CHEM 210 IR Spectroscopy

7/25/2019 CHEM 210 IR Spectroscopy

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Infrared SpectroscopyCHEM 210



IR Spectroscopy

I. IntroductionA. Spectroscopy is the study of the interaction of matter with the

electromagnetic spectrum

1. Electromagnetic radiation displays the properties of bothparticles and waves

2. The particle component is called a photon

3. The energy (E) component of a photon is proportional to thefreuency. !here h is "lanc#$s constant and ν is the freuencyin %ert& (cycles per second)

E = hν

'. The term photon is implied to mean a small* masslessparticle that contains a small wave+pac#et of E, radiation-light we will use this terminology in the course



IR Spectroscopy

I. Introduction5. Because the speed of light, c , is constant, the frequency, , (number

of cycles of the wave per second) can complete in the same time, must

be inversely proportional to how long the oscillation is, or wavelength:

/. 0mplitude* A* describes the wave height* or strength of theoscillation

. ecause the atomic particles in matter also ehibit wave andparticle properties (though opposite in how much) E,

radiation can interact with matter in two ways4• 5ollision particle+to+particle energy is lost as heat and

movement

• 5oupling the wave property of the radiation matches thewave property of the particle and couple to the net

higher uantum mechanical energy level

ν =

c = 3 x 1010 cm/s

___

c

E = hν = ___

hc



IR Spectroscopy

I. Introduction8. The entire electromagnetic spectrum is used by chemists:

UVX-rays IR-rays RadioMicrowave

Energy (kca/mo!

300-30 300-30 "10-#

$ 300 "10-%

Visi&e

're)ency, ν in *+

"101, "1013 "1010 "10,"101"101.

aveeng,

10 nm 1000 nm 0201 cm 100 m"0201 nm"20001 nm

nuclear

ecitation(!"T)

core

electronecitation(#$raycryst.)

electronic

ecitation(π to π%)

molecular

vibration

molecular

rotation

&uclear 'agnetic

esonance &'(')



IR Spectroscopy

I. Introduction5. The I6 7pectroscopic "rocess

1. The uantum mechanical energy levels observed in I6

spectroscopy are those of molecular vibration

2. !e perceive this vibration as heat

3. !hen we say a covalent bond between two atoms is of acertain length* we are citing an average because the bond

behaves as if it were a vibrating spring connecting the twoatoms

'. 8or a simple diatomic molecule* this model is easy to visuali&e4



IR Spectroscopy

I. Introduction5. The I6 7pectroscopic "rocess

9. There are two types of bond vibration4•

Stretch :ibration or oscillation along the line of the bond

• Bend :ibration or oscillation not along the line of the bond

*

*

*

*

scissor

asymmeric

*

*

*

*

*

*

*

*

symmeric

roc* twist wag

in 4ane o) o5 4ane



5. The I6 7pectroscopic "rocess

/.0s a covalent bond oscillates due to the oscillation of the dipole ofthe molecule a varying electromagnetic ;eld is produced

. The greater the dipole moment change through the vibration* themore intense the E, ;eld that is generated

nfrared Spectroscopy



5. The I6 7pectroscopic "rocess

<.!hen a wave of infrared light encounters this oscillating E,;eld generated by the oscillating dipole of the same freuency*

the two waves couple* and I6 light is absorbed

=. The coupled wave now vibrates with twice the amplitude


I6 beam from spectrometer

E, oscillating wavefrom bond vibration

coupled wave



>. The I6 7pectrum1. Each stretching and bending vibration occurs with a

characteristic freuency as the atoms and charges involved are

di?erent for di?erent bonds

The y+ais on an I6spectrum is in unitsof @ transmittance

In regions where theE, ;eld of an osc.bond interacts withI6 light of the same ν transmittance is

low (light isabsorbed)

In regionswhere no osc.bond isinteracting with

I6 light*transmittancenears 1AA@




IR Spectroscopy

>. The I6 7pectrum

2. The +ais of the I6 spectrum is in units of wavenumbers* ν*which is the number of waves per centimeter in units of

cm+1 (6emember E B h ν or E B hc-λ)



IR Spectroscopy

>. The I6 7pectrum3. This unit is used rather than wavelength (microns) because

wavenumbers are directly proportional to the energy of

transition being observed chemists like this, physicistshate it

High frequencies and high wavenumbers equate higherenergy

is quicker to understand than

Short wavelengths equate higher energy

'. This unit is used rather than freuency as the numbers are morereal than the eponential units of freuency

9. I6 spectra are observed for the mid+infrared4 /AA+'AAA cm+1

/. The pea#s are Caussian distributions of the average energy of atransition



IR Spectroscopy

>. The I6 7pectrum. In general4

Dighter atoms will allow the oscillation to be faster higher

energy This is especially true of bonds to hydrogen 5+%* +% and F+%

7tronger bonds will have higher energy oscillations Triple bonds G double bonds G single bonds in energy

Energy- ν of oscillation



". The +pectrum The detection of different bonds

-. s opposed to chromatography or other spectroscopic methods, the

area of a band (or pea*) is not directly proportional to

concentration of the functional group producing the pea*

8. The intensity of an band is affected by two primary factors:

Whether the vibration is one of stretching or bending

Electronegativity difference of the atoms involved in the bond

• /or both effects, the greater the change in dipole moment in a

given vibration or bend, the larger the pea*.

• The greater the difference in electronegativity between the

atoms involved in bonding, the larger the dipole moment

• Typically, stretching will change dipole moment more than

bending




". The +pectrum The detection of different bonds

0. t is important to ma*e note of pea* intensities to show the effect of

these factors:

• Strong (s) pea* is tall, transmittance is low (1$25 3)

• Medium (m) pea* is mid$height (-5$253)

• Weak (w! pea* is short, transmittance is high (01$-53)

• * Broad (r) if the 4aussian distribution is abnormally broad

(*this is more for describing a bond that spans many energies)

Exact transmittance values are rarely recorded


f d S



. nfrared 4roup nalysis

. 4eneral

. The primary use of the is to detect functional grou!s

6. Because the loo*s at the interaction of the "' spectrum with

actual bonds, it provides a uni7ue 7ualitative probe into the

functionality of a molecule, as functional groups are merely different

configurations of different types of bonds

2. +ince most types9 of bonds in covalent molecules have roughly thesame energy, i.e., ; and ;< bonds, $= and &$= bonds they

show up in similar regions of the spectrum

>. emember all organic functional groups are made of multiple bonds

and therefore show up as multiple bands (pea*s)


f d S



. nfrared 4roup nalysis

. 4eneral

5. The four primary regions of the spectrum

#000 cm-1 600 cm-1 6000 cm-1 1%00 cm-1 %00 cm-1

O-H

N-H

C-H

C≡C

C≡N

C=O

C=N

C=C

in!erprint

Re!ion

C-C

C-N

C-O


"onds to H #rip$e %onds&ou%$e %onds Sin!$e "onds

f d S t



12 7kanes combination of $ and $= bonds• $ stretches and bends 2?1$>-1 cm$

• =6$=6 bond >51$>-1 cm$

• =6$=2 bond 2?1$201 cm$

• sp2 $= between 6811$2111 cm$

nfrared Spectroscopy<ctane

'( ) s* '+*

f d S t



62 7kenes addition of the ; and vinyl $= bonds• ; stretch at ?61$?81 cm$ wea*er as

substitution increases

• vinyl $= stretch occurs at 2111$211 cm$

• The difference between al*ane, al*ene or al*yne $=

is important@ f the band is slightly above 2111 it is

vinyl sp6 $= or al*ynyl sp $= if it is below it is al*yl

sp2 $=

$<ctenenfrared Spectroscopy

'( ) +*

'( ) +*

f d S t



32 7kynes addition of the ; and vinyl $= bonds• A stretch 611$66?1 cm$ strength depends on

asymmetry of bond, strongest for terminal al*ynes,

wea*est for symmetrical internal al*ynes

• $= for terminal al*ynes occurs at 2611$2211 cm$

• nternal al*ynes ( $A$ ) would not have this band@

$<ctynenfrared Spectroscopy

'+ ) s*

'(-+*

nfrared Spectroscop



#2 7romaics • Cue to the delocaliDation of e$ in the ring, $ bond

order is .5, the stretching fre7uency for these

bonds is slightly lower in energy than normal ;• These show up as a !air of sharp bands, 511 E

?11 cm$, (lower fre7uency band is stronger)

• $= bonds off the ring show up similar to vinyl $=

at 2111$211 cm$

"thyl benDenenfrared Spectroscopy

'( ) +* '( ) +*




#2 7romaics• f the region between ??-$6111 cm$ (w) is free of interference (;< stretching

fre7uency is in this region) a wea* grouping of pea*s is observed for aromatic systems• nalysis of this region, called the overtone of bending region, can lead to a

determination of the substitution pattern on the aromatic ring

'onosubstituted

,6 disubstituted (ortho or o$)

,6 disubstituted (meta or m$)

,> disubstituted ( para or p$)


8

8

8

8

8

8

8




,2 Unsa)raed 9ysems substitution patterns• The substitution of aromatics and al*enes can also be discerned through the out$

of$plane bending vibration region• =owever, other pea*s often are apparent in this region. These peaks should only

be used for reinforcement of what is known or for hypothesizing as to the functional pattern


R

*

R

*

*6

R

*

R

R

*6

R

R

R

*

R

*

R

*

.:,-..

.0,-.1,

cm-1

.%0-.:0

%%,-30

::,-:.,

.0-:#0

R

R

R

R

R

RR

cm-1

30-0%.0-10

3,-0

:%0-.00,0-:10%:0-6,

:00-:%0




%2 Eers addition of the $<$ asymmetric band and

vinyl $= bonds• +how a strong band for the antisymmetric $<$

stretch at 151$51 cm$

• <therwise, dominated by the hydrocarbon

component of the rest of the molecule

Ciisopropyl ether nfrared Spectroscopy

's*




2 7coos• +trong, broad <$= stretch from 2611$2>11 cm$

• Fi*e ethers, $< stretch from 151$6?1 cm$

• Band position changes depending on the alcohols

substitution: G 1-5$111 6G 1-5$51 2G 11$611

phenol 81$6?1

• The shape is due to the presence of hydrogen bonding

$butanolnfrared Spectroscopy

'+) s*%r

's*




:2 7mines $ !rimary

• +hows the &$= stretch for &=6 as a doulet

between 2611$2511 cm$ (symmetric and anti$

symmetric modes)

• $&=6 has deformation band from 501$?51 cm$

• dditionally there is a wag9 band at -81$861 cm$

that is not diagnostic

6$aminopentanenfrared Spectroscopy

'(* '(*




.2 7mines +econdary

• &$= band for 6&$= occurs at 2611$2511 cm$

as a single sharp pea* wea*er than <$=

• Tertiary amines (2&) have no &$= bond and

will not have a band in this region

pyrrolidinenfrared Spectroscopy

'( ) +*





;a)se and Review• nspect the bonds to = region (6-11 >111 cm$)

• !ea*s from 6851$2111 are simply sp2 $= in most organic molecules

• bove 2111 cm$ Learn shapes, not wavenumbers!!

road H+shape pea#+O,H bond

:+shape pea#+% bond for 2o amine (6

2

N,H)

7harp spi#e

+5≡C,H bond

!+shape pea#+% bond for 1o amine (6NH2)

3AAA cm+1

7mall pea#shouldered Justabove 3AAA cm+1

5BC,H or "h,H




102 7deydes • ;< (carbonyl) stretch from -61$->1 cm$

• Band is sensitive to conHugation, as are all

carbonyls (upcoming slide)

• highly uni7ue sp6 $= stretch appears as a

doublet, 6-61 E 6861 cm$ called a "ermi doublet 9

ycloheyl carboaldehydenfrared Spectroscopy

'(-+*

's*




112 <eones• +implest of the carbonyl compounds as far as

spectrum carbonyl only

• ;< stretch occurs at -15$-65 cm$

2$methyl$6$pentanonenfrared Spectroscopy

's*




162 EsersI ;< stretch at -25$-51 cm$

I +trong band for $< at a higher fre7uency than

ethers or alcohols at 51$651 cm$

nfrared Spectroscopy"thyl pivalate

's*

's*




132 ar&oxyic 7cids• 4ives the messiest of spectra

• ;< band occurs between -11$-65 cm$

• The highly dissociated <$= bond has a broad band from 6>11$

2511 cm$ covering up to half the spectrum in some cases

>$phenylbutyric acidnfrared Spectroscopy

'( ) +*%r

's* 's*




1#2 7cid anydrides• oupling of the anhydride though the ether oygen

splits the carbonyl band into two with a separation

of -1 cm$

• Bands are at ->1$--1 cm$ and 81$8>1 cm$

• 'ied mode $< stretch at 111$11 cm$

!ropionic anhydridenfrared Spectroscopy

's* 's*

>

> >




1,2 7cid aides• lefted band at --1$861 cm$ for ;<

• Bonds to halogens, due to their siDe (see =oo*eJs

Faw derivation) occur at low fre7uencies, only l islight enough to have a band on , $l is at ?11$

811 cm$

!ropionyl chloridenfrared Spectroscopy

's*

's*

3

>




1%2 7mides• Cisplay features of amines and carbonyl compounds• ;< stretch at ?>1$?81 cm$

• f the amide is primary ($&=6) the &$= stretch occursfrom 2611$2511 cm$ as a doublet

• f the amide is secondary ($&=) the &$= stretch

occurs at 2611$2511 cm$ as a sharp singlet

pivalamidenfrared Spectroscopy

'+ ) s* 's*

?*6

>




12 ?iro gro)4 (-?>6!

• !roper Fewis structure gives a bond order of .5

from nitrogen to each oygen

• Two bands are seen (symmetric and asymmetric) at211$281 cm$ and 511$5-1 cm$

• This group is a strong resonance withdrawing group

and is itself vulnerable to resonance effects

6$nitropropanep py

's* 's*

?

>>




1:2 ?iries (e cyano- or @A? gro)4!• !rinciple group is the carbon nitrogen triple

bond at 611$6681 cm$

• This pea* is usually much more intense thanthat of the al*yne due to the electronegativity

difference between carbon and nitrogen

propionitrilep py

's*

?




E55ecs on IR &ands

12 onB)gaion by resonance, conHugation lowers the energy of a double or triple

bond. The effect of this is readily observed in the spectrum:

• onHugation will lower the observed band for a carbonyl from 61$>1 cm$

provided conHugation gives a strong resonance contributor

p py

>

>

1%:# cm-1

11, cm-1

=> =>

*3

>

X X = ?*6

*3

?>6

1% 1%: 1%.6 100 cm-1

*6? *3

>

9rong resonance conri&)or

vs2 ?

>

>*3

>

;oor resonance conri&)or (canno resonae wi =>!




E55ecs on IR &ands

62 9eric e55ecs usually not important in spectroscopy, unless they reduce the

strength of a bond (usually π) by interfering with proper orbital overlap:

• =ere the methyl group in the structure at the right causes the carbonyl

group to be slightly out of plane, interfering with resonance

32 9rain e55ecs changes in bond angle forced by the constraints of a ring will

cause a slight change in hybridiDation, and therefore, bond strength

• s bond angle decreases, carbon becomes more electronegative, as well

as less sp6 hybridiDed (bond angle K 61G)

p py

<

;<: 5?8? cm$5

<

;<: 5?02 cm$5

=2

< < < < <

85 cm$ --5 cm$ -51 cm$ -5 cm$ -15 cm$




E55ecs on IR &ands

#2 *ydrogen &onding• =ydrogen bonding causes a broadening in the band due to the creation of

a continuum of bond energies associated with it

• n the solution phase these effects are readily apparent in the gas phasewhere these effects disappear or in lieu of steric effects, the band appears

as sharp as all other bands:

4as phase spectrum of

$butanol

+teric hindrance to =$bonding

in a di$tert $butylphenol

• =$bonding can interact with other functional groups to lower fre7uencies

p py

<=

;< -1 cm$5

<<=

CHEM 210 IR Spectroscopy

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