1 MPC102 – PHYSICAL METHODS IN CHEMISTRY Course: M. Phil (Chemistry) Unit: I UV - VISIBLE...

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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. SIVAKUMARDepartment of

ChemistrySCSVMV University

[email protected]

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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 = 1/1000000000 meters1A = 10-10 meters = 1/10000000000 meters

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Frequency (): Frequency is the number of waves passing through any point per second.

Frequency is expressed in Hertz (Hz)

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Wave number ( ): Wave number is the number of waves per cm.

1=

c

Where, is wave length

is wave number is frequency

c is velocity of light in vacuum. i.e., 3 x 108 m/s

Wavelength, Wave number and Frequency are interrelated as,

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UVX-rays IRg-rays RadioMicrowaveVisible

nm

EM waves

10-4 to 10-2 10-2 to 100 100 to 102 102 to 103 103 to 105 105 to 107 107 to 109

Electromagnetic Spectral regions

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Electromagnetic Spectrum

E = h h – Planck’s constant

www.spectroscopyNOW.com

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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

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Electromagnetic Spectrum – Type of radiation and Energy change involved

1010


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1212

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

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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

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

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Rotational transition

Microwave and far IR radiations bring about changes in the rotational energies of the molecule

Example: Rotating HCl molecule

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Vibrational transition


Infrared radiations bring about changes in the vibration modes (stretching, contracting and bending) of covalent bonds in a molecule

Example:

Vibrating HCl molecule

Examples:

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Electronic transition

UV and Visible radiations bring about changes in the electronic transition of a molecule

Example: Cl2 in ground and excited states

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Cl2 in Ground state

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Cl2 in Excited state

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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 [380 - 800 nm]

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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 [200 - 380nm]

• 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

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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)

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VISIBLE region in Electromagnetic Spectrum

•Violet : 380 - 420 nm •Indigo : 420 - 440 nm•Blue : 440 - 490 nm•Green : 490 - 570 nm•Yellow : 570 - 585 nm•Orange : 585 - 620 nm•Red : 620 - 800 nm

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• 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

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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

Lambert’s law

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.

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

log (I0/I) = c l = A Where, I0 - the intensity of incident lightI - 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 cmA - absorbance (unitless)

Molar absorptivity

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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 (I0/I)I0 - 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 > 10000

A low transition probability will have max < 1000

max

max = 20000

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Generalizations Regarding max

If spectrum of compound shows, Absorption band of

very low intensity (max = 10-100) in the 270-350nm 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.

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Generalizations Regarding max

If max = 10,000 - 20,000; generally a simple , -unsaturated ketone or diene

If max = 1,000 - 10,000 normally an aromatic system

Substitution on the aromatic nucleus by a functional group which extends the length of the chromophore may give bands with max > 10,000 along with some which still havemax < 10,000.

Bands with max < 100 represent n * transitions.

molar absorptivities vary by orders of magnitude:

values of 104-106 are termed high intensity absorptions

values of 103 -104 are termed low intensity absorptions

values of 0 to 103 are the absorptions of forbidden transitions

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

BouguerActually 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

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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

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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: H2 molecule)

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According to Molecular Orbital Theory

Lower energy than original atomic orbitals

Higher energythan original atomic orbitals and bonding orbital - Because of

repulsion


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: H2 molecule)

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Bonding and anti-bonding formation from p atomic orbitals

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Bonding and anti-bonding formation from p atomic orbitals

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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

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Energy

*s

p

s

*p

nAtomic orbitalAtomic orbital

Molecular orbitals

Occupied levels

Unoccupied levels

Graphically,

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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 theHighest Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO)

Energy

*s

p

s

*p

nAtomic orbitalAtomic orbital

Molecular orbitals

Occupied levels

Unoccupied levels

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Types of Electronic Transitions

Transition between bonding molecular orbitals and anti-bonding molecular orbitals

They are of three types: * * *

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s (bonding)

p (bonding)

n (non-bonding)

*s (anti-bonding)

*p (anti-bonding)

* (bonding to anti-bonding )

• * transition requires large energies in far UV region in 120-200nm range.

• Molar absorptivity: Lowmax = 1000 - 10000

• Examples: Alkanes - transition @ 150nm

Methane Cyclohexane Propane


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C C

CC

*C-C

C-C+

++

_

__

_

_


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s (bonding)

p (bonding)

n (non-bonding)

*s (anti-bonding)

*p (anti-bonding)• * occur in 200-700nm range.

• Molar absorptivity: Highmax = 1000 - 10000.



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

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C C

+

_

CC

+

+ _

_

*

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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

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s (bonding)

p (bonding)

n (non-bonding)

*s (anti-bonding)

*p (anti-bonding)

Transition between non-bonding atomic orbitals and anti-bonding molecular orbitals

They are of two types:

n *n *

n * (non-bonding n to anti-bonding )

• n * occur in 200-700nm range.


• Examples:• Compounds with double bonds involving unshared pair(s) of electrons• Aldehydes, Ketones• C=O, C=S, N=O etc.,

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C C

+

_

C O

+

+ _

_

*

n(py)C O+

_

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• Spectra of aldehydes or ketones exhibit two bands;

A High intense band at 200-250nm 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

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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 150-250nm range.



Example:

Methanol max = 183nm ( = 500)

1-Iodobutane max = 257nm ( = 486)

Trimethylamine max = 227nm ( = 900)

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C

*C-N

C-N

++ __ _

N_ _+C

NC

+

_

N

n

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s (bonding)

p (bonding)

n (non-bonding)

*s (anti-bonding)

*p (anti-bonding)







Energy required for various transitions obey the order: * > n * > *> n *

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• 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.


Energy

*s

p

s

*p

n

s

s

p

n

n

s*

p*

p*

s*

p*

alkanes

carbonyls

unsaturated compounds

O, N, S, halogens

carbonyls

150 nm

170 nm

180 nm

190 nm

300 nm

If conjugated

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• 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

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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 (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.

5454

• 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.

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• 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

5656

• 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).

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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.

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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

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Chromophores: examples

Chromophore Example Excitation λmax, nm ε Solvent

C=C Ethene Π __> Π* 171 15,000 hexane

C≡C 1-Hexyne Π __> Π* 180 10,000 hexane

C=O Ethanal n __> Π*Π __> Π*

290180

1510,000

hexanehexane

N=O Nitromethane n __> Π*Π __> Π*

275200

175,000

ethanolethanol

C-X; X=BrX=I

Methyl bromideMethyl Iodide

n __> σ*n __> σ*

205255

200360

hexanehexane

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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

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Auxochromes: examples

Auxochrome Unsubstitued chromophore

max (nm) Substituted chromophore max

(nm)

-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

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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)ORwhen an auxochrome is attached to a carbon-carbon double bond

Example: Ethylene : max = 175nm1-butene / isobutene : max = 188 nm

The bathochromic shift is progressive as the number of alkyl groups increases.

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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)ORwhen an auxochrome is attached to double bonds where n electrons (eg: C=O) are available

Example: Acetonemax = 279nm in hexane

max = 264.5nm in water

This blue shift results from hydrogen bonding which lowers the energy of the n orbital.

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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 max (nm) max

Phenol C6H5OH 210 6200 270 1450

Phenolate anion C6H5O- 235 9400 287 2600

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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 max (nm) max

Benzoic acid C6H5COOH 230 11600 273 970

Benzoate C6H5COO- 224 8700 268 560

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Effect of substituents on max and max

Shift to Longer

max

Shift to shorter

max

Shift to decreased

max

Shift to increased

max

Graphically,

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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 achemical reaction, then the characteristic behaviour shown in the figure below isobserved.

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.

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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.

AuxochromeA 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 pointpoint common to all curves produced in the spectra of a compound

taken at various pH

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Instrumentation

sam

ple

refe

ren

ce

det

ecto

r

I0

I0 I2

I1

log(I0/I) = A

200 700l, nm

monochromator/beam splitter optics

UV-VIS sources

I

7070

Radiation source, monochromator and detector

Two sources are required to scan the entire UV-VIS band:Deuterium lamp – covers the UV – 200-330Tungsten lamp – covers 330-700

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

Instrumentation…

7171

Virtually all UV spectra are recorded solution-phase

Only quartz is transparent in the full 200-700 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)

Sample Handling

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• 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 190chloroform 240cyclohexane 195 1,4-dioxane 21595% ethanol 205n-hexane 201methanol 205isooctane 195water 190

Solvents

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• 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

*p

p

7474

• 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

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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.

C C

C

HH

HMethyl 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”

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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

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p2*

pp1

[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

7878

2*

p 1 1

2

3*

4*

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

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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.


8080

2*

p 1 1

2

3*

4*

DE for the HOMO LUMO transition is REDUCED

= 176 kcal/mole 136 kcal/mole


8181

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


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

• Exocyclic and Endocyclic double bond:

Exocyclic double bond

Endocyclic double bond


8484

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 = 5000-20000).

• Most acyclic dienes have transoid conformation;• i.e. trans disposition of double bonds about a

single bond. • Base max=217 nm (max = 5000-20000).

BA

Base max=217 nm

max = 5000-20000

Base max=214 nm

max = 5000-20000


8585

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 = 5000-8000).

Homoannular dienes contained in other ring sizes possess different

base absorption values.

Example:

Cyclopentadiene; max=228nm

Cycloheptadiene; max= 241nm

Base max=253 nm

max = 5000-8000

s-cis


8686

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-CH3 1,5-Hexadiene 2,4-Hexadiene(non-conjugated diene) (conjugated diene)

max = 178 nm max = 227 nm


8787


8888

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 = 217 253 220 227 227 256 263 nm

Example:

Longer wavelengths = Lower energy


89

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)

Factors affecting the position of UV bands - 2. Conjugation… with hetero atoms

Y2

p

Y1

A

p*

nA

Y3*

Energy

Here we create 3 MOs – this interaction is not as strong as that of a conjugated -system

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

• 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

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

Absorption of Azobenzene (in ethanol)

Example

*

transitionn *

transition

max max max max

trans-isomer

320 21300

443

510

cis-isomer 281 5260 433

1520

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

HH

Such differences between cis and trans isomers are of some diagnostic value

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

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


, -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 solventPolar solvent

Shorter wavelength

97



(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 solventPolar solvent

Longer wavelength

98



(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

• 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

100100

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

101101

p

*p

n

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


102102

p

*p

n

sCO transitions omitted for clarity

O

O

C O

It has been determined from spectral studies, that carbonyl oxygen more approximates sp rather than sp2 !

Carbonyls – n * transitions (~285 nm); * (188 nm)


103103

For auxochromic substitution on the carbonyl, pronounced hypsochromic (blue) shifts are observed for the n p* transition (lmax):

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


H

O

CH3

O

Cl

O

NH2

O

O

O

OH

O

293 nm

279 nm

235 nm

214 nm

204 nm

204 nm

max

Blue shift

104104

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


105105


Conjugation effects are apparent; from the MO diagram for a conjugated enone:

p

*p

p

*p

n

O

Y1

Y2

Y3*

Y4*

n

O

106106

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 C C

C C

Absorption spectra of Alkanes - Miscellaneous

107107

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

sCN

nN sp3C N

C N

C N

C N

anitbonding orbital

Absorption spectra of Aliphatic compounds - Miscellaneous

108108

Woodward – Fieser rules

Robert B. WoodwardNobel 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

109109

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)

Woodward – Fieser rules

110110

The rules begin with a base value for lmax of the chromophore being observed:

For acyclic butadiene = 217 nm

Group Increment

Extended conjugation

+30

Each exo-cyclic C=C +5

Alkyl +5

-OCOCH3 +0

-OR +6

-SR +30

-Cl, -Br +5

-NR2 +60

Woodward – Fieser rules for Dienes

The incremental contribution of substituents is added to this base value from

the group tables:

or 214 nm

111111

Isoprene - acyclic butadiene = 217 nm

one alkyl subs. + 5 nm

Calculated value222 nm

Observed value 220 nm

Allylidenecyclohexane - acyclic butadiene = 217 nm

one exocyclic C=C + 5 nm

2 alkyl subs. +10 nm

Calculated value 232 nm


Woodward – Fieser rules for Dienes – Examples -1 & 2

112112

Woodward – Fieser rules for Dienes – Problem - 1

Group Increment


+30


Alkyl +5

-OCOCH3 +0

-OR +6

-SR +30

-Cl, -Br +5

-NR2 +60

acyclic butadiene = 217 nmSolution:

acyclic butadiene = 217 nm

extended conjugation

= +30 nm

Calculated value = 247 nm

113113

Woodward – Fieser rules for Dienes – Example-3

114114

Heteroannular (transoid) Homoannular (cisoid)

Group Increment

Additional homoannular

+39

Where both types of diene are present, the one with the longer l 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

Group Increment


+30


Alkyl +5

-OCOCH3 +0

-OR +6

-SR +30

-Cl, -Br +5

-NR2 +60

115115

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


116116


Group Increment


+30


Alkyl +5

-OCOCH3 +0

-OR +6

-SR +30

-Cl, -Br +5

-NR2 +60


Solution:


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

117117


118118


C

O

OH

heteroannular diene = 214 nm

4 alkyl subs. (4 x 5) +20 nm1 exo C=C + 5 nm

239 nm

abietic acid

119119

homoannular diene = 253 nm

4 alkyl subs. (4 x 5) +20 nm1 exo C=C + 5 nm

278 nmC

O

OH


levopimaric acid

120120


Group Increment

Additional homoannular

+39


+30


Alkyl +5

-OCOCH3 +0

-OR +6

-SR +30

-Cl, -Br +5

-NR2 +60

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

121121


122122

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

123123

Be careful with your assignments – three common errors:

R

This compound has three exocyclic double bonds; the indicated bond is exocyclic to two rings

This is not a heteroannular diene; you would use the base value for an acyclic diene

Likewise, this is not a homooannular diene; you would use the base value for an acyclic diene

Woodward – Fieser rules for Cyclic Dienes – PRECAUTIONS

124

Woodward – Fieser rules for Enones

C C C

C C CC

C

O O

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 , , a b g and higher

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 , a b 25, 30

-NR2b 95

Exocyclic double bond 5

Homocyclic diene component 39

125125


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

With a or b alkyl groups 208

With ,a b or ,b b alkyl groups 217

Group value – exocyclic ,a b double bond +5

Group value – endocyclic ,a b bond in 5 or 7 membered ring

+5

126126


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 0

Chloroform -1

Dioxane -5

Ether -7

Hydrocarbon -11

127127

Some examples – keep in mind these are more complex than dienes

cyclic enone = 215 nm 2 x b- alkyl subs. (2 x 12)

+24 nmCalculated value 239 nm

Experimental value 238 nm

cyclic enone = 215 nmextended conj. +30 nmb-ring residue +12 nmd-ring residue +18 nmexocyclic double bond + 5 nm

280 nm

Experimental 280 nm

O

R

O

Woodward – Fieser rules for Enones – Examples – 12 & 13

128128

Woodward – Fieser rules for Enones – Problem – 4

C C C

C C CC

C

O O

Group Position Increment

6-membered ring or acyclic enone

Base 215 nm



Double bond extending conjugation

30


10, 12, 18


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 , a b 25, 30

-NR2b 95



129129

Woodward – Fieser rules for Enones – Solution for Problem – 4

C C C

C C CC

C

O O

Group Increment

6-membered ring or acyclic enone Base 215 nm



Double bond extending conjugation 30


10, 12, 18


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 , a b 25, 30

-NR2b 95



130130

Woodward – Fieser rules for Enones – Example – 14

131131

1. 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

132132

1. Spectroscopy of Organic Compounds, by P.S. Kalsi, 2nd Edition, (1996), pp.7–50.

2. Organic Spectroscopy: Principles and Applications, by Jag Mohan, 2nd Edition, (2009), pp.119–152.

3. Spectrometric Identification of Organic Compounds, by Silverstein, Bassler, Morrill, 5th Edition, (1991), pp. 289–315.

4. Introduction to Spectroscopy, by Pavia, Lampman, Kriz, 3rd Edition, (2001), pp.353-389.

5. Applied Chemistry, by K. Sivakumar, Ist Edition, (2009), pp.8.1–8.14.

6. Instrumental Methods of Chemical Analysis, by Gurdeep.R. Chatwal, Sham Anand, Ist Edition, (1999), pp.180-198.

7. 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, 3rd Edition, (1983).

9. www.spectroscopyNOW.com

UV Spectroscopy - References

134

Dr. K. SIVAKUMARDepartment of

ChemistrySCSVMV University

[email protected]

Good Luck!

1 MPC102 – PHYSICAL METHODS IN CHEMISTRY Course: M. Phil (Chemistry) Unit: I UV - VISIBLE...

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