Suraj C. (MPA)

FACTORS AFFECTING CHEMICAL SHIFT Presented by: Rino M Paul Re-Edited by: Suraj C.

Presented to: Ritu Vivek Kimbahune

TETRAMETHYLSILANE(TMS):

• TMS is added to the sample as internal standard.• TMS protons are all identical, highly shielded providing a single sharp peak always isolated

from peaks of interest.• The TMS was assigned d = 0.00.Organic protons absorb downfield (to the left) of the TMS

signal.• TMS is inert , highly soluble in organic liquids and easily removed from samples by

distillation.

FACTORS AFFECTING CHEMICAL SHIFT:

Four major factors account for the resonance positions (on the ppm scale) of most protons:

• Deshielding by electronegative elementsInductive effect by electronegative groups

• s-character (hybridization effect)• Magnetic Anisotropic effect (magnetic fields usually due to π-bonded electrons in the

molecule)• Hydrogen bonding.

1. DESHIELDING BY ELECTRONEGATIVE ELEMENTS:

1

Suraj C. (MPA)

S-Character:

2

Suraj C. (MPA)

MAGNETIC ANISOTROPIC FIELDS:

• The presence of a nearby pi bond or pi system greatly affects the chemical shift.• Induced magnetic fields due to the π- electrons have greatest effect.

Aromatic protons δ= 7-8 ppm

3

Suraj C. (MPA)

Vinyl (Olefinic) protons, δ = 5-6 ppm :

4

Suraj C. (MPA)

Acetylene protons δ≈ 2.5 ppm

Aldehyde proton δ= 9-10ppm:

Electronegative oxygen atom

HYDROGEN BONDING O-H and N-H Signals:

• The chemical shift depends on how much hydrogen bonding is taking place (observed in high concentrated solutions).

• 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.

• Alcohols vary in chemical shift from 0.5 ppm (free OH) to about 5.0 ppm (lots of H bonding).

SOME MORE EXTREME EXAMPLES:

5

Suraj C. (MPA)

• Carboxylic acids have strong hydrogen bonding – they form dimers. • Resonance, electronegativity of oxygen and the formation of hydrogen bonding

withdraw electron cloud from the acid protons. • Thus, protons attached to carboxylic acids are the least shielded protons and

have a chemical shift of 10-12 ppm. • In methyl salicylate, which has strong internal hydrogen bonding, the NMR

absorption for O-H is at about 14 ppm, (highly downfield)

Summary: Chemical shift:The difference in frequency between the sample and the standard over the operation frequency. BOTTOM LINE: The more the deshielding more downfield.(higher frequency) The more the shielding more upfield.(lower frequency)

R C

O

O H

RC

O

OH δ

δ

δ

δ

δ

δ

I

O

OO

HCH3

6

Suraj C. (MPA)

TYPICAL VALUES:

Resources and references: Textbook: Principles of Instrumental Analysis, Skoog, Holler, Nieman Recommended further reading: “Principles of instrumental analysis, 5th ed. by Skoog, Holler, Nieman” Chapter 19. Extra resources are available on the internet. Relevant web sites http://www.chemguide.co.uk/analysismenu.html

*****THANK YOU*****

7