UV Spectroscopy: Empirical Approach to Molecular Structures

Dr. Mishu Singh

Department of Chemistry

M. P.Govt P. G.College, Hardoi

WHAT IS SPECTROSCOPY ? • Atoms and molecules interact with electromagnetic radiation (EMR) in a

wide variety of ways.

• Atoms and molecules may absorb and/or emit EMR.

• Absorption of EMR stimulates different types of motion in atoms and/or

molecules.

• The patterns of absorption (wavelengths absorbed and to what extent)

and/or emission (wavelengths emitted and their respective intensities) are

called ‘spectra’.

• spectroscopy is the interaction of EMR with matters to get specta ,which

gives information like, bond length, bond angle, geometry and molecular

structure.

The entire electromagnetic spectrum is used by chemists:

UV X-rays IR Radio Microwave

Visible

nuclear excitation (PET)

core electron excitation (X-ray cryst.)

electronic excitation (p to p*)

molecular vibration

molecular rotation

Nuclear Magnetic Resonance NMR (MRI)

g-rays

Electromagnetic radiations, are a form of energy, displays the properties of both

,particles and waves. The particle component is called a photon

The term “photon” is implied to mean a small, massless particle that contains a small

wave-packet of EM radiation/light –

. The important parameters associated with electromagnetic radiation are: Energy (E):

Frequency (n) E = hn Wavelength (l)

l= distance of one wave

n = frequency: waves per unit time (sec-1, Hz)

c = speed of light (3.0 x 108 m • sec-1)

h = Plank’s constant (6.63 x 10-34 J • sec)

5

: Energy is directly proportional to frequency, and inversely proportion to wavelength, as indicated by the equation below

l

UV-Vis: valance electron transitions - gives information about p-bonds and conjugated systems

Infrared: molecular vibrations (stretches, bends) - identify functional groups

Radiowaves: nuclear spin in a magnetic field (NMR) - gives a map of the H and C framework

organic molecule

organic molecule

(excited state)

organic molecule(gro

und state)

relaxation light

hn

Principles of molecular spectroscopy

Ultra-Violet Spectroscopy

• It is a branch of spectroscopy in which transition occur due to the excitation

of electrons from one energy level to higher one by the interaction of

molecules with ultraviolet and visible lights.

• Absorption of photon results in electronic transition of a molecule, and

electrons are promoted from ground state to higher electronic states.

• Since it involves an electron excitation phenomenon, so, also called as

Electronic Spectroscopy.

HOMO

LUMO

hv E

Range of UV-Visible Spectrum

Visible Mid UV Far UV

800 nm 400 nm 200nm 100 nm

• Violet: 400 - 420 nm

• Indigo: 420 - 440 nm

• Blue: 440 - 490 nm

• Green: 490 - 570 nm

• Yellow: 570 - 585 nm

• Orange: 585 - 620 nm

• Red: 620 - 780 nm

Absorption in UV Spectroscopy

Absorption of light in U V region is governed by Lambert’s-Ber’s law.

When a abeam of monochromatic light is passed through the homogenous

medium of substance, a fraction of light is absorbed. The decreased in the intensity of

light with the concentration of medium depends on the thickness and concentration

of the medium and the phenomenon is given by the equation:

A = - logT = ebc A = absorbance ( optical density ) ; and A = log I0 / I, T= Transmittance C= concentration of solutios b = length of the sample tube = molar extinction coefficient I0 = Intensity of incident light I = Intensity of transmitted light

Absorption in UV Spectroscopy

There are three types of electronic transition which can be considered;

1. Transitions involving charge-transfer electrons

2. Transitions involving d and f electrons

3. Transitions involving p, s, and n electrons

Electronic transitions

.

Absorption of ultraviolet and visible radiation in organic molecules is

restricted to certain functional groups (chromophores) that contain valence

electrons of low excitation energy.

Vacuum UV or Far UV (λ<190 nm )

Selection Rules

1. Not all transitions that are possible are observed. Only those transitios which follow certain rule ,are called as allowed transitions.

2. For an electron to transition, certain quantum mechanical

constraints apply – these are called “selection rules”

3. For example, an electron cannot change its spin quantum number during a transition – these 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

Transition Probability ( Allowed and Forbidden Transitions ) The molar extinction coefficient, ξ depends upon:

ξ max = 0.87 x 1020 x P x a

Where, P = transition Probability varies from 0 - 1 a = target area of absorbing system(Chromophores) Depending upon the value of ξ , and symmetry of orbital , the transition may be allowed or forbidden If , ξ > 104 Strong / high intensity peak ( allowed transition) ξ ~ 103 - 104 Medium / Moderate intensity peak( allowed Trans.) ξ < 103

; weak/low intensity peak ( forbidden transition)

Transitions s->s*

– UV photon required, high energy, shorter wavelength

• Methane at 125 nm ( All saturated comp )

• Ethane at 135 nm

n-> s*– Saturated compounds with unshared e-

N, O, S and Halogens, • Absorption between 150 nm to 250 nm • e between 100 and 3000 L cm-1 mol-1 • Shifts to shorter wavelengths with polar solvents

• R-X, R-OH R-NH2

Transitions

• n->p*and p->p*

– Unsaturated Organic compounds, containing non-bonded electrons (O, N , S) and X

– wavelengths 200 to 700 nm

• n->p*low e (10 to 100),

• Shorter wavelengths 200 - 400

– Shorter wavelengths

• p->p*higher e (1000 to 10000) , 200- 260

Longer wavelength as compared to n->p*

Presentation of U V Spectra

TERMINOLOGIES Chromophores and Auxochromes

1. A functional group capable of having characteristic electronic transitions is

called a chromophore (color loving)

C=C ; C=N, C=S , C=C; N=N etc 2. Structural or electronic changes in the chromophore can be quantified

and used to predict shifts in the observed electronic transitions

3. The attachment of substituent groups (other than H) can shift the energy of the transition

4. Substituent's ,that could not absorbed light in UV region, but,increase the intensity and often wavelength of an absorption are called auxochromes

5. Common auxochromes include alkyl, hydroxyl, alkoxy and amino

groups and the halogens

•

200 nm 700 nm

Hy

per

chro

mic

Hy

po

chro

mic

Bathochromic Hypsochromic

Substituents 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 effect – an increase in intensity IV. Hypochromic effect – a decrease in intensity

Ethylene Chromophores

CH2=CH2 174 CH3-(CH2)5-CH=CH2 178 181 184 183 CH2=CH-Cl 186 CH2=CH-OCH3 190 CH2=CH-S-CH3 210

Conjugation And Ethylene Chromophores Ethylene absorbs at 175 nm. Presence of either chromophores or

auxochromes gives bathochromic shift.

lmax nm

175 15,000

217 21,000

258 35,000

465 125,000

Lmax = 2 17 253 220 227 227 256 263 nm

EXPLANATION OF HIGHER WAVE LENHTH FOR CONJUGATION

When we consider butadiene, we are now mixing 4 p orbitals giving 4 MOs of an

as compared to ethylene which has only 2 M Os

Y2*

pY1Y1

Y2

Y3*

Y4*

DE for the HOMO LUMO transition is reduced So, lmax is increased .

HOMO

HOMO

LUMO

LUMO

217 nm

Extending this effect to longer conjugated systems the energy gap becomes progressively smaller:

Energy

ethylene

butadiene

hexatriene

octatetraene

Lower energy = Longer wavelengths

Empirical Approach To Structure Determination

WOODWARD-FEISER RULE

*Woodward ( 1914 ) : gave certain rules for correlate with molecular structures with lmax . *Scott-Feiser(1959):modified rule with more experimental data , the modified rule is known as Woodward-Feiser rule. *A more modern interpretation was compiled by Rao in 1975 – (C.N.R. Rao, Ultraviolet and Visible Spectroscopy, 3rd Ed., Butterworths, London, 1975) It is used to correlate thelmax of for a given structure by relating position and degree of substitution of chromophores.

Woodward-Fieser Rules for Conjugated acyclic Dienes Woodward had predicted an empirical rule for calculating lmax of conjugated acyclic and cyclic

dienes based on base value and contribution of different substituent. The equation is:

lmax = Base value + ∑ Substituent's contribution + ∑ Other contribution

Base value : Take a base value of 214/217 for any conjugated. diene or triene

Incrementals: Add the following to the base value Group Increment

Extended conjugation +30

Each exo-cyclic C=C

Phenyl group

Auxochromes:

+5

+60

Alkyl +5

-OCOCH3 +0

-OR +6

-SR +30

-Cl, -Br +5

-NR2 +60

Examples:

Isoprene - acyclic butadiene = 217 nm

one alkyl subs. + 5 nm 222 nm Experimental value 220 nm Allylidenecyclohexane - acyclic butadiene = 217 nm one exocyclic C=C + 5 nm 2 alkyl subs. +10 nm 232 nm Experimental value 237 nm

Woodward-Fieser Rules – Cyclic Dienes There are two major types of cyclic dienes, with two different base values

Heteroannular (transoid): Homoannular (cisoid): base lmax = 214 base lmax = 253 base lmax = 253

Groups Increment

Extended conjugation

Pheny group

+30

+60

Each exo-cyclic C=C +5

Alkyl / ring residue +5

-OCOCH3 +0

-OR +6

-SR +30

-Cl, -Br +5

-NR2 +60

Additional homoannular +39

Eexamples: 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 234 nm Experimental value 235 nm

C

O

OH

heteroannular diene = 214 nm

1 alkyl subs. (1 x 5) + 05

3 RR + 15

1 exo C=C +0 5 nm

239 nm

homoannular diene = 253 nm

1alkyl subs. (1x 5) +05nm

3 RR +15nm

1 exo C=C + 5 nm

278 nm

C

O

OH

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

Be careful with your assignments – three common errors:

Base - Transoid/Heteroannular + 215 nm

Substituents- 5 Ring Residues 1 Double bond extending conjugation

5 x 5 = + 25 nm + 30 nm

Other Effects- 3 Exocyclic Double Bond + 15 nm

Calculated λmax 285 nm

Observed λmax 283 nm

Component Contribution

Core- Homoannular/Cisoid diene

253 nm

Substituents- 5 alkyl substituents Double bond extending conjugation

5 x 5 = + 25 nm + 30 nm

Other Effects- 3 Exocyclic double bonds

3 x 5 = + 15 nm

Calculated λmax 323 nm

Observed λmax 321

Limitations of Woodwards rule for dienes

The Woodward rule is applicable to conjugated acyclic dienes with a maximum no. of 04 double bonds , it fails beyond that. Conjugated dienes more than 04 double bonds ; Fischer and Khun rule is applicable. λmax = 114 + 5M + n (48.0 – 1.7 n) – 16.5 Rendo – 10 Rexo εmax = (1.74 x 104) n

M = the number of alkyl substituents and ring residues n = the number of conjugated double bonds Rendo = the number of rings with endocyclic double bonds Rexo = the number of rings with exocyclic double bonds

Name of Compound all-trans-lycophene

Base Value 114 nm

M (number of alkyl & RR substituents) 8

n (number of conjugated double bonds) 11

Rendo (number of endocyclic double bonds) 0

Rexo (number of exocyclic double bonds) 0

Substituting in equation λmax = 114 + 5M + n (48.0 – 1.7 n) – 16.5 Rendo – 10 Rexo

= 114 + 5(8) + 11 (48.0-1.7(11)) – 16.5 (0) – 10 (0) = 114 + 40 + 11 (29.3) – 0 – 0 = 114 + 40 + 322.3 – 0 Calc. λmax = 476.30 nm

λmax observed practically 474 nm

Calculate εmax using equation: εmax = (1.74 x 104) n

= (1.74 x 104) 11 Calc. εmax= 19.14 x 104

Practically observed εmax 18.6 x 104

Name of Compound β-Carotene

Base Value 114 nm

M (number of alkyl & RR substituents) 10

n (number of conjugated double bonds) 11

Rendo (number of endocyclic double bonds) 2

Rexo (number of exocyclic double bonds) 0

Substituting in equation λmax = 114 + 5M + n (48.0 – 1.7 n) – 16.5 Rendo – 10 Rexo

= 114 + 5(10) + 11 (48.0-1.7(11)) – 16.5 (2) – 10 (0) = 114 + 50 + 11 (29.3) – 33 – 0 = 114 + 50 + 322.3 – 33

Calc. λmax = 453.30 nm

λmax observed practically 452nm

Calculate εmax using equation: εmax = (1.74 x 104) n

= (1.74 x 104) 11 Calc. εmax= 19.14 x 104

Practically observed εmax 15.2 x 104

p

p*

n

Enones Enone, called as alpha beta unsaturated carbonyl compound shows two

types of electronic transitions. The, p p* and n p* ; Both show longer λmax than isolated enone

Conjugation of a double bond with a

carbonyl group leads to

π→π*, at 200 ~ 250 nm absorption (ε =

8,000 to 20,000), Predictable

And

n→π*, at 210 ~ 330 nm, much less intense

(ε= 50 to 100), not predictable

The effects of conjugation are apparent from the MO diagram for an enone:

pY1

Y2

Y3*

Y4*

p*

n

p

p*

n

OO

E

Woodward-Fieser Rules - Enones

Group Increment

6-membered ring or acyclic enone Base 215 nm

5-membered ring parent enone Base 202 nm

Acyclic dienone Base 245 nm

Aldehyde,Ester & Acid

Substituents:

Base 208 nm

Double bond extending conjugation

Phenyl group

b

30

60

Alkyl group or ring residue a,b,gand higher 10, 12, 18

-OH a,b,gand 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

-NR2 b 95

Exocyclic double bond 5

Homocyclic diene component 39

C C Cb

b a

C C CC

b a

C

gd

dO O

SOLVENT CORRECTION Unlike conjugated alkenes, solvent does have an effect on lmax

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

Common solvent cuttoff:

acetonitrile 190 chloroform 240 cyclohexane 195 1,4-dioxane 215 95% ethanol 205 n-hexane 201 methanol 205 isooctane 195 water 190

Examples keep in mind these are more complex than dienes cyclic enone = 215 nm

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

239 nm

Experimental value 238 nm

cyclic enone = 215 nm

extended conj. +30 nm

b-ring residue +12 nm

d-ring residue +18 nm

exocyclic double bond + 5 nm

280 nm

Experimental 280 nm

O

R

O

Base - cyclopentenone + 202 nm

Substituents at α-position 0

Substituents at β-position- 2 Ring Residue

2 x 12= + 24 nm

Other Effects- 1 Exocyclic Double Bond

+ 5 nm

Calculated λmax 231 nm

Observed λmax 229 nm

Base - cyclohexenone + 215 nm

Substituents at α-position: 0

Substituents at β-position: 1 Ring Residue + 12 nm

Substituents at γ-position

0

Substituents at δ-position: 0

Substituents at ε-position: 1 Ring Residue + 18 nm

Substituents at ζ-position: 2 Ring Residue 2 x 18 = + 36 nm

Other Effects: 2 Double bonds extending conjugation Homoannular Diene system in ring B 1 Exocyclic double bond

2 x 30 = + 60 nm + 35 nm +5

Calculated λmax 381 nm

Observed λmax 388 nm

Problem – can these two isomers be discerned by UV-spectroscopy ?

O

O

Eremophilone allo-Eremophilone

Aromatic Compounds Benzene gives three bands in UV absorption.One would expect there to be four

possible HOMO-LUMO p p* transitions at observable wavelengths

(conjugation) Due to symmetry concerns and selection rules, the actual transition energy

states of benzene are illustrated at the right:

p4* p5*

p6*

p2

p1

p3260 nm (forbidden)

200 nm (forbidden)

180 nm (allowed)

Substituent Effects Aromatic Compound No matter what electronic influence a group exerts, EWG or EDG, their

presence shift l towards longer side.

Substituent lmax

H 203.5

-CH3 207

-Cl 210

-Br 210

-OH 211

-OCH3 217

-NH2 230

-CN 224

C(O)OH 230

-C(O)H 250

-C(O)CH3 224

-NO2 269

50

Polynuclear aromatics • When the number of fused aromatic rings increases, the lmax

for the primary and secondary bands also increased. • For heteroaromatic systems spectra ,become complex

Woodward Rule for Aromatic Ketones

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

Base value = 246 nm

Ring residue in o- position = 1 x 3 = 3 nm

Polar group -OCH3 in p- position = 25 nm

λmax = 274 nm Observed = 274 nm

PROBLEMS

1. The enol acetate (ester) of the following compounds has λmax at 238nm, suggest

the structure of the ester.

2. The following triene on practical hydrogenation gives three products, separated by

GLC. How UV spectra anWoodwards rule will help to identify the products.

3.. Pentene-1 absorbs at λmax 178nm. Three isomeric dienes A,B and C of molecular

composition C5H8 absorbs at:

λmax , for A=179nm

for B=217nm

for C=219nm

Find out A, B and C

4. Four isomeric ketones A, B , C and D having molecular formula C6H8O The

UV spectra gives absorption at 228 nm. Which is probable structure of the

ketone.

Fluorescence is a process that involves a singlet to singlet transition.

Phosphorescence is a process that involves a triplet to singlet transition

• Fluorescence: absorption of radiation to

an excited state, followed by emission of

radiation to a lower state of the same

multiplicity

• Phosphorescence: absorption of

radiation to an excited state, followed by

emission of radiation to a lower state of

different multiplicity

• Singlet state: spins are paired, no net

angular momentum (and no net magnetic

field)

• Triplet state: spins are unpaired, net

angular momentum (and net magnetic

field)

More Complex Electronic Processes