UV - VISIBLE SPECTROSCOPY

UV - VISIBLE SPECTROSCOPY
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

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

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

Electromagnetic wave parameters: Wave number ( ): Wave number is the number of waves per cm. Wavelength, Wave number and Frequency are interrelated as, Where,  is wave length is wave number  is frequency c is velocity of light in vacuum. i.e., 3 x 108 m/s 4

g-rays IR X-rays UV Visible Microwave Radio
Electromagnetic Spectral regions nm 10-4 to 10-2 10-2 to 100 100 to 102 102 to 103 103 to 105 105 to 107 107 to 109 EM waves g-rays X-rays UV Visible IR Microwave Radio 5

Electromagnetic Spectrum
E = h h – Planck’s constant 6

The Electromagnetic wave lengths & Some examples
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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

Electromagnetic Spectrum – Type of radiation and Energy change involved
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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

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: H2O 13

The radiation energies absorbed by molecules may produce Rotational, Vibrational and Electronic transitions. 14

Rotational transition Microwave and far IR radiations bring about changes in the rotational energies of the molecule Example: Rotating HCl molecule 15

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

Electronic transition
Effect of electromagnetic radiations on chemical substances Electronic transition UV and Visible radiations bring about changes in the electronic transition of a molecule Example: Cl2 in ground and excited states

Cl2 in Ground state

Cl2 in Excited state

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

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

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)

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

Lambert 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 Beer’s 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.

log (I0/I) =  c l = A Where, I0 - the intensity of incident light
Beer–Lambert law / Beer–Lambert– Bouguer law / Lambert – Beer law log (I0/I) =  c l = A Where, I0 - the intensity of incident light I - the intensity of transmitted light  - molar absorptivity / molar extinction coefficient in cm2 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

max max Absorption intensity 
Intensity of absorption is directly proportional to the transition probability A fully allowed transition will have max > 10000 A low transition probability will have max < 1000 max max 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 (I0/I) I0 - intensity of the incident light; I - intensity of transmitted light max = 20000 27

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.

molar absorptivities vary by orders of magnitude:
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 103 are the absorptions of forbidden transitions

? Beer–Lambert law / Beer–Lambert– Bouguer law / Lambert – Beer law
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

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

 Bonding and anti-bonding formation from s atomic orbitals (Eg: H2 molecule)
Bonding between two hydrogen atoms One molecular orbital with 2 electrons 2 atomic orbitals of hydrogen atoms According to Molecular Orbital Theory One antibonding orbital without electrons and two nuclei One bonding orbital with 2 electrons 2 atomic orbitals of hydrogen atoms

than original atomic orbitals 2 atomic orbitals of 2 hydrogen atoms
 Bonding and anti-bonding formation from s atomic orbitals (Eg: H2 molecule) According to Molecular Orbital Theory Higher energy than original atomic orbitals and bonding orbital - Because of repulsion Lower energy than original atomic orbitals 2 atomic orbitals of 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 p atomic orbitals

 Bonding and anti-bonding formation from p atomic orbitals

Electronic Energy Levels
s (bonding) p (bonding) n (non-bonding) s* (anti-bonding) p* (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

s* p* Energy n p s Electronic Energy Levels Graphically,
Atomic orbital Molecular orbitals Occupied levels Unoccupied levels

s* p* Energy n p s 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 s* p s p* n Atomic orbital Molecular orbitals Occupied levels Unoccupied levels

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

Transition between bonding molecular orbitals and anti-bonding molecular orbitals   * (bonding  to anti-bonding ) s (bonding) p (bonding) n (non-bonding) s* (anti-bonding) p* (anti-bonding)   * transition requires large energies in far UV region in nm range. Molar absorptivity: Low max = Examples: Alkanes - 150nm Methane Cyclohexane Propane

Transition between bonding molecular orbitals and anti-bonding molecular orbitals   * (bonding  to anti-bonding )

Transition between bonding molecular orbitals and anti-bonding molecular orbitals   * (bonding  to anti-bonding ) s (bonding) p (bonding) n (non-bonding) s* (anti-bonding) p* (anti-bonding)   * occur in nm range. Molar absorptivity: High max = Carbonyl Azo Examples: Unsaturated compounds double or triple bonds aromatic rings Carbonyl groups azo groups Conjugated  electrons 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.

Transition between bonding molecular orbitals and anti-bonding molecular orbitals   * (bonding  to anti-bonding )

Transition between bonding molecular orbitals and anti-bonding molecular orbitals   * (bonding  to anti-bonding ) s (bonding) p (bonding) n (non-bonding) s* (anti-bonding) p* (anti-bonding)   * occur only in <150 nm range. Molar absorptivity: Low   * and   * transitions: high-energy, accessible in vacuum UV (max <150 nm). Not usually observed in molecular UV-Vis. Examples: Carbonyl compounds

Transition between non-bonding atomic orbitals and anti-bonding molecular orbitals They are of two types: s (bonding) p (bonding) n (non-bonding) s* (anti-bonding) p* (anti-bonding) n  * 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.,

Transition between non-bonding atomic orbitals and anti-bonding molecular orbitals n  * (non-bonding n to anti-bonding )

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  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.

Transition between non-bonding atomic orbitals and anti-bonding molecular orbitals n  * (non-bonding n to anti-bonding ) s (bonding) p (bonding) n (non-bonding) s* (anti-bonding) p* (anti-bonding) 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 = Example: Methanol max = 183nm ( = 500) 1-Iodobutane max = 257nm ( = 486) Trimethylamine max = 227nm ( = 900)

Transition between non-bonding atomic orbitals and anti-bonding molecular orbitals n  * (non-bonding n to anti-bonding )

  * (bonding  to anti-bonding )   * (bonding  to anti-bonding ) s* (anti-bonding) n  * (non-bonding n to anti-bonding )   * (bonding  to anti-bonding ) p* (anti-bonding) n  * (non-bonding n to anti-bonding ) n (non-bonding) p (bonding) s (bonding) Energy required for various transitions obey the order:   * > n  * >   *> n  *

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

Singlet state: have electron spin paired
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

Selection Rules 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. In formaldehyde (H2C=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 H2C=O and H3C-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. 54

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 [  *,   *,   *, n  *, n  *] or the letter designation as given below. 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). 56

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, E1 and E2 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. 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

Methyl bromide Methyl Iodide
Chromophores: examples Chromophore Example Excitation λmax, nm ε Solvent C=C Ethene Π __> Π* 171 15,000 hexane C≡C 1-Hexyne 180 10,000 C=O Ethanal n __> Π* Π __> Π* 15 10,000 hexane hexane N=O Nitromethane 17 5,000 ethanol ethanol C-X; X=Br X=I Methyl bromide Methyl Iodide n __> σ* n __> σ*

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: NH2, NHR and NR2, 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

Unsubstitued chromophore Substituted chromophore
Auxochromes: examples Auxochrome Unsubstitued chromophore max (nm) Substituted chromophore -CH3 H2C=CH-CH = CH2 217 H2C=CH-CH=CHCH3 224 -OR H3C-CH=CH-COOH 204 H3C-C(OCH3) = CHCOOH 234 -C1 H2C=CH2 175 H2C = CHCl 185

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.

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.

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 Compound Primary band Secondary band max (nm) max Phenol C6H5OH 210 6200 270 1450 Phenolate anion C6H5O- 235 9400 287 2600

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 Compound Primary band Secondary band max (nm) max Benzoic acid C6H5COOH 230 11600 273 970 Benzoate C6H5COO- 224 8700 268 560

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

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. Example: Absorption spectrum of 3.7×10-4 M methyl red as a function of pH between pH 4.5 and 7.1 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. 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.

UV Spectroscopy (Electronic Spectra) - Terminologies
Beer-Lambert Law A = .c.l Absorbance A, a measure of the amount of radiation that is absorbed Molar absorptivity , absorbance of a sample of molar concentration in 1 cm cell. Extinction coefficicent An alternative term for the molar absorptivity. concentration c, concentration in moles / litre Path length l, the length of the sample cell in cm. max The wavelength at maximum absorbance max The molar absorbance at max Band Term to describe a uv-vis absorption which are typically broad. HOMO Highest Occupied Molecular Orbital LUMO Lowest Unoccupied Molecular Orbital Chromophore Structural unit responsible for the absorption. Auxochrome A group which extends the conjugation of a chromophore by sharing of nonbonding electrons Bathochromic shift The shift of absorption to a longer wavelength. Hypsochromic shift shift of absorption to a shorter wavelength Hyperchromic effect An increase in absorption intensity Hypochromic effect A decrease in absorption intensity Isosbestic point point common to all curves produced in the spectra of a compound taken at various pH

Instrumentation log(I0/I) = A I0 I1 sample detector I I0 I2
UV-VIS sources sample 200 700 l, nm detector I monochromator/ beam splitter optics I0 I2 reference

Radiation source, monochromator and detector
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 (I0) and sends this information to the recorder 70

Virtually all UV spectra are recorded solution-phase
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 380 – 800nm 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 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 215 95% ethanol 205 n-hexane 201 methanol 205 isooctane 195 water 190 72

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

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

Methyl groups also cause a bathochromic shift, even though they
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. 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” 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  * transition than an unconjugated system. Example: Ethylene and Butadiene In conjugated butadiene (max=217nm; max = 21000)  and * orbitals have energies much closer together than those in ethylene, resulting in a lower excitation energy 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. 76

p2* p p1 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  and * in ethylene,] p2* p p1 p p 77

p 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  and * for ethylene and butadiene will be obtained.  4*  2*  3*  2  1 p  1 Ethylene Butadiene 78

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

p Factors affecting the position of UV bands - 2. Conjugated Dienes
 4*  2*  3* 136 kcal/mole  = 176 kcal/mole  2  1 p  1 DE for the HOMO  LUMO transition is REDUCED 80

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 81

Factors affecting the position of UV bands - 2
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

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

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

Homoannular dienes contained in other ring sizes possess different
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 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 85

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. H2C=CH-CH2-CH2-CH=CH2 CH3-CH=CH-CH=CH-CH ,5-Hexadiene ,4-Hexadiene (non-conjugated diene) (conjugated diene) max = 178 nm max = 227 nm 86

87

Longer wavelengths = Lower energy
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 max max = nm Example: Longer wavelengths = Lower energy 88

Factors affecting the position of UV bands - 2
Factors affecting the position of UV bands - 2. Conjugation… with hetero atoms Conjugation with a heteroatom [N, O, S, X] moves the (  *) absorption of ethylene to longer wavelengths Example: CH2=CH-OCH3 (max=190nm) - max~10000 CH2=CH-NMe2 (max=230nm) - max~10000 Methyl vinyl sulphide absorbs at 228 nm (max=8000) Y2 p Y1 p* nA Y3* Energy Here we create 3 MOs – this interaction is not as strong as that of a conjugated -system

Factors affecting the position of UV bands – 3
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 [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 –
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 -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. Verbenene Actual; max =245.5nm Calculated; max =229nm Example-1: Distortion leading to RED shift The strained molecule Verbenene exhibits max=245.5nm whereas the usual calculation shows at max=229 nm.

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

Absorption of Azobenzene (in ethanol)
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 Absorption of Azobenzene (in ethanol) Example  * transition n * transition max max trans-isomer 320 21300 443 510 cis-isomer 281 5260 433 1520 Such differences between cis and trans isomers are of some diagnostic value

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,

n * Band moves to shorter wavelength (blue shift).
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. n * Band moves to shorter wavelength (blue shift).  * Band moves to longer wavelength (red shift)

, -Un saturated carbonyl compounds - For increased solvent polarity
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

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: B * D AB > CD  A Non-polar solvent C Polar solvent Longer wavelength

Factors affecting the position of UV bands – 5. Effect of Solvents… , -Un saturated carbonyl compounds - For increased solvent polarity (iii) In general, 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.

Factors affecting the position of UV bands – 6
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  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.

Absorption spectra of Unsaturated carbonyl compounds……. Enones
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 100

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

p* n p Absorption spectra of Unsaturated carbonyl compounds……. Enones
Carbonyls – n  * transitions (~285 nm);   * (188 nm) p* It has been determined from spectral studies, that carbonyl oxygen more approximates sp rather than sp2 ! n p sCO transitions omitted for clarity 102

For auxochromic substitution on the carbonyl, pronounced hypsochromic (blue) shifts are observed for the n  p* transition (lmax): 293 nm 279 nm 235 nm 214 nm 204 nm max Blue shift 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 p  p* which extends conjugation and causes a bathochromic shift In most cases, this bathochromic shift is not enough to bring the p  p* transition into the observed range 103

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

Conjugation effects are apparent; from the MO diagram for a conjugated enone: Y1 Y2 Y3* Y4* n p p* p p* n 105

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

Absorption spectra of Aliphatic compounds - Miscellaneous
Alcohols, ethers, amines and sulfur compounds – in the cases of simple, aliphatic examples of these compounds the n  s* is the most often observed transition; like the alkane s  s* it is most often at shorter l than 200 nm Note how this transition occurs from the HOMO to the LUMO s*CN nN sp3 sCN 107

Nobel Prize in Chemistry : 1965
Woodward – Fieser rules Robert B. Woodward Nobel Prize in Chemistry : 1965 It is used for calculating λmax Calculated λmax 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 108

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 p  p* 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, 3rd Ed., Butterworths, London, 1975) 109

Woodward – Fieser rules for Dienes
The rules begin with a base value for lmax 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 -OCOCH3 +0 -OR +6 -SR -Cl, -Br -NR2 +60 110

Woodward – Fieser rules for Dienes – Examples -1 & 2
Isoprene - acyclic butadiene = 217 nm one alkyl subs nm Calculated value 222 nm Observed value 220 nm Allylidenecyclohexane - acyclic butadiene = 217 nm one exocyclic C=C nm 2 alkyl subs nm Calculated value 232 nm Observed value 237 nm 111

Woodward – Fieser rules for Dienes – Problem - 1
acyclic butadiene = 217 nm Solution: Group Increment Extended conjugation +30 Each exo-cyclic C=C +5 Alkyl -OCOCH3 +0 -OR +6 -SR -Cl, -Br -NR2 +60 acyclic butadiene = 217 nm extended conjugation +30 nm Calculated value 247 nm 112

Woodward – Fieser rules for Dienes – Example-3
113

Woodward – Fieser rules for Cyclic Dienes
Heteroannular (transoid) Homoannular (cisoid) Base max = 253 Base max = 214 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 l becomes the base Group Increment Extended conjugation +30 Each exo-cyclic C=C +5 Alkyl -OCOCH3 +0 -OR +6 -SR -Cl, -Br -NR2 +60 114

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 nm Observed value nm 115

Heteroannular diene = 214 nm Group Increment Extended conjugation +30 Each exo-cyclic C=C +5 Alkyl -OCOCH3 +0 -OR +6 -SR -Cl, -Br -NR2 +60 Solution: Heteroannular diene = 214 nm Ring residues / Alkyl substitution x 5 + 15 nm Exocyclic C=C bond 1 x 5 + 5 nm Calculated value 234 nm Observed value 247 nm 116

117

heteroannular diene = 214 nm 4 alkyl subs. (4 x 5) +20 nm 1 exo C=C + 5 nm 239 nm abietic acid 118

levopimaric acid homoannular diene = 253 nm 4 alkyl subs. (4 x 5) +20 nm 1 exo C=C + 5 nm 278 nm 119

Homoannular diene = 253 nm Group Increment Additional homoannular +39 Extended conjugation +30 Each exo-cyclic C=C +5 Alkyl -OCOCH3 +0 -OR +6 -SR -Cl, -Br -NR2 +60 Solution: Homoannular diene = 253 nm Extended conjugation 1 x 30 +30 nm Alkyl substitution x 5 + 10 nm Calculated value 293 nm 120

121

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

Woodward – Fieser rules for Cyclic Dienes – PRECAUTIONS
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 123

Woodward – Fieser rules for Enones
Group Increment 6-membered ring or acyclic enone Base 215 nm 5-membered ring parent enone Base 202 nm Acyclic dienone Base 245 nm Double bond extending conjugation 30 Alkyl group or ring residue a, b, g and higher 10, 12, 18 -OH 35, 30, 18 -OR a, b, g, d 35, 30, 17, 31 -O(C=O)R a, b, d 6 -Cl a, b 15, 12 -Br 25, 30 -NR2 b 95 Exocyclic double bond 5 Homocyclic diene component 39

Aldehydes, esters and carboxylic acids have different base values than ketones Unsaturated system Base Value Aldehyde 208 With a or b alkyl groups 220 With a,b or b,b alkyl groups 230 With a,b,b alkyl groups 242 Acid or ester 217 Group value – exocyclic a,b double bond +5 Group value – endocyclic a,b bond in 5 or 7 membered ring 125

Unlike conjugated alkenes, solvent does have an effect on max These effects are also described by the Woodward-Fieser rules Solvent correction Increment Water +8 Ethanol, methanol Chloroform -1 Dioxane -5 Ether -7 Hydrocarbon -11 126

Woodward – Fieser rules for Enones – Examples – 12 & 13
Some examples – keep in mind these are more complex than dienes cyclic enone = nm x b- alkyl subs. (2 x 12) +24 nm Calculated value 239 nm Experimental value 238 nm cyclic enone = nm extended conj nm b-ring residue +12 nm d-ring residue +18 nm exocyclic double bond + 5 nm 280 nm Experimental 280 nm 127

Woodward – Fieser rules for Enones – Problem – 4
Group Position Increment 6-membered ring or acyclic enone Base 215 nm 5-membered ring parent enone Base 202 nm Acyclic dienone Base 245 nm Double bond extending conjugation 30 Alkyl group or ring residue a, b, g and higher 10, 12, 18 -OH 35, 30, 18 -OR a, b, g, d 35, 30, 17, 31 -O(C=O)R a, b, d 6 -Cl a, b 15, 12 -Br 25, 30 -NR2 b 95 Exocyclic double bond 5 Homocyclic diene component 39 128

Woodward – Fieser rules for Enones – Solution for Problem – 4
Group Increment 6-membered ring or acyclic enone Base 215 nm 5-membered ring parent enone Base 202 nm Acyclic dienone Base 245 nm Double bond extending conjugation 30 Alkyl group or ring residue a, b, g and higher 10, 12, 18 -OH 35, 30, 18 -OR a, b, g, d 35, 30, 17, 31 -O(C=O)R a, b, d 6 -Cl a, b 15, 12 -Br 25, 30 -NR2 b 95 Exocyclic double bond 5 Homocyclic diene component 39 129

Woodward – Fieser rules for Enones – Example – 14
130

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

UV Spectroscopy - References
Spectroscopy of Organic Compounds, by P.S. Kalsi, 2nd Edition, (1996), pp.7–50. Organic Spectroscopy: Principles and Applications, by Jag Mohan, 2nd Edition, (2009), pp.119–152. Spectrometric Identification of Organic Compounds, by Silverstein, Bassler, Morrill, 5th Edition, (1991), pp. 289–315. Introduction to Spectroscopy, by Pavia, Lampman, Kriz, 3rd Edition, (2001), pp Applied Chemistry, by K. Sivakumar, Ist Edition, (2009), pp.8.1–8.14. Instrumental Methods of Chemical Analysis, by Gurdeep.R. Chatwal, Sham Anand, Ist Edition, (1999), pp Selected Topics in Inorganic Chemistry, by Wahid U. Malik, G.D. Tuli, R.D. Madan, (1996). Fundamentals of Molecular Spectroscopy, by C.N. Banwell, 3rd Edition, (1983). 132

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

UV - VISIBLE SPECTROSCOPY

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