Top Banner
Spectroscopy copy is the study of the interaction between radiation and matter as of wavelength λ. It also can refer to interactions with particle ra response to an alternating field or varying frequency ν. f the response as a function of wavelength or more commonly frequenc red to as a spectrum . ctrometry is the measurement of these responses and an instrument wh forms such measurements is a spectrometer or spectrograph pplications :: Physical chemistry nalytical chemistry rganic and inorganic chemistry stronomy……..
49
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Page 1: Uv Vis Spectroscopy

Spectroscopy Spectroscopy is the study of the interaction between radiation and matter as a function of wavelength λ. It also can refer to interactions with particle radiation or to a response to an alternating field or varying frequency ν. A plot of the response as a function of wavelength or more commonly frequencyis referred to as a spectrum.

Spectrometry is the measurement of these responses and an instrument which

performs such measurements is a spectrometer or spectrograph

Applications ::

Physical chemistry Analytical chemistry Organic and inorganic chemistry Astronomy……..

Page 2: Uv Vis Spectroscopy

Spectroscopy Classifications

Nature of Excitationmeasured

Measurement Processes Common Types

Fluorescence X-ray Flame Visible Ultra-violet Infrared Raman NMR Photoemission Mössbauer

Absorption Emission Scattering

Electromagnetic Electron Mass Dielectric

Page 3: Uv Vis Spectroscopy

• Ultraviolet (UV)-visible spectroscopy corresponds to excitations of outer shell electron between the energy levels that correspond to the molecular orbital of the systems. 

• Infrared (IR) spectroscopy measures the bond vibration frequencies in a molecule and is used to determine the functional group. What functional groups are present in the compound.

• Mass spectrometry (MS) fragments the molecule and measures the masses. What is the size and formula of the compound.

• Nuclear magnetic resonance (NMR) spectroscopy detects signals from hydrogen atoms and can be used to distinguish isomers. What is the carbon-hydrogen framework of the compound.

• Fluorescence is the emission of photons from electronically excited states of atoms, molecules, and ions it is used to determine the excited state property of the molecule.

• MÖssbauer spectroscopy (or -ray spectroscopy) is concerned with transitions between energy levels within the nuclei of atom. It can be used to determine the valance state of metal in unknown compound.

• Raman effect may be defined as phenomenon due to which scattering of light has a slightly different frequency from that of incident light and there occurs a change in atomic oscillation within the molecule. It is helps in the elucidation of molecular structure, for locating functional groups or chemical bonds in the molecules.

Modern techniques for structure determination of unknown compounds include:

Types of Spectroscopy

Page 4: Uv Vis Spectroscopy

Spectroscopy and the Electromagnetic Spectrum

The electromagnetic spectrum covers a continuous range of wavelengths and frequencies, from radio waves at the low-frequency end to gamma () rays at the high-frequency end

lower frequencylonger wavelength

higher frequencyshorter wavelength

Page 5: Uv Vis Spectroscopy

Spectroscopy and the Electromagnetic Spectrum

Page 6: Uv Vis Spectroscopy

Electromagnetic energy is transmitted in discrete amounts called quanta

Amount of energy, , corresponding to 1 quantum of energy (1 photon) of frequency () is expressed by the Planck equation

The energy of a photon varies directly with its frequency but inversely with its wavelength

-34

= =

= Planck's constant = 6.62 10 s

hch

h J

Page 7: Uv Vis Spectroscopy

E N

N hcE

A

-4

A

is energy of Avogadro's number

of quanta , or, energy of one mole of photons

of wavelength

1.20 10 kJ/mol = =

(m)

It is known as one Einstein of energy

Page 8: Uv Vis Spectroscopy

UV-Visible Spectroscopy

• Region of greatest interest to organic chemists from 2 x 10-7 m to ~8 x 10-7 meters

(i) Far (or vacuum) ultraviolet: The region from 10-200 nm can be studied in evacuatedsystems and is termed as “vacuum ultraviolet”. The atmospheric absorption below200 nm is a blessing to all, including the spectroscopists, since it prevents the hazardous (high energy) ultraviolet radiation in the sunlight from striking the earth’ssurface.(ii) Near or quartz ultraviolet : The region from 200-380 nmThe atmosphere is transparent in this region and quartz optics may be used to scan from 200-380 nm.(iii) Visible region: The region from 380-780 nmA tungsten filament lamp is generally used for the visible region of the spectrum.

Page 9: Uv Vis Spectroscopy

In a typical experiment, the molecules or atoms start at lower energy and go to a higher energy level upon absorption of radiation of appropriate wavelength

After

EBefore

Absorptio

n

Ene

rgy

Page 10: Uv Vis Spectroscopy

After

After

Before

Absorption can only occur when the energy of the radiation (calculated from the frequency or wavelength) matches the energy gap.

Ene

rgy

After

If there are several different upper levels (and there usually are) then several transitions will be observed.

Page 11: Uv Vis Spectroscopy

UV/visible Spectroscopy

Chemical compounds are coloured because they absorb visible light.

In general, even organic compounds that are colourless will absorb UV light.

Where has the energy that was within the photons gone to ?

Absorption of visible light

Page 12: Uv Vis Spectroscopy

In UV/visible spectroscopy the energy of the absorbed photon is used to drive the molecule into an excited electronic state.

In the excitation the energy of the whole molecule increases.

After

EBefore

Absorptio

n

Ene

rgy

Page 13: Uv Vis Spectroscopy

This overall change is typically due to promotion of a single electron from a lower to higher energy orbital. The energy of the transition depends on the gap between the two orbitals.

In organic compounds which have only single bonds between the atoms the excitation energy is very high- lies in deep UV.

This excitation gives a dramatic decrease in bond order due to excitation from

a bonding to an anti-bonding orbital

Page 14: Uv Vis Spectroscopy

With increasing conjugation, the decreasing energy gap is reflected by absorption at longer wavelengths.

Page 15: Uv Vis Spectroscopy

The structures of many coloured compounds show they are very extensively conjugated.

HOOCCOOH

trans-Crocetin

16,17-DimethoxyViolanthrone

O

OMe OMe

ON

NN

N

O

O H

H

NH2

Xanthopterin

beta-Carotene

Page 16: Uv Vis Spectroscopy

Substituents added to the compound may alter the energy of the orbitals which e- is excited from or to.

Auxochromes: substituents that alter the wavelength or intensity of the absorption due to the chromophore

ORANGE

O

O

NH2

PURPLE

O

O

NH2

OH

BLUE

O

O

NHCH3

NHCH3

Page 17: Uv Vis Spectroscopy

O

O

OH

O

O

O

HO O-

Changes in chemical composition can give rise to pronounced colour changes since this can dramatically alter the energies of the orbitals involved in the transitions e.g. pH indicators.

-2H+

Phenolphthalein

pinkcolourless

O

O

OH

O

O

O

HO O-

Page 18: Uv Vis Spectroscopy

N

N N

SO3-

CH3

CH3

N

N N+

SO3-

CH3

CH3

H

N

N N

SO3-

CH3

CH3

N

N N+

SO3-

CH3

CH3

H

Methyl orange

H+

red

orange-yellow

Page 19: Uv Vis Spectroscopy

The basic laws of photochemistry:

1. The Grotthuss-Draper law: Only those radiation which are absorbed by the reacting system are effective in producing chemical change.

But the reverse of this law is not always true i.e. the system on absorbing light mayor may not result into a chemical reaction. In many cases, the absorbed light is converted into the kinetic energy of the absorbing molecules and thereby only heateffects are produced.

2. Laws of photochemical equivalence or Einstein law: Each light absorbing moleculein a photochemical reaction absorbs only one quantum of light which causes the activation.

E = NAhIt is conventionally known as one Einstein of energy.

Page 20: Uv Vis Spectroscopy

The Quantitative Picture• Transmittance:

T = I/I0

B(path through sample)

I0

(power in)I

(power out)• Absorbance:

A = -log10 T = log10 I0/I

• The Beer-Lambert Law:

A = bcWhere the absorbance A has no units, since A = log10 I0 /I

is the molar absorbtivity with units of L mol-1 cm-1

b is the path length of the sample in cm

c is the concentration of the compound in solution, expressed in mol L-1 (or M, molarity)

Page 21: Uv Vis Spectroscopy

How Do UV spectrometers work?

Two photomultiplier inputs, differential voltage drives amplifier.

Matched quartz cuvettes

Sample in solution at ca. 10-5 M.

System protects PM tube from stray light

D2 lamp-UV

Tungsten lamp-Vis

Double Beam makes it a difference technique

Rotates, to achieve scan

Page 22: Uv Vis Spectroscopy

Types of Electronic Transitions

Energy absorbed from UV radiation promotes an electron from highest occupied

molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO)

* *

n * n *

Page 23: Uv Vis Spectroscopy

• An electron in a bonding orbital is excited to the corresponding antibonding orbital. The energy required is large. For example, methane (which has only C-H bonds, and can only undergo * transitions) shows an absorbance maximum at 125 nm. Absorption maxima due to * transitions are not seen in typical UV-Vis spectra (200 - 700 nm)

- * Transitions (120-200 nm)

Page 24: Uv Vis Spectroscopy

n* Transitions• Saturated compounds containing atoms

with lone pairs (non-bonding electrons) are capable of n* transitions. These transitions usually need less energy than * transitions. They can be initiated by light whose wavelength is in the range 150 - 250 nm. The number of organic functional groups with n* peaks in the UV region is small.

Methanol max= 183 nm = 500Trimethylamine max= 227 nm = 900

Page 25: Uv Vis Spectroscopy

n* and * Transitions

• Most absorption spectroscopy of organic compounds is based on transitions of n or electrons to the * excited state. This is because the absorption peaks for these transitions fall in an experimentally convenient region of the spectrum (200 - 700 nm). These transitions need an unsaturated group in the molecule to provide the electrons.

Page 26: Uv Vis Spectroscopy

n* and * Transitions(continue)

• Molar absorbtivities from n* transitions are relatively low because the electrons in the n-orbital are situated perpendicular to the plane of the bond and hence the probability of the jump of an electron is very low and range from 10 to100 L mol-1 cm-1 .

* transitions normally give molar absorbtivities between 1000 and 10,000 L mol-1

cm-1 .

Page 27: Uv Vis Spectroscopy

The corresponding absorption band in UV-Visible spectrum may be designated as follows:

• R band (group type, Radikalartig) originated from n - * transition. єmax < 100

Example:

Acetone

λ max 279 nm, єmax <100

Hypsochromic shift (blue shift) with an increase in solvent polarity

• K band (conjugation band, Konjugierte) form - * transition. High єmax ( > 104)

Example:

Dienes, Acetophenone

B band (Benzene band, Benzenoid bands) from the - * transition of Benzene. Broad band with fine structure between 230 – 270 nm. This band can be used to identify aromatic compound.

Page 28: Uv Vis Spectroscopy

• E band (Ethylenic bands) also from - * transition of ethylenic band in benzene

E1 band and E2 bands of benzene occur near 180 and 200 nm respectively and their molar absorptivity varies between 2000 and 14000.

Page 29: Uv Vis Spectroscopy

Important useful terms in UV-Vis spectroscopy• The absorbing groups in a molecule are called chromophores

• A molecule containing a chromophore is called a chromogen

• An auxochrome does not itself absorb radiation, but can enhance the absorption. If an auxochrome is attached to a C=C a bathochromic effect is observed and if to a double bond where n electron are available (C=O) a hypsochromic effect is observed.

• Bathochromic shift – red shift

• Hypsochromic shift – blue shift

• Hyperchromism – an increase in absorption

• Hypochromism – a decrease in absorption

• Isobestic Point: A point common to all curves produced in the spectra of a compound. e.g. phenol-phenolate conversion as a function of pH can demonstrate the presence of the two species in equm by the appearance of an isobestic point in the spectrum.

Page 30: Uv Vis Spectroscopy

Solvent effect• The solvent in which the absorbing species is

dissolved also has an effect on the spectrum of the species.

• Peaks resulting from n* transitions are shifted to shorter wavelengths (blue shift) with increasing solvent polarity. This arises from increased solvation of the lone pair, which lowers the energy of the n orbital.

• The reverse (i.e. red shift) is seen for * transitions.

Page 31: Uv Vis Spectroscopy

Choice of solvent:

The solvent should be of high purity, generally referred as “spectrograde”

A good solvent should be transparent over the desired range of wavelength. Usuallysolvents which do not contain conjugated system are most suitable for running the Uv-visible spectrum.

The fine structure of an absorption band depend on the polarity of the solvent. A non-polar solvent does not form hydrogen-bond with the solute and the spectrum of the Solute clearly approximate to its spectrum in the gaseous state. In a polar solvent hydrogen bonding forms a solute-solvent complex and the fine structure may disappear.

A solvent should be chosen so that it does not react chemically with the sample.

ON

CF3

O

Coumarin 153 (C-153)

Page 32: Uv Vis Spectroscopy

Solvents for UV (showing high energy cutoffs)

Water 210

CH3CN 210

C6H12 210

Ether 210

EtOH 210

Hexane 210

MeOH 215

Dioxane 225

THF 220

CH2Cl2235

CHCl3 245

CCl4 265

benzene 280

Acetone 330

Various buffers for HPLC, check before using.

Page 33: Uv Vis Spectroscopy

Woodward-Fieser rules for substituted dienes

(i) Parent acyclic diene 217 nm

(ii) Parent heteroannular diene 214 nm

(iii) Parent homoannular diene

Increment for substitution

253 nm

(iv) Alkyl group or ring residue (if the alkyl group is attached to two double bonds, count it twice)

+5 nm

(v) Exocyclic double bond (effect is two-fold if bond is exocyclic to two ring)

+5 nm

(vi) Double bond extending conjugation substituents on vinyl carbons (for each one)

+30 nm

(vii) Halogen (-Cl, -Br) + 5 nm

(viii) -OR, -O-(acyl) (-O-COR) + 6 nm, 0 nm

(ix) S-(alkyl) +30 nm

(x) -NRR- +60 nm

Page 34: Uv Vis Spectroscopy
Page 35: Uv Vis Spectroscopy
Page 36: Uv Vis Spectroscopy
Page 37: Uv Vis Spectroscopy
Page 38: Uv Vis Spectroscopy

The effect of steric hindrance to coplanarity

Woodward rules give reliable result only for those compounds in which there is nostrain around the chromophore.

Longer the conjugation, higher will be the absorption maximum and larger will be the value of extinction coefficient. If in a structure the electron system is prevented from achieving coplanarity, there is a marked shift in the absorption maximum and max..

This departure is due to steric crowding which distorts the geometry of the chromophore. Hence the conjugation is reduced by reduction in the orbital overlap.

Page 39: Uv Vis Spectroscopy

Study of charge transfer complex:

The formation of charge transfer complexes occurs between molecules which, whenmixed, allow transfer of electronic charge through space from an electron rich moleculeto an electron-deficient molecule.

The bond formation between the molecules occurs when filled orbitals (or non-bondingorbital) in the donor overlap with the depleted orbitals in the acceptor resulting in theproduction of new MO.

The transition between these newly formed orbitals are responsible for the new absorption band observed in the charge transfer complex.

The electron transfer from the donor to the acceptor is more complete in the excitedState than in the ground state and the wavelength of absorption is correlated with electron affinity of the acceptor and the ionization potential of the donor.

Page 40: Uv Vis Spectroscopy

The Benesi-Hildebrand method is a mathematical approach used in the determination of the equilibrium constant K and stoichiometry of nonbonding interactions. This methodhas been typically used to study reaction equilibriums that form 1:1 guest-host complexes, where the guest and host are reactants, as shown below.

To observe one-to-one binding between a single host (H) and guest (G) using UV/Vis absorbance, the Benesi-Hildebrand method can be employed. The basis behind this method is that the acquired absorbance should be a mixture of the host, guest, and the host-guest complex.

With the assumption that the initial concentration of the guest (G0) is much largerthan the initial concentration of the host (H0), then the absorbance from H0 shouldbe negligible.

(G0>>H0)

Page 41: Uv Vis Spectroscopy

The absorbance can be collected before and following the formation of the HG complex. This change in absorbance (ΔA) is what is experimentally acquired, with A0 being the initial absorbance before the interaction of HG and A being the absorbance taken at any point of the reaction.

Using the Beer-Lambert law, the equation can be rewritten with the absorption coefficients and concentrations of each component

Due to the previous assumption that [G]0 >> [H]0, one can expect that [G] = [G]0. Δε represents the change in value between εHG and εG.

A binding isotherm can be described as "the theoretical change in the concentration of one component as a function of the concentration of another component at constant temperature." This can be described by the following equation:

Page 42: Uv Vis Spectroscopy

By substituting the binding isotherm equation into the previous equation, the equilibrium constant Ka can now be correlated to the change in absorbance due to the formation of the HG complex.

Further modifications results in an equation where a double reciprocal plot can be made with 1/ΔA as a function of 1/[G]0. Δε can be derived from the intercept while Ka can be calculated from the slope.

Page 43: Uv Vis Spectroscopy

Limitation:

Page 44: Uv Vis Spectroscopy

Conjugation, Color, and the Chemistry of Vision

Colored organic compounds have extended conjugated systems

• “UV” absorptions extend into the visible region

-Carotene has max = 455 nm– When white light strikes -carotene wavelengths in

the blue region are absorbed while the yellow-orange colors are transmitted to our eyes

Page 45: Uv Vis Spectroscopy

Conjugation, Color, and the Chemistry of Vision

-carotene is converted in the human body to 11-cis-retinal, an essential molecule for vision

Page 46: Uv Vis Spectroscopy

Conjugation, Color, and the Chemistry of Vision

In the rod cells of the eye 11-cis-retinal is converted into rhodopsin, a light-sensitive substance

When light strikes rhodopsin, trans-rhodopsin (metarhodopsin II) is produced, which is accompanied by a change in geometry

The change in geometry causes a nerve impulse to be sent through the optic nerve to the brain, where vision is perceived

Metarhodopsin II is then recycled back into rhodopsin

Page 47: Uv Vis Spectroscopy

Conjugation, Color, and the Chemistry of Vision

The cis-trans change in bond geometry accompanying vision

Page 48: Uv Vis Spectroscopy
Page 49: Uv Vis Spectroscopy

Solvent effect (cont)

• The reverse (i.e. red shift) is seen for * transitions. This is caused by attractive polarisation forces between the solvent and the absorber, which lower the energy levels of both the excited and unexcited states. This effect is greater for the excited state, and so the energy difference between the excited and unexcited states is slightly reduced - resulting in a small red shift.

• This effect also influences n* transitions but is overshadowed by the blue shift resulting from solvation of lone pairs.