NMR and IR Spectroscopy - IUPAC

baconossifiedMechanics

Oct 29, 2013 (3 years and 7 months ago)

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Introduction and Scope of this Course



(Mission Impossible
[1]
)



Spectroscopic methods that means:









[1]

The topic means material for several year's courses. Consequently, this course (and text) can rather give a coarse overview a
nd

a collection of literature to go for the details.
No completeness is claimed and no perfection!




NMR spectroscopy





Dielectrical spectroscopy





Infrared spectroscopy





UV
-
vis spectroscopy





X
-
ray spectroscopy






Mass spectroscopy



dynamic
-
mechanic spectroscopy






Material properties of polymers:


Chemical structure


Configuration


Conformation


Physical Structure


Dynamics






in the liquid

and

in the solid state

Infrared Spectroscopy:
l =
760 nm….1mm

near infrared

“quartz
-
infrared”

~ 10,000…4,000 cm
-
1


NIR

middle infrared

“conventional” infrared

~4,000…250 cm
-
1

far infrared

< 250 cm
-
1

use of quartz cuvettes and light pipes

higher order absorptions (lower intensity)

liquids can be measured in thicker layers

4000 cm
-
1
…50 cm
-
1

fundamental vibrations

4000 cm
-
1
…400 cm
-
1

fundamental vibrations

12500 cm
-
1
…4000 cm
-
1

overtones & combinations

scattering

absorption

absorption

monochromatic excitation source

dispersed polychromatic radiation

information from scattered radiation

information from absorbed radiation

homonuclear functionalities

changes in polarizability

polar functionalities

changes in dipol moment

CH/OH/NH

functionalities

high structural selectivity

high structural selectivity

low structural selectivity

I
raman
~ c

Lambert
-
Beer
-
Law

Lambert
-
Beer
-
Law

no sample preparation

required

no sample preparation

required

sample preparation

required (except ATIR)

sample volume µL

sample thickness µm

sample thickness up to cm
-
range

sample volume µL

sample thickness µm

light
-
fiber optics

>100 m

light
-
fibre optics

>100 m

limited

Raman


MIR


NIR

1790
-
1720

very strong

1610
-
1590,

1600
-
1580 and

1510
-
1490

Modif.

Epoxies

Polycarbo=

nates

Alkyd
-
,

Polyesters,

Cellulose=

ether,

PVC

(plasticized)

Polyvinyl=

acetate,

PVC
-
copo=

lymers

Cellulose=

ester

Polyure=

thane

Acrylics,

Polyester

Phenol

derivatives,

Epoxies

Polystyrenes,

Arylsilicones,

Aryl
-
alkyl=

Silicone Co=

polymers


Polyamides,

amines

Nitrocellulose

cellophan

Cellophan,

Alkylcellulose,

PVA, PEO


PAN, PVC,

Polyvinyliden

chlorid,

POM

Alkylsilicone,

aliphatic hy=

drocarbons,

Polytetra=

Fluorethylene,

Thiokol

1450
-
1410

sharp

1680
-

1630

strong

1550
-

1530

1610

1590,

1600


1580 and

1510
-

1490


3500
-

3200

1100
-

1000

1450
-

1410


sharp

840
-

820

3500
-

3200

strong

All numbers have the meaning of wave numbers

and are given in cm
-
1

yes

no

Intensity, arbitrary units

1/d
= 2
/
n
(1
/
l
1

-
1/
l
2
)

n

= number of minima between two maxima
l
1

and

l
2

wave length

3500
-
3200

cm
-
1

1790
-
1720

cm
-
1

1610
-
1590

1600
-
1580
cm
-
1

1510
-
1490

1680
-
1630

cm
-
1

1550
-
1530

cm
-
1

Polyamid


epoxies, polycarbonate,

alkyd resins, polyesters,

cellulose
-
ether, PVC

poly(vinyl acetate), PVC
-
copoly., cellulose ester, PU

acryl polymers



Phenol resins, epoxies, aryl polymers

1790
-
1720

cm
-
1

modified epoxides, polycarbonate, Alkyd resins, polyester, cellulose ester, cellulose ether, PVC (plast),

PVAc, PVC
-
copolym., PU, acrylics


1610
-
1590

1600
-
1580 cm
-
1

1510
-
1490

modified epoxides, polycarbonate, Alkyd resins, polyester, cellulose ester, cellulose ether, PVC (plast)

820
-
840 cm
-
1

modified epoxies, polycarbonate

polycarbonate

?

polycarbonate

1610
-
1590

1600
-
1580 cm
-
1

1510
-
1490

1450
-
1410 cm
-
1

1100
-
1000 cm
-
1

typical pattern of normal PC

typical pattern of PU

C
-
O
-
C
-
ether region

Poly (ether urethane)

? cellulose ester or polyurethane ?

Infrared Spectroscopy:
l =
760 nm….1mm

near infrared

“quartz
-
infrared”

~ 10,000…4,000 cm
-
1


NIR

middle infrared

“conventional” infrared

~4,000…250 cm
-
1

far infrared

< 250 cm
-
1

use of quartz cuvettes and light pipes

higher order absorptions (lower intensity)

liquids can be measured in thicker layers

NIR



Hydrogen
-
containing groups are dominant


Information is often implicid, coupled vibrations


Not suited for trace analysis



Easy analysis of aqueous solutions


Process
-
analysis


Use of light
-
pipes even without cuvette (reflection)


Easy analysis of powders using diffuse reflection


Characterisation of fillers


Determination of water contents in liquids and solids

general pertubation

mechanical, electric, electro
-
magnetic, chemical,…

Electro
-
magnetic probe

IR

X
-
ray, UV
-
vis, NMR,…

Samples are exposed to external pertubations such as:




temperature


pressure


stress


Resolution (the large number) of overlapping NIR bands can be enhanced

and MIR and NIR correlation spectra are very useful for peak assignement



NMR

can

provide

information

about
:


Polymers

in

Solution



The

microstucture

of

polymer

chains


Resonance

assignement


regioisomerism


Stereochemical

configuration


Geometric

isomerism


Isomerism

in

diene

polymers


Asymmetric

centres

in

the

main

chain


Branching

and

cross
-
linking


End

groups


Configurational

statistics


Copolymerization

sequences


Chain

conformation

in

solution


Intermolecular

association









1
H:


natural abundance
99.9844

%


relative sensitivity
1


chemical shift range 10 ppm


1H
-
1H
-
spin
-
spin coupling
chemical environment


chemical structure, regiochemistry, stereochemistry,


conformation


13
C:


natural abundance 1.108%


relative sensitivity 1.59

10
-
2


Chemical shift range
250 ppm


long relaxation times


sensitive to
subtle changes in the near electronic


environment

but insensitive for long
-
range inter
-


actions (solvent effects, diamagnetic anisotropy of


neighbouring groups)


no homonuclear

coupling


Separate resonance for every C in a molecule







But also other e. g.: Si, O, N, P, Al,
Xe (!)
…can be important

Characterization in the Solid State


Chain conformation in the solid state


Solid
-
solid transitions


Organization in the solid state


In multi
-
phase polymers


Orientation


Imaging


Dynamics of Polymers in the Solid State


Semicristalline polymers


Amorphous polymers


Polymer Systems


Polymer blends and miscibility


Multiphase systems



spin
-
echo pulses



selective scalar
-
spin decoupling



off
-
resonance decoupling



selective
13
C
-
excitation



selective multiplet acquisitation (DANTE)



signal enhancement by polarisation tranfer



proton multiplicity on carbons (INEPT, DEPT)



C
-
C connectivity (INADEQUATE)



2
-
Dimensional (and higher) NMR (COSY, NOESY)



“Many of the substantial improvements in NMR are the result of the

spin gymnastics that can be orchestrated by the spectroscopist”*
)

on

the Hamiltonean with a mystic zoo of weird pulse sequences**
)

*
)
T. C. Farrar

**
)
M. Hess

“Advances

in

liquid

and

solid

state

NMR

techniques

have

so

changed



the

picture

that

it

is

now

possible

to

obtain

detailed

information

about




the

mobilities

of

specific

chain

units



domain

structures



end

groups



run

numbers



number
-
average

molecular

weights



minor

structure

aberations


in

many

synthetic

and

natural

products

at

a

level

of


1

unit

per

10
,
000

carbon

atoms

and

below”


J
.

C
.

Randall

(eds
.
)

NMR

and

Macromolecules,

ACS
-
Symp
.

Ser
.

247
,


American

Chemical

Society,

Washington

DC

(
1984
),

p
.

245

10
-
12
10
-
10

10
-
8

10
-
6

10
4
10
2
10
0


T
1
, T
2
, NOE


2D exchange

correlation times [s]

2
H lineshape

CSA lineshape

dipolar lineshape

T
1
r

2
H echo

dynamic range

measured by

different NMR
-
techniques

nuclei with a spin quantum number
I

*)


angular momentum
J

=
ħ

{
I
(
I
+1)}
1/2

magnetic moment
m
I

=
I
,
I
-
1,
I
-
2…0…
-
I




I
H1)=s瑡瑥s
=
ncla=magn瑩c=m潭n琠z
-
c潭p潮n琩=
µ
z

=

**)

ħ m
I

energy of the state:
E

=
µ
z

B
0

=
-


ħ m
I
B
0

Larmor
-
frequency:

0

= 2


0

=


B
0

m
I

an ensemble of isolated spins
I

=


1/2

in an external field
B
0

split up into two states


(lower) and


(higher)

energy difference:

E

=
E

-
E

=
h

0
=
ħ


0
=


ħ

B
0

in an external field
B
0
:


*)

integers are Bosons others are Fermions

**)
(experimental)

magnetogyric ratio


E

=
h




=

0
ħ

=




ħ

B
0

N


=
N


exp(
-





k
b

T
)


N



N




E(B
0
=14.1T)
@

0.5 J

m

B
0

B
1


E
=
mB
0
=

0

ħ
(spin =1/2 nuclei)

Q


0

B =


H

B

= magnetic flux density (induction) [T]
*)

H

= magnetic field strength [A m
-
1
]



= permeability

B

or
H
??

A question of “taste”

B
~ origin of the field

H

~ field properties

T = kg s
-
2

A
-
1
= V s m
-
2

=10
4
G

H
0

rf
-
transmission and

Detection coil (antenna)

laboratory (static) frame:

coordinates x, y, z


rotating frame:

coordinates x’, y’, z’



branching in PE



thermal oxidation in PE



stereoregularity e.g. PMMA, PP



directional isomerism (regio
-
isomerism: head
-
tail,…)



copolymer structure



comparison of the chemical shift with known model
-
compounds



calculation of
13
C
-
shifts by the (additive) increment method



synthesis of polymers with known structure or compositional features



selective
13
C
-
enrichment



comparison of experimental results with calculated intensities


(simulation of the polymerisation kinetics)



determination of C
-
H bonds (INEPT)*, C
-
C bonds (INADEQUATE)*



2
-
dimensional techniques

* These are specific pulse sequences for particular spectral editing

“The concept of the rotating frame is of paramount importance in

NMR
-
spectroscopy. For almost all classical descriptions of NMR
-

experiments are described using this frame of reference”







D. E. Traficante

Q

rotating frame y’

X’

m

B
0

B
1


0

static laboratory frame

y

x

z
=z’

the frame is rotating with

the frequency of the applied

rf
-
field

a nucleus with the Larmor frequency

equal to the rotation of the frame is

static

with respect to the frame

the

original

focussed

and

in
-
phase

in

the

x
-
y

plane

rotating

magnetisation

decreases

by

two

effects
:



interaction

with

the

environment

(“the

lattice”)



Relaxation

time

T
1

(spin
-
lattice

relaxation,

longitudinal

relaxation)



interaction with neighbouring spins (dephasing)


Relaxation time T
2

(spin
-
spin relaxation, transversal relaxation)

M
z
(t)
-
M
z
(t=0) ~ exp [
-
t/T
1
]

M
y
(t)
-
M
y
(t=0) ~ exp [
-
t/T
2
]


the “effective” T
2

(
T
2
*
)

is responsible for the
line broadening (transversal relaxation +
inhomogeneous field broadening):


1/2
= 1/(

T
2
*
)


1/2
= line
-
width at ½ of the peak
-
height [Hz]

T
1
r

is the longitudinal relaxation time i
n the rotating frame. T
1
r


>
T
1


in solids and in solutions of high polymers: T
1
>>T
2

T
2

is affected by molecular motion at the Larmor frequency

and low
-
frequency motions around 10
2
-
10
3

Hz


non
-
viscous liquids T
1

= T
2

T
1

sensitive to motions 5
-
500 MHz
*)

T1
r

is sensitive to motions in the tens kHz
-
range

13
C
-
Relaxations

*)

short range, high frequency segmental motions, local environment is reflected

90
°
-
pulse

dephasing

slow

fast

180
°
-
pulse

Re
-
focussing re
-
focussed and echo

slow

fast

only the
“inhomogeneous”
contribution of
the T
2
is refocus=
sed

the result

of the “true” T
2
-
process is not
re
-
focussed

the echo decays
according to the
true T
2

Direct chemical exchange (e. g.: the hydroxyl proton


of an alcohol with water protons)

Magnetization exchange (e. g.: cross
-
polarization, NOE)

Decoupling of heteronuclear spin coupling causes the


NUCLEAR OVERHAUSER EFFECT (NOE)

Decoupling
1
H
-
13
C saturates
1
H and changes the
13
C
-
spin population

excess
13
C in the lower level compared

with the equilibrium distribution

more energy is absorbed


E = 1 + (

H
/2
C
)

NOE depends on the specific resonance makes quantification difficult

better S/N

The NOE is sensitive to the distance


NOESY experiment

The total Hamilton operator is given by the sum of the individual
interactions:





=

z

+

q

+

dd

+



+

k

+

J




z

= zeeman interaction with the external magnetic field (constant
term)


q

= quadrupol interaction


dd

= direct dipolar interaction




= magnetic shielding (chemical shift) reflects the chemical
environment


k

= knight shift


J

= indirect coupling



z




q




dd









k




J


A
-
B

A
-
B

2 single crystals AB with different orientation

with respect to
B
0

B
0

powder pattern of a polycrystalline AB

“iso” corresponds to an (isotropic) solution

powder pattern of a polycrystalline AB

but fully anisotropic, tensor components

as indicated

The relaxation times are correlated with molecular motions

C
ross
-
P
olarization and
M
agic
A
ngle

S
pinning

lattice

T
CH

T
1
r
H

T
1
H

1
H spins

T
H

T
1
C

T
1
r
C

13
C spins

T
C


H
B
rf
1H

=

C
B
rf
1C

Hartmann
-
Hahn condition

spin temperature

spin temperature

Broad lines

narrow is beautiful

…but line shape can also tell us a lot

2D
-
Experiments can be useful:


for the separation of shifts and scalar couplings in isotropic phase


in particular in weakly coupled homo
-
and heteronuclear systems


in oriented phase, especially in static powders or magic
-
angle spinning


samples information can be extracted by separation of dipolar


couplings and anisotropic chemical shifts that cannot be obtained (easily)


from 1D
-
spectra


isotropic and anisotropic chemical shift components can be


separated in two frequency domains in the solid state

2D
-
Experiments can be designed to


separate different interactions (shifts, couplings)


Correlate transitions of coupled spins


Study dynamic processes (chemical exchange, cross
-
relaxation,


transient Overhauser effects, spin diffusion…)

2
-
dimensional
*)

NMR

*)

and higher



correlated 2D
-
NMR



exchange 2D
-
NMR



resolved 2D
-

NMR

molecular connectivities, distances

interactions

In fact projections (contour plots) of 3D
-
spectra


The many possible experiments can be categorised as:

molecular motion, environment

Advantage over e. g. decoupling: no loss of information, just unravelling

of overlapping signals


The properties and motions of a spin system are represented by the

Hamilton Operator
H
*)

*)
in fact in NMR a reduced Hamiton spin operator
H
s
is sufficient


When
H

contains contributions of different physical origin

(e. g.: chemical shift, dipolar or scalar couplings…)

it is sometimes possible

to separate these effects in a multi
-
dimensional plot

preparation

evolution

mixing

detection


system is prepared in a coherent non
-
equilibrium state


System evolves under the influence


of what ever modification (pulse sequence)


Transformation into transversal magnetization


Measurement of the transversal magnetization

collect incremented FIDs

type of
experiment

X
-
axis

Y
-
axis

information

heteronuclear
J
-
resolved

d
C

J
CH

heteronuclear coupling
constants

homonuclear
J
-
resolved

d
H

J
HH

homonuclear J and
d
=
ht潮捬a=
捨浩捡l=shi晴
=
d
C

d
H

correlation of
d
H

and

d
C


COSY

d
C

d
H

correlation of all scalar
coupling interactions

NOESY

d
H
, J
HH

d
H
, J
HH

spatial proximity of non
-

bonded protons

INADEQUATE

d
X

d
A

+

d
X

heteronuclear connecti
-

vities

2D
-
spectra, if the spins are modulated

By spin
-
spin, chemical shift or dipole
-
dipole

Interactions during the evolution time

Coupling resolved spectra:


y
-

axis


coupling information


x
-

axis


chem shift information

Coupling correlated spectra:


y
-

axis


chem shift information


x
-

axis


chem shift information

correlated through homo
-
or heteronuclear

or dipolar coupling

Exchange spectra:



y
-

axis


chem shift information



x
-

axis


chem shift information

correlated through chemical exchange,

conformational or motional effects, or

Overhauser effects from non
-
bonded H

J
-
coupling

COSY
-
experiment

Evolution:

/2
-
t
1
-

/2
-
t
2

protons arrange according to their phases

controlled by their individual chemical shift

sampling of the magnetization

components

diagonal: all auto
-
correlated H

off
-
diagonal: cross
-
correlated H with another H, shows connectivities

C

CH
2

CH
3

CH
3

n

PIB

CH
2

CH
3

ppm (1H)

ppm 13C

CH
3

CH
2

C

Hetero
-
nuclear
-
2D
-

spectrum, no diagonal

peaks

The off
-
diagonal (cross
-
peaks) indicate

that the nuclei A and B are geometrically

closer than about 0.5 nm.


The density of cross
-
peaks can be fairly

high in real spectra (e.g.: proteins).

In this case further dimensions can help

to resolve the interactions


Combined with molecular modelling and

Conformation
-
energy studies the distance

Information can lead to 3
-
dimensional

Molecule structures in solution

Parella T, Sánchez
-
Fernando F, Virgili A (1997) J Magn Reson
125
, 145

when there is a cross
-
peak it shows

that the spins A and B are geometrically

closer than ~0.5 nm

a real 2D proton NOE spectrum

of a protein*)

For further peak assignment higher dimensions of spectroscopy are required

*)After G. W. Vuister, R. Boelens, R. Kaptein (1988) J. Magn. Res.
80
, 176

Example of a 2D
-
NOE Spectrum

PS/PVME blend cast from CHCl
3

PS/PVME blend cast from toluene

after: P. Carvatti, P. Neuenschwander, R. R. Ernst (1985) Macromolecules
18
, 119

Diagonal: spin exchange, same types

of spins

Off
-
diagonal peaks: spin diffusion among

chemically different spins

no cross
-
peaks between PS and PVME

cross
-
peaks between PS and PVME

indicating (molecular) mixed domains

Self
-
diffusion

Spin
-
echo experiment

aquisition

Hahn
-
echo pulse
sequence



= diffusion time

d

= gradient pulse length



= magnetogyric ratio

g

= gradient strength

E

= signal amplitude

unrestricted diffusion

D

= diffusion coefficient

R
h
= apparent hydrodynamic radius



= viscosity of the pure solvent


r
2


= mean
-
square diffusion length

Stokes
-
Einstein

Einstein
-
Smoluchowsky

6


r
h
3