 30, 17, 31 -O(C=O)R 6 -Cl 15, 12 -Br 25, 30 -NR 2 95 Exocyclic double bond5 Homocyclic diene component39

130 130 Woodward – Fieser rules for Enones – Example – 14

131 Absorption spectra of Polyenes – Lycopene, Carotene etc.. 2. Woodward Fieser rules for Polyenes – Rules and calculation for atleast 2 polyenes 3. Applications of UV spectra - with specific examples UV Spectroscopy – For Assignment

132 132 1.Spectroscopy of Organic Compounds, by P.S. Kalsi, 2nd Edition, (1996), pp.7–50. 2.Organic Spectroscopy: Principles and Applications, by Jag Mohan, 2 nd Edition, (2009), pp.119– Spectrometric Identification of Organic Compounds, by Silverstein, Bassler, Morrill, 5 th Edition, (1991), pp. 289– Introduction to Spectroscopy, by Pavia, Lampman, Kriz, 3 rd Edition, (2001), pp Applied Chemistry, by K. Sivakumar, I st Edition, (2009), pp.8.1– Instrumental Methods of Chemical Analysis, by Gurdeep.R. Chatwal, Sham Anand, I st Edition, (1999), pp Selected Topics in Inorganic Chemistry, by Wahid U. Malik, G.D. Tuli, R.D. Madan, (1996). 8.Fundamentals of Molecular Spectroscopy, by C.N. Banwell, 3 rd Edition, (1983). 9.www.spectroscopyNOW.com UV Spectroscopy - References

133 133

134 134 Dr. K. SIVAKUMAR Department of Chemistry SCSVMV University Good Luck!


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