Kuliah Uv Vis

8/11/2019 Kuliah Uv Vis

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Rurini RetnowatiMolecule Organic Spectroscopy

7 MARET 2011

UV Spectroscopy UV & electronic transitions Usable ranges & observations

Selection rules Band Structure Instrumentation & Spectra Beer-Lambert LawApplication of UV-spec


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UV Spectroscopy

I. IntroductionA. UV radiation and Electronic Excitations

1. The difference in energy between molecular bonding, non-bonding and

anti-bonding orbitals ranges from 125-650 kJ/mole

2. This energy corresponds to EM radiation in the ultraviolet (UV) region,100-350 nm, and visible (VIS) regions 350-700 nm of the spectrum

3. For comparison, recall the EM spectrum:

4. Using IR we observed vibrational transitions with energies of 8-40 kJ/molat wavelengths of 2500-15,000 nm

5. For purposes of our discussion, we will refer to UV and VIS spectroscopyas UV

UVX-rays IR-rays RadioMicrowave

Visible


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UV Spectroscopy

I. IntroductionB. The Spectroscopic Process

1. In UV spectroscopy, the sample is irradiated with the broad spectrum of

the UV radiation2. If a particular electronic transition matches the energy of a certain band

of UV, it will be absorbed3. The remaining UV light passes through the sample and is observed4. From this residual radiation a spectrum is obtained with gaps at these

discrete energiesthis is called an absorption spectrum


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UV Spectroscopy

I. IntroductionC. Observed electronic transitions

1. The lowest energy transition (and most often obs. by UV) is typically that

of an electron in the Highest Occupied Molecular Orbital (HOMO) to theLowest Unoccupied Molecular Orbital (LUMO)

2. For any bond (pair of electrons) in a molecule, the molecular orbitals area mixture of the two contributing atomic orbitals; for every bondingorbital created from this mixing (s, ), there is a corresponding anti-

bonding orbital of symmetrically higher energy (s*, *)

3. The lowest energy occupied orbitals are typically the s; likewise, thecorresponding anti-bonding sorbital is of the highest energy

4. -orbitals are of somewhat higher energy, and their complementary anti-

bonding orbital somewhat lower in energy than s*.

5. Unshared pairs lie at the energy of the original atomic orbital, most oftenthis energy is higher than or s(since no bond is formed, there is nobenefit in energy)


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UV Spectroscopy


6. Here is a graphical representation

Energy

s

s

nAtomic orbitalAtomic orbital

Molecular orbitals

Occupied levels

Unoccupied levels


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UV Spectroscopy


7. From the molecular orbital diagram, there are several possible electronic

transitions that can occur, each of a different relative energy:

Energy

s

s

n

s

s

n

n

s

s

alkanes

carbonyls

unsaturated cmpds.

O, N, S, halogens

carbonyls


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UV Spectroscopy


7. Although the UV spectrum extends below 100 nm (high energy), oxygen

in the atmosphere is not transparent below 200 nm

8. Special equipment to study vacuumor far UVis required

9. Routine organic UV spectra are typically collected from 200-700 nm

10. This limits the transitions that can be observed:

s

s

n

n

s

s

alkanes

carbonyls

unsaturated cmpds.

O, N, S, halogens

carbonyls

150 nm

170 nm

180 nm - if conjugated!

190 nm

300 nm


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UV Spectroscopy

I. IntroductionD. Selection Rules

1. Not all transitions that are possible are observed

2. For an electron to transition, certain quantum mechanical constraintsapplythese are called selection rules

3. For example, an electron cannot change its spin quantum number duringa transitionthese are forbidden

Other examples include: the number of electrons that can be excited at one time symmetry properties of the molecule symmetry of the electronic states

4. To further complicate matters, forbidden transitions are sometimes

observed (albeit at low intensity) due to other factors


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UV Spectroscopy

I. IntroductionE. Band Structure

1. Unlike IR (or later NMR), where there may be upwards of 5 or more

resolvable peaks from which to elucidate structural information, UV tendsto give wide, overlapping bands

2. It would seem that since the electronic energy levels of a pure sample ofmolecules would be quantized, fine, discrete bands would be observedfor atomic spectra, this is the case

3. In molecules, when a bulk sample of molecules is observed, not all bonds(readpairs of electrons) are in the same vibrational or rotational energystates

4. This effect will impact the wavelength at which a transition is observed

very similar to the effect of H-bonding on the O-H vibrational energylevels in neat samples


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UV Spectroscopy

I. IntroductionE. Band Structure

5. When these energy levels are superimposed, the effect can be readily

explainedany transition has the possibility of being observed

Energy

Vo

V4

V3

V2

V1

Disassociation

R1- Rn

R1- Rn

R1- Rn

R1- Rn

R1- RnE0

E1 Vo

V4

V3

V2

V1

Disassociation

R1- Rn

R1- Rn

R1- Rn

R1- Rn

R1- Rn


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UV Spectroscopy

II. Instrumentation and SpectraA. Instrumentation

1. The construction of a traditional UV-VIS spectrometer is very similar to an

IR, as similar functionssample handling, irradiation, detection andoutput are required

2. Here is a simple schematic that covers most modern UV spectrometers:

sample

reference

de

tector

I0

I0 I0

Ilog(I0/I) = A

200 700l

, nm

monochromator/

beam splitter optics

UV-VIS sources


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UV Spectroscopy


3. Two sources are required to scan the entire UV-VIS band:

Deuterium lampcovers the UV200-330 Tungsten lampcovers 330-700

4. As with the dispersive IR, the lamps illuminate the entire band of UV orvisible light; the monochromator (grating or prism) gradually changes thesmall bands of radiation sent to the beam splitter

5. The beam splitter sends a separate band to a cell containing the samplesolution and a reference solution

6. The detector measures the difference between the transmitted lightthrough the sample (I) vs. the incident light (I0) and sends this

information to the recorder


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UV Spectroscopy


7. As with dispersive IR, time is required to cover the entire UV-VIS band

due to the mechanism of changing wavelengths

8. A recent improvement is the diode-array spectrophotometer - here aprism (dispersion device) breaks apart the full spectrum transmittedthrough the sample

9. Each individual band of UV is detected by a individual diodes on a siliconwafer simultaneouslythe obvious limitation is the size of the diode, sosome loss of resolution over traditional instruments is observed

sample

Polychromator

entrance slit and dispersion device

UV-VIS sources

Diode array


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UV Spectroscopy

II. Instrumentation and SpectraB. InstrumentationSample Handling

1. Virtually all UV spectra are recorded solution-phase

2. Cells can be made of plastic, glass or quartz

3. Only quartz is transparent in the full 200-700 nm range; plastic and glassare only suitable for visible spectra

4. Concentration (we will cover shortly) is empirically determined

A typical sample cell (commonly called a cuvet):

S


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UV Spectroscopy


5. Solvents must be transparent in the region to be observed; the

wavelength where a solvent is no longer transparent is referred to as thecutoff

6. Since spectra are only obtained up to 200 nm, solvents typically onlyneed to lack conjugated systems or carbonyls

Common solvents and cutoffs:acetonitrile 190chloroform 240cyclohexane 1951,4-dioxane 21595% ethanol 205

n-hexane 201methanol 205isooctane 195water 190

UV S t


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UV Spectroscopy


7. Additionally solvents must preserve the fine structure (where it is actually

observed in UV!) where possible

8. H-bonding further complicates the effect of vibrational and rotationalenergy levels on electronic transitions, dipole-dipole interacts less so

9. The more non-polar the solvent, the better (this is not always possible)

UV S t


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UV Spectroscopy

II. Instrumentation and SpectraC. The Spectrum

1. The x-axis of the spectrum is in wavelength; 200-350 nm for UV, 200-700

for UV-VIS determinations

2. Due to the lack of any fine structure, spectra are rarely shown in theirraw form, rather, the peak maxima are simply reported as a numerical listof lamba max values or lmax

lmax= 206 nm

252

317

376

O

NH2

O

UV S t


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UV Spectroscopy


1. The y-axis of the spectrum is in absorbance,A

2. From the spectrometers point of view, absorbance is the inverse oftransmittance: A= log10(I0/I)

3. From an experimental point of view, three other considerations must bemade:

i. a longer path length, lthrough the sample will cause moreUV light to be absorbedlinear effect

ii. the greater the concentration, cof the sample, the more UVlight will be absorbedlinear effect

iii. some electronic transitions are more effective at theabsorption of photon than othersmolar absorptivity, ethis may vary by orders of magnitude

UV S t


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UV Spectroscopy


4. These effects are combined into the Beer-Lambert Law: A= ec l

i. for most UV spectrometers, lwould remain constant (standard cellsare typically 1 cm in path length)

ii. concentration is typically varied depending on the strength ofabsorption observed or expectedtypically dilutesub .001 M

iii. molar absorptivities vary by orders of magnitude: values of 104-106104-106are termed high intensity

absorptions values of 103-104are termed low intensity absorptions values of 0 to 103are the absorptions of forbidden transitions

A is unitless, so the units for eare cm-1 M-1 and are rarely expressed

5. Since path length and concentration effects can be easily factored out,absorbance simply becomes proportional to e, and the y-axis is expressedas edirectly or as the logarithm of e

UV Spectroscop


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UV Spectroscopy

II. Instrumentation and SpectraD. Practical application of UV spectroscopy

1. UV was the first organic spectral method, however, it is rarely used as a

primary method for structure determination

2. It is most useful in combination with NMR and IR data to elucidate uniqueelectronic features that may be ambiguous in those methods

3. It can be used to assay (via lmaxand molar absorptivity) the proper

irradiation wavelengths for photochemical experiments, or the design ofUV resistant paints and coatings

4. The most ubiquitous use of UV is as a detection device for HPLC; sinceUV is utilized for solution phase samples vs. a reference solvent this iseasily incorporated into LC design

UV is to HPLC what mass spectrometry (MS) will be to GC

UV Spectroscopy


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UV Spectroscopy

III. ChromophoresA. Definition

1. Remember the electrons present in organic molecules are involved in

covalent bonds or lone pairs of electrons on atoms such as O or N

2. Since similar functional groups will have electrons capable of discreteclasses of transitions, the characteristic energy of these energies is morerepresentative of the functional group than the electrons themselves

3. A functional group capable of having characteristic electronic transitions iscalled a chromophore(color loving)

4. Structural or electronic changes in the chromophore can be quantifiedand used to predict shifts in the observed electronic transitions

UV Spectroscopy


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UV Spectroscopy

III. ChromophoresB. Organic Chromophores

1. Alkanesonly 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

UV Spectroscopy


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UV Spectroscopy


2. Alcohols, ethers, amines and sulfur compoundsin the cases of simple,

aliphatic examples of these compounds the n

s* is the most oftenobserved transition; like the alkane ss* it is most often at shorter lthan 200 nm

Note how this transition occurs from the HOMO to the LUMO

s CN

sCN

nN sp

3C N

C N

C N

C N

anitbonding

orbital

UV Spectroscopy


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UV Spectroscopy


3. Alkenes and Alkynesin the case of isolated examples of these

compounds the

* is observed at 175 and 170 nm, respectively

Even though this transition is of lower energy than ss*, it is still in

the far UVhowever, the transition energy is sensitive to substitution

UV Spectroscopy


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UV Spectroscopy


4. Carbonylsunsaturated systems incorporating N or O can undergo

n

* transitions (~285 nm) in addition to

*

Despite the fact this transition is forbidden by the selection rules (e=15), it is the most often observed and studied transition for carbonyls

This transition is also sensitive to substituents on the carbonyl

Similar to alkenes and alkynes, non-substituted carbonyls undergo the* transition in the vacuum UV (188 nm, e= 900); sensitive tosubstitution effects

UV Spectroscopy


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UV Spectroscopy


4. Carbonylsn* transitions (~285 nm); * (188 nm)

n

sCOtransitions omitted for clarity

O

O

C O

It has been determinedfrom spectral studies,that carbonyl oxygenmore approximates sprather than sp2!

UV Spectroscopy


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UV Spectroscopy

III. ChromophoresC. Substituent Effects

Generalfrom our brief study of these general chromophores, only the weak

n

* transition occurs in the routinely observed UV

The attachment of substituent groups (other than H) can shift the energyof the transition

Substituents that increase the intensity and often wavelength of an

absorption are called auxochromes

Common auxochromes include alkyl, hydroxyl, alkoxy and amino groupsand the halogens

UV Spectroscopy


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UV Spectroscopy


GeneralSubstituents may have any of four effects on a chromophore

i. Bathochromic shift(red shift)a shift to longer l; lower energy

ii. Hypsochromic shift (blue shift)shift to shorter l; higher energy

iii. Hyperchromic effectan increase in intensity

iv. Hypochromic effecta decrease in intensity

200 nm 700 nm

e

Hypochromic

Hypsochromic

Hyperchromic

Bathochromic

UV Spectroscopy


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UV Spectroscopy


1. Conjugationmost efficient means of bringing about a bathochromic and

hyperchromic shift of an unsaturated chromophore:H2C

CH2

-carotene

O

O

lmax nm e175 15,000

217 21,000

258 35,000

n* 280 27* 213 7,100

465 125,000

n* 280 12* 189 900

UV Spectroscopy


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UV Spectroscopy


1. ConjugationAlkenes

The observed shifts from conjugation imply that an increase inconjugation decreases the energy required for electronic excitation

From molecular orbital (MO) theory two atomic porbitals, f1and f2fromtwo sp2hybrid carbons combine to form two MOs Y1and Y2* in ethylene

Y

2

1

f

1

f

2

UV Spectroscopy


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UV Spectroscopy



When we consider butadiene, we are now mixing 4 p orbitals giving 4MOs of an energetically symmetrical distribution compared to ethylene

Y

2

1

Y

1

Y

2

Y

3

Y

4

DE for the HOMOLUMO transition isreduced

UV Spectroscopy


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UV Spectroscopy



Extending this effect out to longer conjugated systems the energy gapbecomes progressively smaller:

Energy

ethylene

butadiene

hexatriene

octatetraene

Lower energy =Longer wavelengths

UV Spectroscopy


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UV Spectroscopy



Similarly, the lone pairs of electrons on N, O, S, X can extend conjugatedsystemsauxochromesHere we create 3 MOsthis interaction is not as strong as that of aconjugated -system

Y

2

Y

1

A

nA

Y

3

Energy


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UV Spectroscopy


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p py

Next time We will find that the effect of substituent groups can be reliably quantified

from empirical observation of known conjugated structures and applied to new

systems

This quantification is referred to as the Woodward-Fieser Rules which we will apply

to three specific chromophores:

1. Conjugated dienes

2. Conjugated dienones

3. Aromatic systems

max= 239 nm

UV Spectroscopy


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p py

IV. Structure DeterminationA. Dienes

1. General Features

For acyclic butadiene, two conformers are possibles-cis and s-trans

The s-cisconformer is at an overall higher potential energy than the s-trans; therefore the HOMO electrons of the conjugated system have lessof a jump to the LUMOlower energy, longer wavelength

s-trans s-cis

UV Spectroscopy


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p py


1. General Features

Two possible

* transitions can occur for butadiene Y2

Y3

and Y2Y4*

The Y2Y4* transition is not typically observed:

The energy of this transition places it outside the region

typically observed175 nm

For the more favorable s-transconformation, this transition isforbidden

The Y2Y3* transition is observed as an intense absorption

s-trans s-cis

175 nmforb.

217 nm 253 nm

175 nm

Y

4

Y

2

Y

1

Y

3

UV Spectroscopy


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p py


1. General Features

The Y2

Y3*

transition is observed as an intense absorption (e=20,000+) based at 217 nm within the observed region of the UV

While this band is insensitive to solvent (as would be expected) it issubject to the bathochromic and hyperchromic effects of alkylsubstituents as well as further conjugation

Consider:

lmax = 217 253 220 227 227 256 263 nm

UV Spectroscopy


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p py


2. Woodward-Fieser Rules

Woodward and the Fiesers performed extensive studies of terpene andsteroidal alkenes and noted similar substituents and structural featureswould predictably lead to an empirical prediction of the wavelength forthe lowest energy * electronic transition

This work was distilled by Scott in 1964 into an extensive treatise on the

Woodward-Fieser rules in combination with comprehensive tables andexamples(A.I. Scott, Interpretation of the Ultraviolet Spectra of NaturalProducts, Pergamon, NY, 1964)

A more modern interpretation was compiled by Rao in 1975(C.N.R.Rao, Ultraviolet and Visible Spectroscopy, 3rdEd., Butterworths, London,

1975)

UV Spectroscopy


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2. Woodward-Fieser Rules - Dienes

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

acyclic butadiene = 217 nm

The incremental contribution of substituents is added to this base valuefrom the group tables:

Group Increment

Extended conjugation +30

Each exo-cyclic C=C +5

Alkyl +5-OCOCH3 +0

-OR +6

-SR +30

-Cl, -Br +5

-NR2 +60

UV Spectroscopy


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2. Woodward-Fieser Rules - Dienes

For example:

Isoprene - acyclic butadiene = 217 nmone alkyl subs. + 5 nm

222 nmExperimental value 220 nm

Allylidenecyclohexane- acyclic butadiene = 217 nm

one exocyclic C=C + 5 nm2 alkyl subs. +10 nm

232 nmExperimental value 237 nm

UV Spectroscopy


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3. Woodward-Fieser RulesCyclic Dienes

There are two major types of cyclic dienes, with two different base values

Heteroannular (transoid): Homoannular (cisoid):

e= 5,00015,000 e= 12,000-28,000base lmax = 214 base lmax = 253

The increment table is the same as for acyclic butadienes with a couple

additions: Group Increment

Additional homoannular +39

Where both types of dieneare present, the one withthe longer lbecomes thebase

UV Spectroscopy


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In the pre-NMR era of organic spectral determination, the power of themethod for discerning isomers is readily apparent

Consider abietic vs. levopimaric acid:

C

O

OHC

O

OH

levopimaric acidabietic acid

UV Spectroscopy


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For example: 1,2,3,7,8,8a-hexahydro-8a-methylnaphthaleneheteroannular diene = 214 nm

3 alkyl subs. (3 x 5) +15 nm

1 exo C=C + 5 nm234 nm

Experimental value 235 nm

UV Spectroscopy


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C

O

OH

heteroannular diene = 214 nm


1 exo C=C + 5 nm

239 nm

homoannular diene = 253 nm


1 exo C=C + 5 nm

278 nm

C

O

OH

UV Spectroscopy


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3. Woodward-Fieser RulesCyclic DienesBe careful with your assignmentsthree common errors:

R

This compound has three exocyclicdouble bonds; the indicated bond isexocyclic to tworings

This is nota heteroannular diene; you woulduse the base value for an acyclic diene

Likewise, this is nota homooannular diene;you would use the base value for an acyclicdiene

UV Spectroscopy


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IV. Structure DeterminationB. Enones

1. General FeaturesCarbonyls, as we have discussed have two primary electronic transitions:

n

Remember, the * transition isallowed and gives a high e, but liesoutside the routine range of UVobservation

The n* transition is forbidden andgives a very low e, but can routinely be

observed


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UV Spectroscopy


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1. General FeaturesConversely, if the C=O system is conjugated both the n* and *

bands are bathochromically shifted

Here, several effects must be noted:i. the effect is more pronounced for *

ii. if the conjugated chain is long enough, the much higherintensity * band will overlap and drown out the n*

band

iii. the shift of the n* transition is not as predictable

For these reasons, empirical Woodward-Fieser rules for conjugatedenones are for the higher intensity, allowed * transition

UV Spectroscopy


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1. General FeaturesThese effects are apparent from the MO diagram for a conjugated enone:

Y

1

Y

2

Y

3

Y

4

n

n

OO


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UV Spectroscopy


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2. Woodward-Fieser Rules - EnonesAldehydes, esters and carboxylic acids have different base values thanketones

Unsaturated system Base Value

Aldehyde 208

With aor alkyl groups 220

With a,or ,alkyl groups 230With a,,alkyl groups 242

Acid or ester

With aor alkyl groups 208

With a,or ,alkyl groups 217

Group valueexocyclic a,double bond +5

Group valueendocyclic a,bond in 5or 7 membered ring

+5

UV Spectroscopy


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2. Woodward-Fieser Rules - EnonesUnlike conjugated alkenes, solvent does have an effect on l

max

These effects are also described by the Woodward-Fieser rules

Solvent correction Increment

Water +8

Ethanol, methanol 0Chloroform -1

Dioxane -5

Ether -7

Hydrocarbon -11

UV Spectroscopy


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2. Woodward-Fieser Rules - EnonesSome exampleskeep in mind these are more complex than dienes cyclic enone = 215 nm

2 x - alkyl subs. (2 x 12) +24 nm

239 nm

Experimental value 238 nm

cyclic enone = 215 nm

extended conj. +30 nm

-ring residue +12 nmd-ring residue +18 nm

exocyclic double bond + 5 nm

280 nm

Experimental 280 nm

O

R

O

UV Spectroscopy


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2. Woodward-Fieser Rules - EnonesTake home problemcan these two isomers be discerned by UV-spec

O

O

Eremophilone allo-Eremophilone

Problem Set 1: (text)1,2,3a,b,c,d,e,f,j, 4, 5, 6 (1st, 2ndand 5thpairs),8a, b, c

Problem Set 2: outside problems/key -Tuesday

UV Spectroscopy


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IV. Structure DeterminationC. Aromatic Compounds

1. General FeaturesAlthough aromatic rings are among the most widely studied and observedchromophores, the absorptions that arise from the various electronictransitions are complex

On first inspection, benzene has six -MOs, 3 filled , 3 unfilled *

4

5

6

2

1

3

UV Spectroscopy


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1. General FeaturesOne would expect there to be four possible HOMO-LUMO *

transitions at observable wavelengths (conjugation)

Due to symmetry concerns and selection rules, the actual transitionenergy states of benzene are illustrated at the right:

4

5

6

2

1

3

A

1

g

B

2u

B

1

u

E

1

u

260 nm(forbidden)

200 nm(forbidden)

180 nm(allowed)

UV Spectroscopy


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1. General FeaturesThe allowed transition (e= 47,000) is not in the routine range of UV obs.at 180 nm, and is referred to as the primary band

The forbidden transition (e= 7400) is observed if substituent effects shiftit into the obs. region; this is referred to as the second primary band

At 260 nm is another forbiddentransition (e= 230), referred toas the secondary band.

This transition is fleetingly alloweddue to the disruption of symmetry

by the vibrational energy states,the overlap of which is observedin what is called fine structure

UV Spectroscopy


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1. General FeaturesSubstitution, auxochromic, conjugation and solvent effects can causeshifts in wavelength and intensity of aromatic systems similar to dienesand enones

However, these shifts are difficult to predictthe formulation of empiricalrules is for the most part is not efficient (there are more exceptions than

rules)

There are some general qualitative observations that can be made byclassifying substituent groups --

UV Spectroscopy


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2. Substituent Effectsa. Substituents with Unshared Electrons

If the group attached to the ring bears nelectrons, they caninduce a shift in the primary and secondary absorption bands

Non-bonding electrons extend the -system throughresonancelowering the energy of transition *

More available n-pairs of electrons give greater shifts

GG G G

UV Spectroscopy


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The presence of n-electrons gives the possibility of n*transitions

If this occurs, the electron now removed from G, becomes anextra electron in the anti-bonding * orbital of the ring

This state is referred to as a charge-transfer excited state

GG G G

*

- *

*

*

UV Spectroscopy


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pH can change the nature of the substituent group deprotonation of oxygen gives more available n-pairs,

loweringtransition energy protonation of nitrogen eliminates the n-pair,

raisingtransition energy

Primary SecondarySubstituent lmax e lmax e

-H 203.5 7,400 254 204

-OH 211 6,200 270 1,450

-O- 235 9,400 287 2,600

-NH2 230 8,600 280 1,430

-NH3+ 203 7,500 254 169

-C(O)OH 230 11,600 273 970

-C(O)O- 224 8,700 268 560

UV Spectroscopy


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2. Substituent Effectsb. Substituents Capable of -conjugation

When the substituent is a -chromophore, it can interact withthe benzene -system

With benzoic acids, this causes an appreciable shift in theprimary and secondary bands

For the benzoate ion, the effect of extra n-electrons from theanion reduces the effect slightly

Primary Secondary

Substituent lmax e lmax e

-C(O)OH 230 11,600 273 970

-C(O)O- 224 8,700 268 560

UV Spectroscopy


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2. Substituent Effectsc. Electron-donating and electron-withdrawing effects

No matter what electronic influence a group exerts, thepresence shifts the primary absorption band to longer l

Electron-withdrawing groups exert no influence on theposition of the secondary absorption band

Electron-donating groups increase the land eof thesecondary absorption band

UV Spectroscopy


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2. Substituent Effectsc. Electron-donating and electron-withdrawing effects

Primary Secondary

Substituent lmax e lmax e

-H 203.5 7,400 254 204

-CH3 207 7,000 261 225

-Cl 210 7,400 264 190-Br 210 7,900 261 192

-OH 211 6,200 270 1,450

-OCH3 217 6,400 269 1,480

-NH2 230 8,600 280 1,430

-CN 224 13,000 271 1,000

C(O)OH 230 11,600 273 970

-C(O)H 250 11,400

-C(O)CH3 224 9,800

-NO2 269 7,800

Electrondonating

Electronwithdrawi

ng

UV Spectroscopy


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2. Substituent Effectsd. Di-substituted and multiple group effects

With di-substituted aromatics, it is necessary to consider bothgroups

If both groups are electron donating or withdrawing, theeffect is similar to the effect of the stronger of the two groupsas if it were a mono-substituted ring

If one group is electron withdrawing and one group electrondonating and they are para- to one another, the magnitude ofthe shift is greater than the sum of both the group effects

Consider p-nitroaniline:

H2N N

O

O

H2N N

O

O

UV Spectroscopy


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If the two electonically dissimilar groups are ortho- or meta-to one another, the effect is usually the sum of the twoindividual effects (meta- no resonance; ortho-steric hind.)

For the case of substituted benzoyl derivatives, an empiricalcorrelation of structure with observed l

maxhas been

developed

This is slightly less accurate than the Woodward-Fieser rules,but can usually predict within an error of 5 nm

RO

G

UV Spectroscopy


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RO

G

Substituent increment

G o m p

Alkyl or ring residue 3 3 10

-O-Alkyl, -OH, -O-Ring 7 7 25

-O- 11 20 78

-Cl 0 0 10

-Br 2 2 15

-NH2 13 13 58

-NHC(O)CH3 20 20 45

-NHCH3 73

-N(CH3)2 20 20 85

Parent Chromophore lmax

R = alkyl or ring residue 246

R = H 250

R = OH or O-Alkyl 230

UV Spectroscopy


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2. Substituent Effectsd. Polynuclear aromatics

When the number of fused aromatic rings increases, thelforthe primary and secondary bands also increase

For heteroaromatic systems spectra become complex with theaddition of the n* transition and ring size effects and are

unique to each case

UV Spectroscopy


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V. Visible SpectroscopyA. Color

1. General The portion of the EM spectrum from 400-800 is observable to

humans- we (and some other mammals) have the adaptation ofseeing color at the expense of greater detail

400 500 600 800700

l, nm

Violet 400-420

Indigo 420-440

Blue 440-490

Green 490-570

Yellow 570-585

Orange 585-620

Red 620-780

UV Spectroscopy


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1. General When white (continuum of l) light passes through, or is reflected

by a surface, those ls that are absorbed are removed from thetransmitted or reflected light respectively

What is seen is the complimentary colors (those that are notabsorbed)

This is the origin of the color wheel

UV Spectroscopy


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1. General Organic compounds that are colored are typically those with

extensively conjugated systems (typically more than five)

Consider -carotene

-carotene, max= 455 nm

lmaxis at 455in the far blue region of the

spectrumthis is absorbed

The remaining light has the complementarycolor of orange

UV Spectroscopy


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1. General Likewise:

lmaxfor lycopene is at 474in the near blue region of thespectrumthis is absorbed, the compliment is now red

lmaxfor indigo is at 602in the orange region of thespectrumthis is absorbed, the compliment is now indigo!

lycopene, max = 474 nm

NH

HN

O

O

indigo

UV Spectroscopy


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1. General One of the most common class of colored organic molecules are

the azo dyes:

From our discussion of di-subsituted aromatic chromophores, theeffect of opposite groups is greater than the sum of the individual

effectsmore so on this heavily conjugated system

Coincidentally, it is necessary for these to be opposite for theoriginal synthetic preparation!

N N

EDGsEWGs

UV Spectroscopy


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OH

N

N

NO2

Para Red

NN

NH2

H2N

Fast Brown

NNO3S

HO

SO3

Sunset Yellow (Food Yellow 3)


1. General These materials are some of the more familiar colors of our

environment

The colors of M&Ms


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Bright Blue

Common Food Uses

Beverages, dairy products, powders, jellies, confections,condiments, icing.

Royal Blue

Common Food Uses

Baked goods, cereals, snack foods, ice-cream, confections,cherries.

Orange-red

Common Food UsesGelatins, puddings, dairy products, confections, beverages,condiments.

Lemon-yellow

Common Food UsesCustards, beverages, ice-cream, confections, preserves,cereals.

OrangeCommon Food Uses

Cereals, baked goods, snack foods, ice-cream, beverages,dessert powders, confections


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UV Spectroscopy

bl S


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NNO3S NCH3

CH3

NHNO3S N

CH3

CH3

Yellow, pH > 4.4 Red, pH < 3.2

Methyl Orange


1. General In the chemical sciences these are the acid-base indicators used for

the various pH ranges:

Remember the effects of pH on aromatic substituents

UV-Visible Absorption Chromophores


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ff f l i l Ch h


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Effects of Multiple Chromophores

The effects of substitution


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The effects of substitution

Auxochrome

func t ion group

Auxochrome is a funct ional group that does not abso rb in UV region but

as the effect of sh if t ing ch romoph ore peaks to longer wavelength as wel l

As in creas in their in ten sit .


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O i C d


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Organic Compounds

Most organic spectra are complex Electronic and vibration transitions superimposed Absorption bands usually broad Detailed theoretical analysis not possible, but semi-quantitative or

qualitative analysis of types of bonds is possible

Kuliah Uv Vis

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