8/3/2019 Molecular Structure and Chemical Shifts

http://slidepdf.com/reader/full/molecular-structure-and-chemical-shifts 1/20

Molecular structureMolecular structureand chemical shiftsand chemical shiftsMolecular structureMolecular structureand chemical shiftsand chemical shifts

NMR spectroscopyNMR spectroscopy

8/3/2019 Molecular Structure and Chemical Shifts

http://slidepdf.com/reader/full/molecular-structure-and-chemical-shifts 2/20

Local diamagnetic

shielding Associated with the circulation of

electrons caused by the applied

magnetic field around the protonitself

Electron density««..

Electronegativity of the groupattached

8/3/2019 Molecular Structure and Chemical Shifts

http://slidepdf.com/reader/full/molecular-structure-and-chemical-shifts 3/20

ElectronegativityElectronegativity

DependenceDependence

of Chemical Shift of Chemical Shift 

Compound CH3X

Element X

Electronegativity of X

Chemical shift H

CH3F  CH3OH  CH3Cl CH3Br CH3I  CH4 (CH3)4Si

F  O  Cl Br I  H  Si

4.0 3.5 3.1 2.8 2.5 2.1 1.8

4.26 3.40 3.05 2.68 2.16 0.23 0

Dependence of the Chemical Shift of CH3X on the Element X

deshielding increases with theelectronegativity of atom X

T MSmostdeshielded

8/3/2019 Molecular Structure and Chemical Shifts

http://slidepdf.com/reader/full/molecular-structure-and-chemical-shifts 4/20

Substitution Effects on ChemicalSubstitution Effects on Chemical

Shift Shift 

CHCl3

CH2

Cl2

CH3

Cl

7.27 5.30 3.05 ppm

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

3.30 1.69 1.25 ppm

most 

deshielded

most 

deshielded

The effect decreases

with incresing distance.

The effect 

increases with

greater numbersof electronegative

atoms.

8/3/2019 Molecular Structure and Chemical Shifts

http://slidepdf.com/reader/full/molecular-structure-and-chemical-shifts 5/20

A proton may experience additionalshielding or deshielding

Magnetic anisotropic effects operatethrough the space no matter theinfluenced group is attached to the

anisotropic group or not

Magnetic ANISOTROPIC Effect Magnetic ANISOTROPIC Effect 

DUE TO THE PRESENCE OF PI BONDS

The presence of a nearby pi bond or pi system

greatly affects the chemical shift.

8/3/2019 Molecular Structure and Chemical Shifts

http://slidepdf.com/reader/full/molecular-structure-and-chemical-shifts 6/20

ANISOT ROPIC FIELDANISOT ROPIC FIELD

FOR AN ALKYNEFOR AN ALKYNE

Bo

secondary

magnetic

(anisotropic)

 field

H

H

CC

hydrogens

are shielded

Shielded

 fields subtract 

8/3/2019 Molecular Structure and Chemical Shifts

http://slidepdf.com/reader/full/molecular-structure-and-chemical-shifts 7/20

Acetylene Protons, H2.5

8/3/2019 Molecular Structure and Chemical Shifts

http://slidepdf.com/reader/full/molecular-structure-and-chemical-shifts 8/20

Shielding of acetylene

protons If oriented perpendicular to AMF the electrons are not free circulateand cause deshielding

Net effect average over all possibleorientations is affected by thesealigned molecules

Chemical shift observed is sum oflocal diamagnetic and magneticanisotropic effects

8/3/2019 Molecular Structure and Chemical Shifts

http://slidepdf.com/reader/full/molecular-structure-and-chemical-shifts 9/20

ANISOT ROPIC FIELDANISOT ROPIC FIELD

IN AN ALKENEIN AN ALKENE

C=C

HH

H H

Bo

protons are

deshielded

shifted

downfield

secondary

magnetic

(anisotropic)

 field lines

Deshielded

 fields add

8/3/2019 Molecular Structure and Chemical Shifts

http://slidepdf.com/reader/full/molecular-structure-and-chemical-shifts 10/20

Vinyl Protons, H5-H6

8/3/2019 Molecular Structure and Chemical Shifts

http://slidepdf.com/reader/full/molecular-structure-and-chemical-shifts 11/20

Aldehyde Proton, H9-H10

8/3/2019 Molecular Structure and Chemical Shifts

http://slidepdf.com/reader/full/molecular-structure-and-chemical-shifts 12/20

Aromatic Protons, H7-H8

8/3/2019 Molecular Structure and Chemical Shifts

http://slidepdf.com/reader/full/molecular-structure-and-chemical-shifts 13/20

Secondary magnetic field

generated by circulating T

electrons deshields aromatic

protons

Circulating T electrons

Ring Current in BenzeneRing Current in Benzene

Bo

Deshielded

H H

8/3/2019 Molecular Structure and Chemical Shifts

http://slidepdf.com/reader/full/molecular-structure-and-chemical-shifts 14/20

Ring current

Ring current in aromatic compoundshas greater deshielding influence ascompared to conjugated alkenes

In T oluene methyl protons resonateat 2.34 whereas methyl groupattached to acyclic conjugated alkeneappears at 1.95

acetophenone

8/3/2019 Molecular Structure and Chemical Shifts

http://slidepdf.com/reader/full/molecular-structure-and-chemical-shifts 15/20

Toluene at Higher Field

Splitting patterns in aromatic groups can be confusingA monosubstituted aromatic ring can appear as an apparent

singlet or a complex pattern of peaks

8/3/2019 Molecular Structure and Chemical Shifts

http://slidepdf.com/reader/full/molecular-structure-and-chemical-shifts 16/20

T erms Used

Diamagnetic Field:an induce magnetic field against AMF

Paramagnetic Field:

an induced magnetic field along AMF Diamagnetic shielding:

Local shielding or deshielding

Paramagnetic Shielding:shielding or deshielding due to neighboringgroup

8/3/2019 Molecular Structure and Chemical Shifts

http://slidepdf.com/reader/full/molecular-structure-and-chemical-shifts 17/20

T erms Used Diamagnetic Shift:

Downfield shift of absorption signal

Paramagnetic shift:Up field shift of absorption signal

8/3/2019 Molecular Structure and Chemical Shifts

http://slidepdf.com/reader/full/molecular-structure-and-chemical-shifts 18/20

A Simplified 60MHzA Simplified 60MHz

NMR SpectrometerNMR Spectrometer

Transmitter 

Receiver 

Probe

hR

SN

RF

DetectorRecorder

RF (60 MHz)

Oscillator

~ 1.41 Tesla

(+/-) a few ppm

absorption

signal

MAGNETMAGNET

8/3/2019 Molecular Structure and Chemical Shifts

http://slidepdf.com/reader/full/molecular-structure-and-chemical-shifts 19/20

Sample Handling

0.5 ml of approximately 5% solutionof the sample in a suitable solvent

A few percent of the reference

A liquid sample is used without asolvent if it is not viscous

Ideal solvent: no viscous, inexpensive

capable of dissolving the sample,chemically inert and devoid odprotons of its own

8/3/2019 Molecular Structure and Chemical Shifts

http://slidepdf.com/reader/full/molecular-structure-and-chemical-shifts 20/20

Sample handling commonly used solvents

Carbon tetrachloride, carbon disulfide,deuterium oxide, deuterated chloroformand deuterated benzene

occasionally usedCD3CN, (CD3)2SO, CD3COCD3 

It is advisable to quote the solvent used