NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY

PRINCIPLE AND APPLICATION IN STRUCTURE ELUCIDATION

Second order spectra

Professor S. SANKARARAMAN

Department of Chemistry

Indian Institute of Technology Madras

Chennai – 600 036

sanka@iitm.ac.in

Second order spectra:

When the chemical shift values are very close and

the difference in chemical shifts are comparable to J values

one finds second order effects in the NMR spectra

Typically when (∆δ∆δ∆δ∆δ)/J is less than 10, second order effects

are seen in the spectra

Unusual intensities of multiplets

More than expected number of lines in multiplets

Number of lines ( frequency and intensities) can be theoretically

calculated

These are characteristic features of a second order spectrum

Examples of spin systems that show second order effects:

Hm

Ha Hx

R

mutually coupled amx spin system

δa, δm, δx, Jam, Jax, Jmx

Ha

Hb

X

Y

R

Rmutually coupled ab spin system

δa, δb, Jab

X

R

Ha

Hb

Y Z

Ha'

Hb'Hb

Ha

X

Y

X

YHa Ha'

Hb Hb'

mutually coupled aa'bb' spin system

δa, δb, six different coupling constants

Note: Ha and Ha' are chemicallyequivalent but magnetically non-equivalent protons. similarlyHb and Hb' are.

AB spin system – second order spin system

Has 4 lines, but line spacing and intensities depend on ∆δ∆δ∆δ∆δ/J ratio

The multiplet consists of two chemical shift values (δδδδA and δδδδB)

and one coupling constant (JAB)

First order quartet

Second order

AB quartet

First order quartet AB quartet

All 4 lines equally spaced and the

spacing between any adjacent

lines is J value.

Spacing between lines 2 and 3 varies

considerably. Spacing between 1 and 2 or 3

and 4 is J value. Spacing between 2 and 3 is

not J value.

Intensity ratio always 1:3:3:1 Intensity ratios varies widely

Contains only one δ and one J Contains two δ values and one J

Spectral pattern is symmetical

with respect to center of the

multiplet

Pattern does not depend on

spectrometer frequency.

Spectral pattern is symmetical with respect to

center of the multiplet

Pattern dependents on spectrometer

frequency. Multiplet can look like AB or AM or

AX system depending on spectrometer

frequency

First order simple quartet

second order AB quartet

( 4 - 1)( 3 - 2)

a = (c -

b = (c +

Jab =( 4 - 3) = ( 2 - 1)

Calculation of δδδδ and J values from a AB spectrum

( 4 - 1)( 3 - 2)

( - )( - )

( )( ) = 18 Hz

c = (20.5 + 28.0)/2 = 24.25

a = (c -

b = (c +

= 24.25 - 9 = 15.25 Hz= 24.25 + 9 = 33.25 Hz

Jab = ( 4 - 3) = ( 2 - 1) = 10.5 Hz

S

COOHH

H

Jab = 14 Hz

Ha

Ha

Hb

HbHa and Hb are

diastereomeric protons

ABC, ABX and AMX type 3 spin systems – examples

Computer simulated 3 spin system

F1 = 528.5 Hz, F2 = 594.5 Hz

F3 = 626.1 Hz, J1,2 = 10.54 Hz

J1,3 = 1.59 Hz, J2,3 = 17.28 Hz

AB

X

AB portion X portion

Dependence of ABX system on ∆δAB

JAB = 15.7 Hz, JAX = 0 Hz, JBX = 7.7 Hz

∆δAB =56.7 Hz

∆δAB =18.7 Hz

∆δAB =5.0 Hz

AB portion of the 60 MHz 1H NMR spectrum of

100 MHz 1H NMR of (olefinic region)

HaHb

Hc

100 MHz 1H NMR spectrum (aromatic region) of

60 MHz 1H NMR spectrum of

HaHx Hm

AA’BB’ and AA’XX’ type 4 spin systems – examples

Cl

Cl

Ha

Hb

Hb'

Ha'

Cl

OH

Hb

Ha Ha'

Hb'

NH2

Hb Hb'

OH

Ha'Ha

S

Hb Hb'

Ha Ha'Spectral pattern is complex, but

Symmetric with respect to mid-point

Of the multiplet

Pattern recognition is easy for AA’BB’ system

although extraction of δδδδ and J values from

The spectrum is not easy

4 spin spectrum can be simulated

Dependence of AA’BB’ spectrum on ∆δ

> 30 Hz

30 Hz

20 Hz

15 Hz

10 Hz

5 Hz

(J = 8.2 Hz, J’ = 1.5 Hz, Ja = 7.5 Hz, Jb = 3.0 Hz)

THANK YOU