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Organic Spectroscopy 1 Michaelmas 2011

Lecture 5

Dr Rob Paton

[email protected]://paton.chem.ox.ac.uk


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Organic Spectroscopy 1: Outline of Lectures 5-8

In lectures 5-6 of this course, aspects ofUltraviolet-visible and Infra red techniques will be introduced that are important inassigning organic structures. Coverage of the underlying theory and instrumentation associated with each method will be keptto a bare minimum since these aspects are covered elsewhere.

We will look at a variety of real spectra and learn to correlate distinguishing features in these spectra with functional groups.

UV-vis and IR spectroscopy provide direct experimental data to support of a number of the underlying concepts in organicchemistry introduced last year, such as conjugation and the mesomeric effect. We will also take a moment to consider thesepoints.

In lectures 7-8 we will go through worked examples to illustrate how to combine 13C and 1H NMR with UV-vis and IR spectrato assign structures.

Digital copies of all handouts, problems and slides are available through the web: http://paton.chem.ox.ac.uk

O OO

N

O

N


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Organic Spectroscopy 1: Outline of Lectures 5-8

Further Reading:Chemical Structure and Reactivity: an Integrated Approach J. Keeler and P. D. Wothers, OUP(Chapter 11)Introduction to Organic Spectroscopy- L. M. Harwood and T. D.W. Claridge, Oxford Chemistry PrimersOrganic Chemistry Clayden, Greeves, Warren and Wothers, OUP (Chapter 3)

Organic Spectroscopic Analysis R. J. Anderson. D. J. Bendell and P. W. Groundwater, RSCFor more complete coverage including many more real examples of spectra, tables of spectroscopic data that will be useful instructural elucidation, and worked examples consult the following:Organic Structure Analysis P. Crews, J. Rodriguez and M. Jaspers, OUPSpectroscopic Methods in Organic Chemistry (6thedition) D. H. Williams and I. Fleming, Mcgraw-Hill

A wealth of experimental spectra may be found on the internet, in openly accessible repositories. The following may be ofinterest:NMRshift DB - NMR database for organic structures: http://www.ebi.ac.uk/nmrshiftdb/

The Japanese Spectral Database for Organic Compounds (SDBS) has free access to IR, Raman, 1H and 13C NMR and MS data:http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi?lang=eng

Sigma-Aldrich has IR, Raman and 1H and 13C NMR spectra for many of their commericially available compounds:http://www.sigmaaldrich.com

Problems in structure, combining IR with 1H and 13C NMR courtesy of Prof Craig Merlic, UCLA:http://www.chem.ucla.edu/~webspectra/

Past Paper Questions containing to NMR/IR/UV-vis spectroscopy:NB Since 2011 Mass Spectrometry has been shifted to 1BPart IA: 2004 (Q7), 2005 (Q2), 2006 (Q1), 2007 (Q8), 2008 (Q9), 2009 (Q1), 2010 (Q1), 2011 (Q7).General Paper I: 1993 Q6, 2000 (Q1), 2001 (Q5) and 2004 (Q8)General Paper II: 1991 (Q3, Q5), 1992 (Q8), 1993 (Q3), 1994 (Q1), 1995 (Q3), 1996 (Q7), 1997 (Q5), 1998, Q3), 1999(Q6), 2000 (Q9), 2002 (Q1) and 2003 (Q3)


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The Electromagnetic Spectrum

By irradiating molecules at different frequencies, it is possible to gain different types of information about their structure,since these frequencies bring into resonance various modes of molecular motion, or electronic or nuclear excitation. In modernlaboratories, NMR spectroscopy is the first choice method for gaining structural information, with Infrared (IR) and massspectroscopy (MS) techniques acting in a supporting capacity and UV spectra only being required in specialized circumstances

(e.g. analysis of specific compound classes such as polymers or porphyrins).


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The Electromagnetic Spectrum

100000

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0

-

-

-

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-

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0

-

-

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-

-

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1500

1000

500

0

-

-

-

-

-

Electronic States Vibrational energy levels Rotational energy levels

(energies in wavenumbers, cm-1)

0.01 cm-1

Nuclear spin states

(400 MHz)


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UV-vis SpectroscopyUV-vis is a form of absorption spectroscopy. Radiation in the UV-visible region of the EM spectrum is absorbed, causing anelectron to be excited to a higher energy level.

UV and visible spectra of organic compounds are associated with excitations of electrons from the ground state to an excitedstate higher in energy. The transition occurs from a filled bonding or non-bonding orbital to a formerly empty antibondingorbital.The energy gap is proportional to the frequency of absorption, and so UV-vis spectroscopy is a source of bonding information UV spectroscopy is most important in the structural analysis of compounds containing -bonds, in particular conjugatedsystems.

h

ground state excited state

E

1

2

3

4

1

2

(900 kJ/mol)(750 kJ/mol)

(500 kJ/mol)


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UV-vis Spectroscopy

Recording UV-vis spectraThe ultraviolet or visible spectrum is usually taken using a dilute solution of the sample in a glass or quartz tube, or cuvette.Typically the sides of the cuvette are 1 cm, and the total volume is 2-3 cm 3. UV or visible light is passed through the sampleand the intensity of the transmitted beam is recorded across the wavelength range of the instrument ( I). First the intensity ofthe light is recorded with pure solvent in the cuvette (I0) the absorbance due to the sample can then be computed as log10(I0/I).

200

wavelength, (nm) 800

600 150

400 600

300 200 Energy gap (kJ/mol)

molarextinctioncoefficient,

hypsos= height

bathos =depth

hyper = above

hypo = below

light source detectorI0 I

l

*


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UV-vis Spectroscopy

The Beer-Lambert law states that the absorption of light by a given sample is proportional to the number of absorbingmolecules, and independent of the source intensity.

I0 and I are the intensities of the incident and transmitted light, respectively, lis the path length of the absorbing solution in cm

and c is the concentration in moles/litre. is the molar extinction coefficient in 1000 cm2 mol-1. log10 (I0/I) is called theabsorbance.

Example:A 1.12 x 10-4 M solution of paranitroaniline, in a cuvette of path length 1cm, has a measured absorbance maximum of 1.55 at227 nm. This means the intensity of the transmitted light is 101.55 = 35 times the intensity of the incident light.

The value for this absorption is:

This would be quoted as:


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UV-vis Spectroscopy

The solvent and vessels must be transparent in the range of interest.

UV-vis absorptions of common functional groups:

Functional groups such as polyenes and poly-ynes that give rise to diagnostic absorptions in the UV-vis region of the EMspectrum are referred to as chromophores

150 170 190 210 230 290 310 330 350wavelength (nm)

chloroform

95% ethanol

water

quartz

glass

cyclohexane

150 170 190 210 230 290 310 330 350wavelength (nm)

*

n*

single bonds

lone pairs (O, N, S)

isolated*double bonds

n*

conjugated *

Vacuum UV UV

*

*

n (LP)


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UV-vis Spectroscopy

Selection Rules and Intensity

Irradiation of organic compounds does not always give rise to excitations of electrons from any filled to unfilled orbital,because there are rules based on symmetry governing which transitions are allowed. The intensity of absorption is related tothe allowedness of a particular transition

A chromophore with two double bonds conjugated together possesses a fully allowed transition, and has associated values ofabout 10,000

Forbidden absorptions are in practice observed with weak absorptions, as the symmetry may be broken by a molecularvibration or by unsymmetrical substitution.

The most important point to be made is that, in general:

> 10,000

O

= 10 - 100 = 100 - 1000

- * n - * - *

allowed "forbidden"


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UV-vis Spectroscopy

Example: conjugated dienes:

The most important point to be made is that, in general:

345678

275310342380401411

30,00076,500

122,000146,000

--

358384403420435

-

75,00086,50094,000

113,000135,000

n max(nm) max(nm)

MeMe

nPh

Ph

n


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UV-vis Spectroscopy

Absorption maxima for substituted benzene rings (Ph-R)

pH induced shifts: an acid induced blue (i.e. hypsochromic) shift

HNHMe

IClBrOHOMeSO NHCNCOCO HNHONHAcCOMeCH=CH

CHOPhOPhNOCH=CHCO HCH=CHPh

203.5203

206.5

207209.5210

210.5217

217.5224224230230235238

245.5248

249.5251.5255

268.5273

295.5

7,4007,5007,000

7,0007,4007,9006,2006,4009,700

13,0008,700

11,6008,6009,400

10,5009,800

14,000

11,40018,30011,0007,800

21,00029,000

254254261

257263.5261270269

264.5271268273280287

282

272

204160225

700190192

14501480740

1000560970

14302600

750

2000

max

(nm) max

(nm)

2 2

2

2

2

2

2

2

3

254254261

257263.5261270269

264.5271268273280287

291

278

204160225

700190192

14501480740

1000560970

14302600

500

1800

max

(nm)R

NH2H

NH3

max 230 nm max 203 nm


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UV-vis Spectroscopy

pH induced shifts: a base induced red (i.e. bathochromic) shift

Effects of complementary EWG/EDG substituents

Acid base indicators: e.g phenolphthalein

OH

-H

O

max210.5 nm max235 nm

NH2 NO2

max

230 nm max

269 nm

7800 8600

max

229 nm

14800

NH2

NO2NH2

O2N

NO2

max

235 nm

16000

max

375 nm

16000

H2N NO2

max

260 nm

1300

O2N

OO

HO

HO OO

O

HOpKa 9.4

OH

max 231 nm (25,800)

max 275 nm (4,200)

max 230 nm (25,800)

max 553 nm (26,000)


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UV-vis SpectroscopyCarbonyls:

1

2

3

4

1

2

2pO 2pO

O O

1

2


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UV-vis Spectroscopy

Predicting UV absorptions of conjugated dienes:Alkyl substitution of a diene extends the chromophore through hyperconjugative interactions, causing a small red shift to

longer values for max.

The effect of alkyl substitution on open chain dienes and dienes in six-membered rings is approximately additive, so a few rules(first formulated by R. B. Woodward in 1941) can be used to predict absorption. Woodwards rules have since been refined asa result of experience by Fieser.

Woodwards rules may be applied to predict the absoroption of a diene that is either homoannular with both double bondscontained in one ring or heteroannular with two double bonds distributed between two rings.

Base value for parent s-transdiene (heteroannular)Base value for parent s-cisdiene (homoannular)

Increments for:

(a) each alkyl substituent or ring residue(b) exocyclic nature of any double bond

(c) additional double bond extending conjugation(d) auxochrome:-OAcyl-OAlkyl-SAlkyl-Cl or -Br-NAlkyl

Woodward's rules for diene and triene absorption

214 nm253 nm

+5 nm+5 nm

+30 nm

+0 nm -OAcyl+6 nm -OAlkyl+30 nm - -SAlkyl+5 nm -Cl or -Br+60 nm -NAlkyl


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UV-vis Spectroscopy

Example of applying the Woodward-Fieser rules:

Less empirical treatment particle in a box: En = n2h2/8mL2


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UV-vis Spectroscopy

Example:

Parent chromophore:X

alkyl or ring residue

HOH or Oalkyl

Increment for each substituent:

Rules for the principal band of substituted benzenes RC6H4OX

246 nm250 nm230 nm

X

O

R

o, mpo, mpomp

o, mp

+3

+10

+7

+25

+11

+20+78

0

+10

-alkyl/ring residue-OH, OMe, OAlkylo, m-Oom-Cl

o, mp

o, mpo, mpo, mpp

o, mp

+2

+15

+13

+58

+20

+45

+73

+20+85

-Br-OH, OMe, OAlkyl-NHom-NHAco, m-NHMe

-NMe

2

2

O

MeO


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UV-vis Spectroscopy

trans-stilbene and cis-stilbene

2,4,6-trimethylacetophenone and para-methylacetophenone

Strain release in the hydrolysis of a dilactone produced from shelloic acid.

max 296 nm ( 29,000) max 280 nm ( 10,500)

max 242 nm ( 3,200) max 252 nm ( 15,000)

O O

O

H

H

O

O

O

HH2O

O

H

H

O H

O

OH

OH

no strong absoprtion >210 nm max 227 nm ( 5,500)


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UV-vis SpectroscopyTomatoes are a deeper red than carrots. Given that the conjugated systems of -carotene and lycopene are both elevendouble bonds conjugated together with a similar number of alkyl substituents, why might lycopene absorb at a longerwavelength and with greater intensity?

Dehydration of graphene oxide to grapheme (Chem. Mater. 2009, 21, 2950)

Expanding a porphyrin -system (Org. Lett. 2008, 10, 3945)

l

OHH

H+


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Electronic States Vibrational energy levels Rotational energy levels

(energies in wavenumbers, cm-1)

IR Spectroscopy

cortisone acetate

E = h c / i .e. C-H bonds absorb at around 3000 cm-1 :6.63x10-34 x 3x108 x 3000x102 x Na

= 36 kJ/mol

Me

Me

H

H H

O

O

O

O

H

O


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IR Spectroscopy

High-resolution IR spectrum of CO in the gas phase:

Modelling a vibration: Hookes law (as the extension, so the force)

The frequency depends on the mass and the stiffness of the spring

When applying this model to a pair of bonded atoms, the force constant corresponds to the strength of the covalent bond.Stronger bonds are harder to stretch.

Tamiso

2000 2050 2100 2150 2200 2250

wavenumber (cm-1)

m2

kf

oscillation

about COM

m1


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IR Spectroscopy

Unlike a mass hanging from a spring, when a diatomic molecule vibrates, both of the atoms move. We take this into account byusing the reduced mass for the system to compute the frequency of oscillation:

For a vibrating diatomic molecule, the frequency of vibration (expressed as a wavenumber, in cm -1) is given by:

When one of the two masses is considerably larger than the other, as in a X-H bond, this expression approximates to thelighter of the two masses:

Due to the inverse relationship between reduced mass and frequency, the stretching frequencies for X-H bonds areconsiderably greater than those for other bonds.

For atoms with similar masses, the stretching frequencies of triple bonds are greater than double bonds, which in turn aregreater than for single bonds. This is a consequence of force constants following the bond strengths.


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IR Spectroscopy

Compare the C-H region of the IR spectra of fluorobenzene and d 5-fluorobenzene.

C-H - 3050 cm-1 C-D - 2280 cm-1

C-H/C-D = 1.34

reduced mass C-H: 1 x 12 / (1+ 12) 1

reduced mass C-D: 2 x 12 / (2+ 12) 2

ratio of C-H to C-D stretching frequency = = 1.4

1H

1H

1H

1H

1H

F

2

D2D

2D

2D

2D

F


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IR Spectroscopy

The complex vibrational motion of polyatomic molecules can be resolved into a series of simpler normal modes.There are 3N 6 (non-linear molecule) or 3N 5 (linear molecule) normal modes.

Normal modes of sulfur dioxide, SO2:

Different bends of a methylene group:

bending mode symmetric stretching mode antisymmetric mode519 cm-1 1151 cm-1 1361 cm-1


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IR Spectroscopy

Normal modes of carbon dioxide:

symmetric stretching mode antisymmetric mode


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IR Spectroscopy

It is helpful to divide the IR spectrum into regions:

Example: cyanoacetamide

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