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INTEGRATION. NMR Spectrum of Phenylacetone. RECALL. Each different type of proton comes at a different place . You can tell how many different types of hydrogen there are in the molecule. from last time. INTEGRATION OF A PEAK. Not only does each different type of hydrogen give a

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Slide2 l.jpg

NMR Spectrum of Phenylacetone

RECALL

Each different type of proton comes at a different place .

You can tell how many different types of hydrogen

there are in the molecule.

from last

time


Slide3 l.jpg

INTEGRATION OF A PEAK

Not only does each different type of hydrogen give a

distinct peak in the NMR spectrum, but we can also tell

the relative numbers of each type of hydrogen by a

process called integration.

Integration = determination of the area

under a peak

The area under a peak is proportional

to the number of hydrogens that

generate the peak.


Slide4 l.jpg

Benzyl Acetate

The integral line rises an amount proportional to the number of H in each peak

METHOD 1

integral line

integral

line

simplest ratio

of the heights

55 : 22 : 33 = 5 : 2 : 3


Benzyl acetate ft nmr l.jpg
Benzyl Acetate (FT-NMR)

Actually : 5 2 3

21.215 / 11.3

= 1.90

33.929 / 11.3

= 3.00

58.117 / 11.3

= 5.14

METHOD 2

assume CH3

33.929 / 3 = 11.3

digital

integration

Integrals are

good to about

10% accuracy.

Modern instruments report the integral as a number.


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DIAMAGNETIC ANISOTROPY

SHIELDING BY VALENCE ELECTRONS


Slide7 l.jpg

Bo applied

Diamagnetic Anisotropy

The applied field

induces circulation

of the valence

electrons - this

generates a

magnetic field

that opposes the

applied field.

valence electrons

shield the nucleus

from the full effect

of the applied field

magnetic field

lines

B induced

(opposes Bo)

fields subtract at nucleus


Slide8 l.jpg

SPECTRUM

DOWNFIELD

UPFIELD

Less shielded protons

appear here.

Highly shielded

protons appear here.

It takes a higher field

to cause resonance.

PROTONS DIFFER IN THEIR SHIELDING

All different types of protons in a molecule

have a different amounts of shielding.

They all respond differently to the applied magnetic

field and appear at different places in the spectrum.

This is why an NMR spectrum contains useful information

(different types of protons appear in predictable places).



Slide10 l.jpg

PEAKS ARE MEASURED RELATIVE TO TMS

Rather than measure the exact resonance position of a

peak, we measure how far downfield it is shifted from TMS.

reference compound

tetramethylsilane

“TMS”

Highly shielded

protons appear

way upfield.

TMS

Chemists originally

thought no other

compound would

come at a higher

field than TMS.

shift in Hz

downfield

n

0


Slide11 l.jpg

g

2p

REMEMBER FROM OUR EARLIER DISCUSSION

Stronger magnetic fields (Bo) cause

the instrument to operate at higher

frequencies (n).

field

strength

frequency

hn = Bo

NMR Field

Strength

1H Operating

Frequency

constants

60 Mhz

1.41 T

2.35 T

100 MHz

n = ( K) Bo

7.05 T

300 MHz


Slide12 l.jpg

HIGHER FREQUENCIES GIVE LARGER SHIFTS

The shift observed for a given proton

in Hz also depends on the frequency

of the instrument used.

Higher frequencies

= larger shifts in Hz.

TMS

shift in Hz

downfield

n

0


Slide13 l.jpg

THE CHEMICAL SHIFT

The shifts from TMS in Hz are bigger in higher field

instruments (300 MHz, 500 MHz) than they are in the

lower field instruments (100 MHz, 60 MHz).

We can adjust the shift to a field-independent value,

the “chemical shift” in the following way:

parts per

million

shift in Hz

chemical

shift

= d =

= ppm

spectrometer frequency in MHz

This division gives a number independent

of the instrument used.

A particular proton in a given molecule will always come

at the same chemical shift (constant value).


Slide14 l.jpg

HERZ EQUIVALENCE OF 1 PPM

What does a ppm represent?

1 part per million

of n MHz is n Hz

1H Operating

Frequency

Hz Equivalent

of 1 ppm

1

(

)

n MHz = n Hz

60 Mhz 60 Hz

106

100 MHz 100 Hz

300 MHz 300 Hz

ppm

3

2

1

0

7

6

5

4

Each ppm unit represents either a 1 ppm change in

Bo (magnetic field strength, Tesla) or a 1 ppm change

in the precessional frequency (MHz).


Nmr correlation chart l.jpg
NMR Correlation Chart

-OH

-NH

DOWNFIELD

UPFIELD

DESHIELDED

SHIELDED

CHCl3 ,

TMS

d (ppm)

12

11

10

9

8

7

6

5

4

3

2

1

0

H

CH2Ar

CH2NR2

CH2S

C C-H

C=C-CH2

CH2-C-

CH2F

CH2Cl

CH2Br

CH2I

CH2O

CH2NO2

C-CH-C

RCOOH

RCHO

C=C

C

C-CH2-C

C-CH3

O

Ranges can be defined for different general types of protons.

This chart is general, the next slide is more definite.


Slide16 l.jpg

O

O

O

O

O

O

O

APPROXIMATE CHEMICAL SHIFT RANGES (ppm) FOR SELECTED TYPES OF PROTONS

R-CH3 0.7 - 1.3

R-N-C-H 2.2 - 2.9

R-C=C-H

R-CH2-R 1.2 - 1.4

4.5 - 6.5

R-S-C-H 2.0 - 3.0

R3CH 1.4 - 1.7

I-C-H 2.0 - 4.0

H

R-C=C-C-H 1.6 - 2.6

Br-C-H 2.7 - 4.1

6.5 - 8.0

Cl-C-H 3.1 - 4.1

R-C-C-H 2.1 - 2.4

R-C-N-H

RO-C-H 3.2 - 3.8

5.0 - 9.0

RO-C-C-H 2.1 - 2.5

HO-C-H 3.2 - 3.8

R-C-H

HO-C-C-H 2.1 - 2.5

9.0 - 10.0

R-C-O-C-H 3.5 - 4.8

N C-C-H 2.1 - 3.0

O2N-C-H 4.1 - 4.3

R-C-O-H

R-C C-C-H 2.1 - 3.0

11.0 - 12.0

F-C-H 4.2 - 4.8

C-H 2.3 - 2.7

R-N-H 0.5 - 4.0 Ar-N-H 3.0 - 5.0 R-S-H

R-O-H 0.5 - 5.0 Ar-O-H 4.0 - 7.0

1.0 - 4.0

R-C C-H 1.7 - 2.7


Slide17 l.jpg

YOU DO NOT NEED TO MEMORIZE THE

PREVIOUS CHART

IT IS USUALLY SUFFICIENT TO KNOW WHAT TYPES

OF HYDROGENS COME IN SELECTED AREAS OF

THE NMR CHART

C-H where C is attached to an

electronega-tive atom

CH on C

next to

pi bonds

aliphatic

C-H

acid

COOH

aldehyde

CHO

benzene

CH

alkene

=C-H

X=C-C-H

X-C-H

12

10

9

7

6

4

3

2

0

MOST SPECTRA CAN BE INTERPRETED WITH

A KNOWLEDGE OF WHAT IS SHOWN HERE


Slide18 l.jpg

DESHIELDING AND ANISOTROPY

Three major factors account for the resonance

positions (on the ppm scale) of most protons.

1. Deshielding by electronegative elements.

2. Anisotropic fields usually due to pi-bonded

electrons in the molecule.

3. Deshielding due to hydrogen bonding.

We will discuss these factors in the sections that

follow.


Slide19 l.jpg

DESHIELDING BY

ELECTRONEGATIVE ELEMENTS


Slide20 l.jpg

DESHIELDING BY AN ELECTRONEGATIVE ELEMENT

d-

d+

Chlorine “deshields” the proton,

that is, it takes valence electron

density away from carbon, which

in turn takes more density from

hydrogen deshielding the proton.

Cl

C

H

d-

d+

electronegative

element

NMR CHART

“deshielded“

protons appear

at low field

highly shielded

protons appear

at high field

deshielding moves proton

resonance to lower field


Electronegativity dependence of chemical shift l.jpg
Electronegativity Dependence of Chemical Shift

Dependence of the Chemical Shift of CH3X on the Element X

Compound CH3X

CH3F CH3OH CH3Cl CH3Br CH3I CH4 (CH3)4Si

Element X

F O Cl Br I H Si

Electronegativity of X

4.0 3.5 3.1 2.8 2.5 2.1 1.8

Chemical shift d

4.26 3.40 3.05 2.68 2.16 0.23 0

most

deshielded

TMS

deshielding increases with the

electronegativity of atom X


Substitution effects on chemical shift l.jpg
Substitution Effects on Chemical Shift

most

deshielded

The effect

increases with

greater numbers

of electronegative

atoms.

CHCl3 CH2Cl2 CH3Cl

7.27 5.30 3.05 ppm

most

deshielded

-CH2-Br -CH2-CH2Br -CH2-CH2CH2Br

3.30 1.69 1.25 ppm

The effect decreases

with incresing distance.


Slide23 l.jpg

ANISOTROPIC FIELDS

DUE TO THE PRESENCE OF PI BONDS

The presence of a nearby pi bond or pi system

greatly affects the chemical shift.

Benzene rings have the greatest effect.



Slide25 l.jpg

ANISOTROPIC FIELD IN AN ALKENE

protons are

deshielded

H

H

Deshielded

shifted

downfield

fields add

C=C

H

H

secondary

magnetic

(anisotropic)

field lines

Bo


Slide26 l.jpg

ANISOTROPIC FIELD FOR AN ALKYNE

H

C

C

H

secondary

magnetic

(anisotropic)

field

Shielded

hydrogens

are shielded

Bo

fields subtract



Slide28 l.jpg

HYDROGEN BONDING DESHIELDS PROTONS

The chemical shift depends

on how much hydrogen bonding

is taking place.

Alcohols vary in chemical shift

from 0.5 ppm (free OH) to about

5.0 ppm (lots of H bonding).

Hydrogen bonding lengthens the

O-H bond and reduces the valence

electron density around the proton

- it is deshielded and shifted

downfield in the NMR spectrum.


Slide29 l.jpg

SOME MORE EXTREME EXAMPLES

Carboxylic acids have strong

hydrogen bonding - they

form dimers.

With carboxylic acids the O-H

absorptions are found between

10 and 12 ppm very far downfield.

In methyl salicylate, which has strong

internal hydrogen bonding, the NMR

absortion for O-H is at about 14 ppm,

way, way downfield.

Notice that a 6-membered ring is formed.


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