Files 3-Lecture Notes CHEM-303 UV Spectroscopy

8/2/2019 Files 3-Lecture Notes CHEM-303 UV Spectroscopy

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Ultraviolet SpectroscopyUltraviolet Spectroscopy

The wavelength of UV and visible light are substantially shorter than thewavelength of infrared radiation.

The UV spectrum ranges from 100 to 400 nm.

A UV-Vis spectrophotometer measures the amount of light absorbed at eachwavelength of the UV and visible regions of the electromagnetic spectrum.

A UV or visible spectrophotometer has the same basic design as an infrared

spectrophotometer. In a standard UV-Vis spectrophotometer, a beam of light is split; one half of the

beam (the sample beam) is directed through a transparent cell containing asolution of the compound being analyzed, and one half (the reference beam) isdirected through an identical cell that does not contain the compound but contains

the solvent.

Solvents are chosen to be transparent in the region of the spectrum being usedfor analysis.


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The instrument is designed so that it can make a comparison of the

intensities of the two beams as it scans over the desired region of the

wavelengths.

If the compound absorbs light at a particular wavelength, the intensity of

the sample beam (IS) will be less than that of the reference beam (IR).

Absorption of radiation by a sample is measured at various wavelengths

and plotted by a recorder to give the spectrum which is a plot of the

wavelength of the entire region versus the absorption (A) of light at each

wavelength.


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A broad absorption band in the region between 210 and 260 nm.

The absorption is at a maximum at 242.5 nm. It is this wavelength that is

usually reported in the chemical literature max.


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Expressions Used in Ultraviolet SpectrometryExpressions Used in Ultraviolet Spectrometry

The spectrum shows that the scan is from 200-400 nm.

Because absorption by atmospheric carbon dioxidebecomes significant below 200 nm, the 100-200 nm region is

usually not scanned unless special air-free techniques areemployed.


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The wavelength of absorption is usually reported as max whichrepresents the wavelength at the highest point of the curve.

The absorption of energy is reported as absorbance (nottransmittance as in infrared spectra).

The absorbance at a particular wavelength is defined by the

equation:

The absorbance by a compound at a particular wavelengthincreases with an increasing number of molecules undergoing

transitions.


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Therefore, the absorbance depends on the electronic structure of thecompound and also upon the concentration of the sample and the lengthof the sample cell.

Usually, energy absorption is reported as molarmolar absorptivityabsorptivity (alsocalled molar extinction coefficient) rather as the actual absorbance.

The molar absorptivity (usually reported at max) is a reproducible value

that makes into account concentration and cell length.

It is simply the proportionality constant that relates the observedabsorbance (A) at a particular wavelength () to the molar concentration

(c) of the sample and length (l) (in centimeter) of the path of the lightbeam through the sample cell.


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Absorption of infrared radiation by a molecule leads to

increased vibrations of covalent bonds.

Molecular transitions from the ground state to an excited state requiresabout 2-15 kcal/mol.

Both UV and visible radiation are of higher energy than IR

radiation.

Absorption of UV or visible light results in electronicelectronic

transitionstransitions; electrons are promoted from low-energy ground

state or orbitals to higher-energy excited-state orbitals.

These transitions require about 40-300 kcal/mol.

The energy absorbed is subsequently dissipated as heat, as light(e.g., fluorescence), or in chemical reactions (such as isomerization or

free-radical reactions) or in dissociation or ionization of the molecule.

The structural unit associated with an electronic transitionin the UV-Vis spectroscopy is called a chromophore.


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The wavelength of UV or visible light absorbed depends on

the ease of electron promotion.

Molecules that require more energy for electron promotionabsorb at shorter wavelengths.

Compounds that absorb light in the visible region (that is

colored compounds) have more-easily promoted electronsthan compounds that absorb at shorter UV wavelengths.

colorless


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Although the energy absorption by a molecule is quantized, aUV or visible spectrum consists of not a spectrum of lines orsharp peaks but rather of broad absorption bands over a

wide range of wavelength.

The reason for the broad absorption is that the energy levelsof both the ground state and the excited state of a molecule

are subdivided into rotational and vibrational sublevels.

Electronic transitions may occur from any of the sublevels ofthe ground state to any one of the sublevels of an excited

state.

That is, a discrete line is not obtained since electronicabsorption is superimposed on rotational and vibrational

sublevels.

Since these various transitions differ slightly in energy, theirwavelengths of absorption also differ slightly and give rise to

the broad band observed in the spectrum.


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Types of Electron TransitionsTypes of Electron Transitions

The ground state of an organic molecule contains valence

electrons in three principal types of molecular orbitals: sigma

() orbitals; pi () orbitals; and filled but nonbonded orbitals (n).

Both and orbitals are formed from the overlap of twoatomic or hybrid orbitals. Each of these molecular orbitals

therefore has an antibonding * or* orbital associated with it.

An orbital containing n electrons does not have an antibonding

orbital (because it was not formed from two orbitals).


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Electron transitions involve the promotion of an electron from

one of the three ground states (, , or n) to one of the two

excited states (,

or

). There are six possible transitions; the four important

transitions and their relative energies are:


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The most useful region of the UV spectrum is at wavelengths

longer than 200 nm.

The following transitions give rise to absorption in thenonuseful 100-200 nm range:

* for an isolated double bond, and

* for an ordinary carbon-carbon bond.

The useful transitions (200 nm-400 nm) are * for

compounds with conjugated double bonds, and some n*

and some n* transitions.

Alkenes and nonconjugated dienes usually have absorption

maxima below 200 nm.

Example:Example:

Ethene gives an absorption maximum at 171 nm,

1,4-pentadiene gives an absorption maximum at 178 nm.

Ab ti bAb ti b P lP l


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Absorption byAbsorption by PolyenesPolyenes

Compounds whose molecules contain conjugated multiple bonds have

absorption maxima at wavelengths longer than 200 nm. For example, less energy is required to promote a electron of 1,3-

butadiene than is needed to promote a electron of ethylene.

The reason is that the energy gap between the HOMO and the LUMO for

conjugated double bonds is less than the energy difference for an isolateddouble bond.

Resonance-stabilization of the excited state of a conjugated diene is one

factor that decreases the energy of the excited state.

max at 217 nm

max at 171 nm

Because less energy is needed for a * transition of 1,3-butadiene, this diene absorbs

UV radiation of longer wavelengths than does ethylene.


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The UV spectrum

of

cis,trans-1,3-cyclooctadiene

The * transition in cis,trans-1,3-

cyclooctadiene involves excitation ofan electron from the HOMO to LUMO

S ffi i t j ti hift th b ti t l th th t h i t


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Sufficient conjugation shifts the absorption to wavelengths that reach into

the visible region of spectrum.

The compound responsible for the red color of tomatoes

The compound responsible for the color of carrot


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General Rule:General Rule: the greater the number of conjugated multiple


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General Rule:General Rule:the greater the number of conjugated multiplebonds a compound contains, the longer will be the

wavelength at which the compound absorbs light.

Absorption b Compo nds ith C O BondsAbsorption by Compounds with C O Bonds


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Absorption by Compounds with C=O BondsAbsorption by Compounds with C=O Bonds

The carbonyl groups of saturated aldehydes and ketones

give a weak absorption band in the UV region between 270and 300 nm.

Aldehydes and ketones have two absorption bands in theultraviolet region. Both involve excitation of an electron to an

antibonding * orbital (n* and * ).

This band is shifted to longer wavelengths (300-350 nm)

when the carbonyl group is conjugated with a double bond.


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Also compounds in which the carbon-oxygen double bond is conjugated


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Also, compounds in which the carbon oxygen double bond is conjugated

with a carbon-carbon double bond have absorption maxima

corresponding to n* excitation and * excitations.

The n* absorption maxima occur at larger wavelengths but are much

weaker (i.e., smaller molar absorptivities)

Absorption by Aromatic SystemsAbsorption by Aromatic Systems


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Absorption by Aromatic SystemsAbsorption by Aromatic Systems

The conjugated electrons of a benzene ring give

characteristic ultraviolet absorptions that indicate thepresence of a benzene ring in an unknown compound.

Benzene and other aromatic compounds exhibit more-

complex spectra than can be explained by simple *transitions.

The complexity arises from the existence of several low-lying

excited states.

One absorption band of moderate intensity occurs near 205nm and another, less intense band appears in the 250-275

nm range.

Conjugation outside the benzene ring leads to absorptions at

other wavelengths.


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Absorption Arising from Transitions ofAbsorption Arising from Transitions of nn ElectronsElectrons


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Absorption Arising from Transitions ofAbsorption Arising from Transitions of nnElectronsElectrons

Compounds that contain nitrogen, sulfur, phosphorous, or one

of the halogens all have unshared n electrons.

If the structure contains no bonds, these n electrons can

undergo only n* transitions.

Because the n electrons are of higher energy than either or

electrons, less energy is required to promote an n electrons,

and transitions occur at longer wavelengths than *transitions.

The * orbital is of lower energy than the * orbital;


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The orbital is of lower energy than the orbital;

consequently, n* transitions require less energy than

n* transitions and often are in the range of a normalinstrument scan.

The n electrons are in a different region of space from * and

* orbitals, and the probability of an n transition is low.

Since molar absorptivity depends on the number of electrons

undergoing transitions, values forn transitions are low, in

the 10-100 range(compared to about 10,000 for a * transition).

A compound such as acetone that contains both a bond andn electrons exhibits both * and n* transitions.

Acetone shows absorption at 187 nm (*) and 270 nm

(n*)


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187 nm270 nm

Absorption by AlcoholsAbsorption by Alcohols


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Absorption by AlcoholsAbsorption by Alcohols

Unless the molecule has other chromophores, alcohols are

transparent above about 200 nm. Example:max for methanol is 177 nm.

Absorption by Ethers andAbsorption by Ethers and EpoxidesEpoxides

Simple ethers have their absorptions maximum at about 185

nm and are transparent to ultraviolet radiation above about

220 nm.

Absorption by AminesAbsorption by Amines


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Absorption by AminesAbsorption by Amines

In the absence of any other chromospheres, the UV-Vis spectrum of an

alkylamine is not very informative.

The longest wavelength absorption involves promoting one of the

unshared electrons of nitrogen to an antibonding * orbital (n *) with a

max in relatively inaccessible region near 200 nm. In arylamines the interaction of the nitrogen lone pair with the -electron

system of the ring shifts the rings absorptions to longer wavelengths.

Tying up the lone pair by protonation causes the UV-Vis spectrum ofanilinium to resemble benzene.

Absorption by PhenolsAbsorption by Phenols


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Absorption by PhenolsAbsorption by Phenols

An OH group affects the UV-Vis spectrum of benzene in a

way similar to that of an NH2 group, but to a smaller extent.

In basic solution, in which OH is converted to O_, however,

the shift to longer wavelengths exceeds that of an NH2group.

Absorption by Carboxylic Acid and CarboxylicAbsorption by Carboxylic Acid and Carboxylic


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Absorption by Carboxylic Acid and Carboxylicbso pt o by Ca bo y c c d a d Ca bo y c

Acid DerivativesAcid Derivatives

In the absence of any additional chromophores, carboxylic

acids absorb at a wavelength (210 nm) that is not very useful

for diagnostic purposes.

The following values are typical for the n * absorptionassociated with C=O group of carboxylic acid derivatives.

Analytical Uses of UVAnalytical Uses of UV--VisVis SpectroscopySpectroscopy


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In practice, ultraviolet spectrometry is limited to conjugated

systems for the most part, and UV-Vis spectroscopy can beused in the structure elucidation of organic molecules toindicate whether conjugation is present in a given sample.

Although conjugation in a molecule may be indicated by data

from IR, NMR, or mass spectrometry, UV-Vis analysis canprovide corroborating information.

A more widespread use of UV-Vis spectroscopy, however,

has to do with determining concentration of an unknownsample.

The relationship A =A =ClClindicates that the amount ofabsorption by a sample at a certain wavelength is dependent

on its concentration.

Using calibration curve ofmax versus concentration ofstandards, concentration of an unknown sample could be

determined.

Quantitative analysis using UV-Vis spectroscopy is routinelyd i bi h i l t di t th t f


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used in biochemical studies to measure the rates ofenzymatic reactions (kinetics).

The concentration of a species involved in the reaction (asrelated to its UV-Vis absorbance) is plotted versus time todetermine the rate of reaction.

UV-Vis spectroscopy is also used in environmental chemistryto determine the concentration of various metal ions

(sometimes involving absorption spectra for organiccomplexes with the metal), as a setection method in HPLC.

There is an advantage tothe selectivity of ultraviolet


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the selectivity of ultravioletabsorption; characteristicgroups may be recognized

in molecules of widelyvarying complexities.

A large portion of a

relatively complex moleculemay be transparent in theultraviolet so that we mayobtain a spectrum similar tothat of a much simpler

molecules.

The absorption results from the

conjugated enone portion of the

two compounds


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THE ENDTHE END

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Files 3-Lecture Notes CHEM-303 UV Spectroscopy