instrumental analysis spectroscopy.ppt12

Instrumentation and Interpretation of Spectra

1

Tarekegn Berhanu (Ph. D)

1. Absorption molecular spectrophotometric techniques

Introduction to spectroscopy Light interacting with matter as an analytical tool

The Electromagnetic SpectrumThe visible spectrum constitutes but a small part of the total radiation spectrum. Most of the radiation that surrounds us cannot be seen, but can be detected by dedicated sensing instruments. This electromagnetic spectrum ranges from very short wavelengths (including gamma and x-rays) to very long wavelengths (including microwaves and broadcast radio waves).

2

X-ray: core electron excitation

UV: valance electronic excitation

IR: molecular vibrations

Radio waves:Nuclear spin states(in a magnetic field)

Electronic Excitation by UV/Vis Spectroscopy :

3

Spectroscopic Techniques and Chemistry they Probe

UV-vis UV-vis region bonding electrons

Atomic Absorption UV-vis region atomic transitions (val. e-)

FT-IR IR/Microwave vibrations, rotations

Raman IR/UV vibrations

FT-NMR Radio waves nuclear spin states

X-Ray Spectroscopy X-rays inner electrons, elemental

X-ray Crystallography X-rays 3-D structure

4

Spectroscopic Techniques and Common Uses

UV-vis UV-vis regionQuantitative

analysis/Beer’s Law

Atomic Absorption UV-vis regionQuantitative analysis

Beer’s Law

FT-IR IR/Microwave Functional Group Analysis

Raman IR/UVFunctional Group

Analysis/quant

FT-NMR Radio waves Structure determination

X-Ray Spectroscopy X-rays Elemental Analysis

X-ray Crystallography X-rays 3-D structure Anaylysis

5

EM regions related to different Analytical Techniques(a) The visible spectrum constitutes a small portion of the complete electromagnetic radiation spectrum that extends from the ultra-short

wave gamma rays at one end to that of the radio-waves at the other (400-700 nm),

(b) The wave length scale is nonlinear,(c) -Rays Region : Mossbauer Spectroscopy (due to absorption) and

- Ray Spectroscopy (due to emission) are used as analytical means.(d) Inner-shell Electrons : X-Ray absorption spectroscopy (due to absorption) and X-Ray Fluorescence spectroscopy (XRF) (due to emission) are employed as analytical means.(e) From Vacuum-UV to Infra-Red Region : UV-VIS, IR-spectroscopy, spectrophotometry, atomic absorption spectroscopy (AAS) (due to absorption) and atomic emission spectroscopy (AES, ESS, ICP) ; atomic fluorescence spectroscopy (AFS) (due to emission) are used as analytical techniques.(f) Microwave Region : Microwave spectroscopy and electron spin resonance (ESR) (due to absorption) are employed as analytical methods.(g) Radiowave Region : Nuclear Magnetic Resonance (NMR) (due to absorption) is used as analytical method.

6

The UV Absorption process• The lowest energy transition (and most often obs. by UV) is

typically that of an electron in the Highest Occupied Molecular

Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital

(LUMO)

• * and * transitions: high-energy, accessible

in vacuum UV (max <150 nm). Not usually observed in

molecular UV-Vis.

• n * and * transitions: non-bonding electrons (lone

pairs), wavelength (max) in the 150-250 nm region.

• n * and * transitions: most common transitions observed

in organic molecular UV-Vis, observed in compounds with lone

pairs and multiple bonds with max = 200-600 nm.

• Any of these require that incoming photons match in energy the gap

corresponding to a transition from ground to excited state.

• Energies correspond to a 1-photon of 300 nm light are ca. 95

kcal/mol

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9

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

2. If a particular electronic transition matches the energy of a certain band of UV, it will be absorbed

3. The remaining UV light passes through the sample and is observed

4. From this residual radiation a spectrum is obtained with “gaps” at these discrete energies – this is called an absorption spectrum

The Spectroscopic Process

Electronic TransitionIn all compounds other than alkanes, the electrons may undergo several possibleTransitions of different energies. Some of the most important transitions are:

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Selection Rules 1. Not all transitions that are possible are observed

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

4. To further complicate matters, “forbidden” transitions are sometimes observed (albeit at low intensity) due to other factors

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MOLAR ABSORPTIVITY

The molar absorptivity is mostly controlled by two vital factors, namely :(i)polarity of the excited state, and (ii) probability of the electronic transition. So as to materialize an interaction, a photon should evidently strike a molecule very closely within the space of the molecular dimensions.The probability of the electronic transition, designated as ‘g’, shall be responsible for the target hits that may ultimately lead to absorption. However, the molar absorptivity may be expressed as follows :

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SPECTRAL PRESENTATIONAbsorption spectra may be presented in a number of fashions as depicted in Figure 21.2, namely :(a) Wavelength Vs Absorbance,(b) Wavelength Vs Molar Absorptivity, and(c) Wavelength Vs Transmittance.

A few important features related to spectral presentation are enumerated below :(a) In order to simplify the conversion of spectra in qualitative identification the spectral data should be plotted either as log A or as log ∈ Vs wavelength, thereby giving rise to the following expression

where, b = Cell-length, and c = Sample concentration

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Absorption Bands

1. four different types of absorption bands have so far gained cognizance in the spectra of organic compounds, which are namely : K-bands ; R-bands ; B-bands ; and E-bands.

These bands will be discussed briefly here with regard to the structural features.(a) K-bands : They normally arise from π-π structures and result from π → π* transitions. These are invariably characterized by high molar absorptivity.Examples :(i) A diene : C = C—C = C to C+—C = C—C– ; where K-band is due to the resonance transition,(ii) Vinyl benzene or acetophenone : i.e., aromatic compounds having chromophoric substitution.(b) R-bands : They usually arise from n → π* transitions. They seldom display very noticeable results in aliphatic compounds, but marked and pronounced bathochromic shifts (i.e., shifting of absorption towards longer wavelengths—as in extended open-chain-conjugated systems) do take place when—SH, —OH and —NH2 replace hydrogen atom in unsaturated groups. Thus, R-bands help in the confirmation of a particular structure whereby additional bands are obtained by appropriate modifications in the electronic-structure of the parent compound.

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(c) B-bands : These are rather weak-type of absorption bands. They are characteristic of both heteroatomic and aromatic molecules and may also consist of fine vibrational sub-bands.(d) E-bands : They usually result from oscillations of electrons in aromatic-ring systems,Conjugated Systems :It is quite evident that the conjugated systems might fail to display the expected conjugated bands due to the following two reasons, namely :(a) Orbitals of adjacent multiple bonds are at right angles instead of being parallel, and(b) Resonating dipolar structures cannot be envisaged.The resulting spectrum may seem to appear as a mere superimposition of the spectra of the individual chromophoric groups

Absorption Bands ..

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17

UV Spectroscopy

When the energy levels are superimposed, the effect can be readily explained – any 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

18

UV Spectroscopy

II. Instrumentation and SpectraA. Instrumentation

1. The construction of a traditional UV-VIS spectrometer is very similar to an IR, as similar functions – sample handling, irradiation, detection and output are required

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

sam

ple

refe

renc

e

dete

ctor

I0

I0 I0

Ilog(I0/I) = A

200 700, nm

monochromator/beam splitter optics

UV-VIS sources

1919

UV Spectroscopy


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

4. As with the dispersive IR, 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

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

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

20

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 a prism (dispersion device) breaks apart the full spectrum transmitted through the sample

9. Each individual band of UV is detected by a individual diodes on a silicon wafer simultaneously – the obvious limitation is the size of the diode, so some loss of resolution over traditional instruments is observed

sam

ple

Polychromator – entrance slit and dispersion device

UV-VIS sources

Diode array

21

UV Spectroscopy

II. Instrumentation and SpectraB. Sample 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 glass are only suitable for visible spectra

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

A typical sample cell (commonly called a cuvet):

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Spectrometric instruments have a common set of general features. Often, one technique is distinguished from another by differences in these features. Here we look at specific features for the UV/Visible experiment.Sources: D2 lamp, W filament (halogen lamp), and Xe arc lamp.Wavelength Selectors: Filters and Monochromators.Detectors: Phototube, PMT, photodiode, photodiode array

II. Instrumentation and Spectra

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UV Spectroscopy

II. Instrumentation and SpectraC. Solvents

5. Solvents must be transparent in the region to be observed; the wavelength where a solvent is no longer transparent is referred to as the cutoff

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

Common solvents and cutoffs:acetonitrile 190chloroform 240cyclohexane 195 1,4-dioxane 21595% ethanol 205n-hexane 201methanol 205isooctane 195water 190

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Solvents can interact with the analyte molecules and shift absorbance peaks and intensities. Red Shift (Bathochromic) – Peaks shift to longer wavelength. Blue Shift (Hypsochromic) – Peaks shift to shorter wavelength. n → π* generally blue shifted by solvent; solvation of and hydrogen bonding to the lone pair. Large shifts (up to 30 nm). Both n→π* and π→π* red shifted; attractive polarization forces, increase with increasing solvent polarity. Small shifts (less than 5 nm).

II. Instrumentation and Spectra

25

UV Spectroscopy

II. Instrumentation and SpectraSolvents …

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 rotational energy levels on electronic transitions, dipole-dipole interacts less so

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

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UV Spectroscopy

II. Instrumentation and SpectraD. 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 their raw form, rather, the peak maxima are simply reported as a numerical list of “lamba max” values or max

max = 206 nm 252

317376

O

NH2

O

27

UV Spectroscopy


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

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

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

i. a longer path length, l through the sample will cause more UV light to be absorbed – linear effect

ii. the greater the concentration, c of the sample, the more UV light will be absorbed – linear effect

iii. some electronic transitions are more effective at the absorption of photon than others – molar absorptivity,

this may vary by orders of magnitude…

UV Spectroscopy


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

i. for most UV spectrometers, l would remain constant (standard cells are typically 1 cm in path length)

ii. concentration is typically varied depending on the strength of absorption observed or expected – typically dilute – sub .001 M

iii. molar absorptivities vary by orders of magnitude:• values of 104-106 104-106 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

A is unitless, so the units for are 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 , and the y-axis is expressed as directly or as the logarithm of

29

UV Spectroscopy

II. Instrumentation and SpectraE. 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 unique electronic features that may be ambiguous in those methods

3. It can be used to assay (via max and molar absorptivity) the proper irradiation wavelengths for photochemical experiments, or the design of UV resistant paints and coatings

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

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

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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 discrete classes of transitions, the characteristic energy of these energies is more representative of the functional group than the electrons themselves

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

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

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UV Spectroscopy

III. ChromophoresB. Organic Chromophores

1. Alkanes – only posses -bonds and no lone pairs of electrons, so only the high energy * transition is observed in the far UV

This transition is destructive to the molecule, causing cleavage of the -bond

C C

C C

32

UV Spectroscopy


2. Alcohols, ethers, amines and sulfur compounds – in the cases of simple, aliphatic examples of these compounds the n * is the most often observed transition; like the alkane * it is most often at shorter than 200 nm

Note how this transition occurs from the HOMO to the LUMO

CN

CN

nN sp3C N

C N

C N

C N

anitbonding orbital

33

UV Spectroscopy


3. Alkenes and Alkynes – in 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 *, it is still in the far UV – however, the transition energy is sensitive to substitution

34

UV Spectroscopy


4. Carbonyls – unsaturated 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 ( = 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, = 900); sensitive to substitution effects

35

UV Spectroscopy


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

n

CO transitions omitted for clarity

O

O

C O

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

UV Spectroscopy

III. ChromophoresC. Substituent Effects

General – from 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 energy of the transition

Substituents that increase the intensity and often wavelength of an absorption are called auxochromes

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

37

UV Spectroscopy


General – Substituents may have any of four effects on a chromophore

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

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

iii. Hyperchromic effect – an increase in intensity

iv. Hypochromic effect – a decrease in intensity

200 nm 700 nm

H

ypochromic

Hypsochromic

Hyp

erch

rom

ic

Bathochromic

38

UV Spectroscopy


1. Conjugation – most efficient means of bringing about a bathochromic and hyperchromic shift of an unsaturated chromophore:

H2CCH2

-carotene

O

O

max nm 175 15,000

217 21,000

258 35,000

n * 280 27 * 213 7,100

465 125,000

n * 280 12 * 189 900

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UV Spectroscopy


1. Conjugation – AlkenesThe observed shifts from conjugation imply that an increase in conjugation decreases the energy required for electronic excitation

From molecular orbital (MO) theory two atomic p orbitals, 1 and 2 from two sp2 hybrid carbons combine to form two MOs 1 and 2* in ethylene

40

UV Spectroscopy


2. Conjugation – AlkenesWhen we consider butadiene, we are now mixing 4 p orbitals giving 4 MOs of an energetically symmetrical distribution compared to ethylene

E for the HOMO LUMO transition is reduced

41

UV Spectroscopy


2. Conjugation – AlkenesExtending this effect out to longer conjugated systems the energy gap becomes progressively smaller:

Energy

ethylene

butadiene

hexatriene

octatetraene

Lower energy =Longer wavelengths

42

UV Spectroscopy


2. Conjugation – AlkenesSimilarly, the lone pairs of electrons on N, O, S, X can extend conjugated systems – auxochromesHere we create 3 MOs – this interaction is not as strong as that of a conjugated -system

A

nA

Energy

43

UV Spectroscopy


2. Conjugation – AlkenesMethyl groups also cause a bathochromic shift, even though they are devoid of - or n-electronsThis effect is thought to be through what is termed “hyperconjugation” or sigma bond resonance

C C

C

HH

H

44

UV Spectroscopy

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 dienes2. Conjugated dienones3. Aromatic systems

max = 239 nm

45

UV Spectroscopy

IV. Structure Determination A. Dienes

1. General FeaturesFor acyclic butadiene, two conformers are possible – s-cis and s-trans

The s-cis conformer is at an overall higher potential energy than the s-trans; therefore the HOMO electrons of the conjugated system have less of a jump to the LUMO – lower energy, longer wavelength

s-trans s-cis

46

UV Spectroscopy


1. General FeaturesTwo possible * transitions can occur for butadiene

and 2 4*

The 2 4* transition is not typically observed:

• The energy of this transition places it outside the region typically observed – 175 nm

• For the more favorable s-trans conformation, this transition is forbidden

The 2 3* transition is observed as an intense absorption

s-trans s-cis

175 nm –forb.

217 nm 253 nm

175 nm

47

UV Spectroscopy


1. General FeaturesThe 2 3

* transition is observed as an intense absorption ( = 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 is subject to the bathochromic and hyperchromic effects of alkyl substituents as well as further conjugation

Consider:

max = 217 253 220 227 227 256 263 nm

48

UV Spectroscopy


2. Woodward-Fieser RulesWoodward 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 * 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)

50

UV Spectroscopy


2. Woodward-Fieser Rules - DienesThe rules begin with a base value for max of the chromophore being observed:

acyclic butadiene = 217 nm

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

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UV Spectroscopy


2. Woodward-Fieser Rules - DienesFor example:

Isoprene - acyclic butadiene = 217 nm

one alkyl subs.+ 5 nm

222 nmExperimental value

220 nm

Allylidenecyclohexane- acyclic butadiene =

217 nmone exocyclic C=C

+ 5 nm2 alkyl subs.

+10 nm

232 nmExperimental value

237 nm

52

UV Spectroscopy


3. Woodward-Fieser Rules – Cyclic DienesIn the pre-NMR era of organic spectral determination, the power of the method for discerning isomers is readily apparent

Consider abietic vs. levopimaric acid:

C

O

OHC

O

OH

levopimaric acidabietic acid

53

UV Spectroscopy


3. Woodward-Fieser Rules – Cyclic DienesBe 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

54

UV Spectroscopy

IV. Structure Determination B. Enones

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

n

Remember, the * transition is allowed and gives a high , but lies outside the routine range of UV observation

The n * transition is forbidden and gives a very low e, but can routinely be observed

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UV Spectroscopy


1. General FeaturesFor auxochromic substitution on the carbonyl, pronounced hypsochromic shifts are observed for the n * transition (max):

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 * which extends conjugation and causes a bathochromic shift

In most cases, this bathochromic shift is not enough to bring the * transition into the observed range

H

O

CH3

O

Cl

O

NH2

O

O

O

OH

O

293 nm

279

235

214

204

204

UV Spectroscopy


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 higher intensity * 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 conjugated enones are for the higher intensity, allowed * transition

57

UV Spectroscopy


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

n

n

OO

58

UV Spectroscopy


2. 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

Double bond extending conjugation 30

Alkyl group or ring residue and higher

10, 12, 18

-OH and higher

35, 30, 18

-OR 35, 30, 17, 31

-O(C=O)R 6

-Cl 15, 12

-Br 25, 30

-NR2 95

Exocyclic double bond 5

Homocyclic diene component 39

C C C

C C CC

C

O O

59

UV Spectroscopy


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

Unsaturated system Base Value

Aldehyde 208

With or alkyl groups 220


With alkyl groups 242

Acid or ester



Group value – exocyclic double bond

+5

Group value – endocyclic bond in 5 or 7 membered ring

+5

60

UV Spectroscopy


2. Woodward-Fieser Rules - EnonesUnlike 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

61

UV Spectroscopy


2. Woodward-Fieser Rules - EnonesSome examples – keep in mind these are more complex than dienes cyclic enone = 215 nm 2 x - alkyl subs.(2 x 12) +24 nm

239 nmExperimental value 238 nm

cyclic enone = 215 nm

extended conj.+30 nm

-ring residue+12 nm -ring residue

+18 nm exocyclic double bond + 5 nm

280 nm

Experimental280 nm

O

R

O

62

UV Spectroscopy


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

O

O

Eremophilone allo-Eremophilone

63

UV Spectroscopy

IV. Structure Determination C. Aromatic Compounds

1. General FeaturesAlthough aromatic rings are among the most widely studied and observed chromophores, the absorptions that arise from the various electronic transitions are complex

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

64

UV Spectroscopy


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 transition energy states of benzene are illustrated at the right:

g

u

u

u

260 nm(forbidden)

200 nm(forbidden)

180 nm(allowed)

Expected Actual

65

UV Spectroscopy


1. General FeaturesThe allowed transition ( = 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 ( = 7400) is observed if substituent effects shift it into the obs. region; this is referred to as the second primary band

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

This transition is fleetingly alloweddue to the disruption of symmetryby the vibrational energy states,the overlap of which is observedin what is called fine structure

66

UV Spectroscopy


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

However, these shifts are difficult to predict – the formulation of empirical rules is for the most part is not efficient (there are more exceptions than rules)

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

67

UV Spectroscopy


2. Substituent Effectsa. Substituents with Unshared Electrons

• If the group attached to the ring bears n electrons, they can induce a shift in the primary and secondary absorption bands

• Non-bonding electrons extend the -system through resonance – lowering the energy of transition *

• More available n-pairs of electrons give greater shifts

GG G G

68

UV Spectroscopy



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

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

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

GG G G

*- *

*

*

69

UV Spectroscopy



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

lowering transition energy• protonation of nitrogen eliminates the n-pair,

raising transition energy

Primary Secondary

Substituent

max max

-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

70

UV Spectroscopy


2. Substituent Effectsb. Substituents Capable of -conjugation

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

• With benzoic acids, this causes an appreciable shift in the primary and secondary bands

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

Primary Secondary

Substituent

max max

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

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

71

UV Spectroscopy


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

• With di-substituted aromatics, it is necessary to consider both groups

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

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

• Consider p-nitroaniline:

H2N N

O

O

H2N N

O

O

72

UV Spectroscopy



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

• For the case of substituted benzoyl derivatives, an empirical correlation of structure with observed max has been developed

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

G

73

UV Spectroscopy



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 max

R = alkyl or ring residue

246

R = H 250

R = OH or O-Alkyl 230

74

UV Spectroscopy


2. Substituent Effectsd. Polynuclear aromatics

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

• For heteroaromatic systems spectra become complex with the addition of the n * transition and ring size effects and are unique to each case

75

It is pertinent to mention here that an excited electron normally returns to the ground state in about 10–9 to 10–8 seconds. Consequently, energy must now be released to compensate for the energy absorbed by the system. In actual practice however, the following three situations arise, namely : Firstly, if the electron returns directly to the ground state, the net effect would be evolution of heat. Secondly, if the electron returns to the ground state by passing through a second excited state, the net outcome would be release of energy in the form of heat and light. Thirdly, if a large amount of energy is absorbed by certain substances, bonds may be ruptured and thereby giving rise to altogether new compounds.For instance : ergosterol on being subjected to UV radiation yields cholecalciferols which are, in fact, altogether new substances.

What happens after absorbing UV-Vis radiation?

77

ASSAY METHODSMEASUREMENT OF TRANSMITTANCE AND ABSORBANCE

Transmittance and absorbance, as outlined in Table 13-1 (above), cannot normally be measured in the laboratory because the analyte solution must be held in a transparent Container or cell.

78

FIGURE 13-1 Reflection and scattering losses with a solution contained in a typical glass cell. Losses by reflection can occur at all the boundaries that separate the different materials. In this example, the light passes through the air-glass, glass-solution, solution-glass, and glass-air interfaces.

MEASUREMENT OF TRANSMITTANCE AND ABSORBANCE …

79

Qualitative Analysis

Great deal of information from UV spectrum used by itself cannot be extracted UV spectrum is most useful when at least a general idea of the structure is already known There are several generalizations can serve to guide our use of UV data. These generalizations will be more meaningful if combined with IR and NMR data In the absence of IR and NMR data these generalizations should be taken only as guidelines

80

Some Important Generalizations…

81


82


83


84

Not all radiations are effective Some of ways of loss for radiation are:

- loss by reflection in passing through a sample container containing solvent or water.

- Attenuation of a beam may occur as a result of scattering by large molecules and sometimes from absorption by the container walls (see above figure). To compensate for these effects, the power of the beam transmitted by the analyte solution is usually compared with the power of the beam transmitted by an identical cell containing only solvent. An experimental transmittance and absorbance that closely approximate the true transmittance and absorbance are then obtained with the equations

The terms Po and P, refer to the power of radiation after it has passed through cells containing the solvent and the analyte solutions, respectively.

MEASUREMENT OF TRANSMITTANCE AND ABSORBANCE …..

85

Beer- Lamber Law

Beer's law also applies to a medium containing more than one kind of absorbing substance. Provided that the species do not interact, the total absorbance for a multicomponent system is given by

where the subscripts refer to absorbing components 1,2, ... , n.

Quantitative Analysis

86

Ultraviolet (UV) Spectroscopy – Analysing the Output

wavelength (nm)

Absorbance

450400350

1.0

0.5

0.0

Handling samples of known concentration

If you know the structure of your compound X and you wish to acquire UV data you would do the following.

Prepare a known concentration solution of your sample.

Run a UV spectrum (typically from 500 down to 220 nm).

From the spectrum read off the wavelength values for each of the maxima of the spectra (see left)

Read off the absorbance values of each of the maxima (see left).

Then using the known concentration (in moles L-1 ) and the known pathlength (1 cm) calculate the molar absorptivity () for each of the maxima.

Finally quote the data as follows (for instance for the largest peak in the spectrum to the left and assuming a concentration of 0.0001 moles L-1 ).

max = 487nm A= 0.75

= 0.75 /(0.001 x 1.0) = 7500 moles-1 L cm -1

Beer Lambert Law

A = .c.l

87

Determining concentration of samples with known molar absorptivity ().

Having used the calculation in the yellow box to work out the molar absorptivity of a compound you can now use UV to determine the concentration of compound X in other samples (provided that these sample only contain pure X).

Simply run the UV of the unknown and take the absorbance

reading at the maxima for which you have a known value of . In

the case above this is at the peak with the highest wavelength (see

above). Having found the absorbance value and knowing and l

you can calculate c.This is the the principle used in many experiments to determine the

concentration of a known compound in a particular test sample –

for instance monitoring of drug metabolites in the urine of drug

takers; monitoring biomolecules produced in the body during

particular disease states

88

Spectrum Analysis

89

Typical Beer’s Law Plot

y = 0.02x

0

0.20.4

0.6

0.81

1.2

0.0 20.0 40.0 60.0

concentration (uM)

A

90

• 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)

Other Electronic Transition absorptions

91

1. During an assay of the thiamine (vitamin B1) content of a pharmaceutical preparation, the percent transmittance scale was accidentally read, instead of the absorbance scale of the spectrophotometer. One sample gave a reading of 82.2% T, and a second sample gave a reading of 50.7% T at a wavelength of maximum absorbance. What is the ratio of concentrations of thiamine in the two samples?

2. (a) A 3.73 × 10-5 M solution of Compound A from a spectrophotometric analysis has a maximum absorbance of 0.494 at 401 nm in a 1.000-cm cell, while a reagent blank from the same analysis has an absorbance of 0.053 at 401 nm. Find the molar absorptivity of Compound A.

(b) A 5.00 mL aliquot of unknown solution containing Compound A was mixed with color forming reagents and diluted to a final volume of 250.0 mL to give an absorbance of 0.777 at 401 nm in a 1.000-cm cell. The reagent blank had an absorbance of 0.053. Find the concentration of Compound A in the unknown solution.

Exercises

92

Answers

1.

2.

93

Limitations to Beer's Law Few exceptions are found to the generalization that absorbance is linearly related to path length. On the other hand, deviations from the direct proportionality between the measured absorbance and concentration frequently occur when b is constant. Some of these deviations are called real deviatiolls, are fundamental and represent real limitations of the law. Others are a result of how the absorbance measurements are made (instrumental deviations) or a result of chemical changes that occur when the concentration changes (chemical deviations).

94

• Linearity is observed in the low concentration ranges(<0.01), but may not be at higher concentrations.

• This deviation at higher concentrations is due to intermolecular interactions.

• As the concentration increases, the strength of interaction increases and causes deviations from linearity.

• The absorptivity not really constant and independent of concentration but is related to the refractive index (ɳ ) of the solution by the expression:

• At low concentrations the refractive index is essentially constant-so constant and linearity is observed.

Real Limitations

)22 + (true

95

Apparent Chemical Deviations Apparent deviations from Beer's law arise when an analyte dissociates, associates, or reacts with a solvent to produce a product with a different absorption spectrum than the analyte. A common example of this behavior is found with aqueous solutions of acid-base indicators. For example, the color change associated with a typical indicator Hln arises from shifts in the equilibrium

E.g. K2Cr2O7 solutions exist as a dichromate, chromate equilibrium: At max of 350 (and 450) nm and 372 nm respectively. There is a strong dependence of position of this equilibrium on relative pH. Absorbance at one of these wavelengths for a given initial concentration of [K2Cr2O7] strongly depends upon the pH. When plotting absorbance as a function of [K2Cr2O7], the plot will not be linear since dilutions will affect the equilibrium and thus the relative amounts of the two.

96

Instrument Deviation

Beer's law strictly applies only when measurements are made with monochromatic source radiation. In practice, polychromatic sources that have a continuous distribution of wavelengths are used in conjunction with a grating or with a filter to isolate a nearly symmetric band of wavelengths surrounding the wavelength to be employed Consider a beam of radiation consisting of just two wavelengths ' and “ Assuming that Beer's law applies strictly for each wavelength, we may write for '

97Beer’s law is followed

99

As shown in Figure 13-4, however, the relationship between Am and concentration no longer linear when the molar absorptivities differ. In addition, as the difference between ' and “ increases, the deviation from linearity increases. Thisderivation can be expanded to include additional wavelengths: the effect remains the same.

100

Instrumental Deviations in the Presence of Stray Radiation

The radiation exiting from a monochromator is usually contaminated with small amounts of scattered or stray radiation. This radiation, commonlv called stray light, is defined as radiation from the instrument that is outside the nominal wavelength band chosen for the determination. This stray radiation often is the result of scattering and reflection off the surfaces of gratings, lenses or mirrors. filters, and windows. The wavelength of stray radiation often differs greatly from that of the principal radiation and, in addition, the radiation may not have passed through the sample.

Must account for the effect of stray light on the measured absorbance. The measured absorbance is where Ps = radiant power of the stray light. Negative deviations in the Beer's law plot observed since are the result since the measured absorbance will be smaller than it should be.

P + PP + P logA

s

som

101

• errors in the measurement of the transmittance; can have a dramatic affect on the estimation of concentration.

• Normal Error Analysis starting with Beer's law equation: • C = A/b = .

• C = f(T). • General error equation is

• Take the derivative of both sides to get:• Substituting we get:

or

• Conclusion: Optimum transmittance. • sT related to type of noise.

Photometric errors

1

b I

I =

T

b=

0.424 T

bo

log

log ln

Cs

C=

s

T TT

ln

C2

2

T2s =

C

Ts

CT

= 0.434

bT

C2

2

T2

2

T2s =

0.434

bTs =

C

T Ts

ln

102

APPLICATIONS• Mixtures:Determining the concentration of

mixtures the components of which absorb in the same spectral regions is possible.

• Strategy of the analysis. Total absorption at some wavelength of a two component mixture: Atotal,1 = A + AN1.

• Each should obey Beer's law at this wavelength as long as concentration is sufficiently low. The contribution from each would then be:

• AM,1 = M,1bCM and AN1 = N,1bCN. and • Atotal,1 = M,1bCM + N,1bCN. • Similarly at some other wavelength we

would have, • Atotal,2 = M,2bCM + N,2bCN. • .b can be determined for each using

standard solutions.• Take absorbance readings of mixture at the

two s.• Substitute into above so that there are two

equations with two unknowns.

103

• Absorbance measured during titration of analyte.

• The endpoint can be determined by extrapolation of the lines that result from before and after the endpoint.

• The shape of the titration curves depends upon the molar absorptivities of reactants, products and titrants.

• All absorbing species must obey Beer's law for this method to be successful.

PHOTOMETRIC TITRATIONS

104

Standard Addition Method

• Standard addition method reduces problems with matrix; analyte added to the matrix to change the signal; signal change enables the determination of the original concentration of the analyte.

• Another linear procedure with volume correction: • Add volume, Vx, of the unknown solution with a

concentration cx to a series of separate containers – Add variable amounts, Vs, of a standard solution with

concentration cs of the same compound. – Dilute these to constant final volume, Vt.

• Beer's law predicts the absorbance will vary according to .

• A should vary linearly with Vs; the slope and intercept should be

. • Ratio of intercept and slope is:.

•Skoog & Leary

A = bV c

V +

bV c

Vs s

T

x x

T

S = slope = bc

V I = intercept =

bV c

Vs

T

x x

T

I

S = V c

c c = c

I

V Sx x

sx s

x

105Chapter 14 - 105

STOICHIOMETRY OF COMPLEX IONS

• Ligand to metal ratio in can be determined from absorption measurements. Equilibrium not affected significantly!

• Assuming reactant or product absorbs radiation, we can – determine the composition of complex ions in solutions

and – determine formation constants.

• Stoichiometry: mole ratio, continuous variation, and slope ratio methods. One complex only!

106

STOICHIOMETRY OF COMPLEX IONS

• Ligand to metal ratio in can be determined from absorption measurements. Equilibrium not affected significantly!

• Assuming reactant or product absorbs radiation, we can – determine the composition of complex ions in solutions

and – determine formation constants.

• Stoichiometry: mole ratio, continuous variation, and slope ratio methods. One complex only!

107

Mole-ratio method

• Concentration of one of the components held constant while other is varied giving a series of [L]/[M] ratios.

• The absorbance of each of these solutions is measured and plotted against the above mole ratio.

• The ratio of ligand to metal can thus be obtained from the plot.

Instrumental Methods of Analysis, Ewing, p. 69.

108

MOLE RATIO METHOD (cont’d)

Determination Kf (ML only) non-linear portion of the plot. Let:• Fm = [M] + [ML] = the total metal concentration at equilibrium and • FL = [L] + [ML] = the total ligand concentration at equilibrium • at any point on the curved part of the plot: A = Mb[M] + MLb[ML]:

assuming L = 0. • Determine b for both the metal and ligand. • Metal Let FL = 0 and [ML] = 0; AM = MbFm or Mb = AM/Fm. • Ligand:With a large excess of ligand, [ML] >> [M] and AML = eMLbFM or

eMLb = AML/FM. • Known equations: • Fm = [M] + [ML]; • FL = [L] + [ML]• A = Mb[M] + MLb[ML]• Determine [ML],[M],[L]• Kf = [ML]/[M][L]

109

SLOPE-RATIO METHOD

• Makes it possible to determine ratio of ligand to metal. Two plots performed with large excess of either ligand or metal.

• Absorbance vs. FM : large excess .ligand: [L] >> [M] – [MnLp] = FM/n – Beer's law will be AM = b[MnLp] = bFm/n – Metal Concentration varied and plotted.

• Absorbance vs. FL: large excess of metal the [M]o >> [L] – [MnLp] = FL/p and AL = bFL/p.– Beers law : AM = b[MnLp] = bFL/n – Ligand Concentration varied and plotted.

• Slopes will be b/p and b/n. The ratio of the slopes gives the ratio of p/n.

instrumental analysis spectroscopy.ppt12

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