NMR spectra of some simple molecules Effect of spinning: averaging field inhomogeneity (nmr1.pdf pg...

NMR spectra of some simple molecules

Effect of spinning: averaging field inhomogeneity (nmr1.pdf pg 2)

Because the protons have a magnetic field associated with them, the field changes as across the nmr tube. Diffusion tends to offset this field gradient

Ho

Chemical Shifts

Heff = The magnetic field felt at the proton

Heff = Hext + Hlocal ; Heff : magnetic field felt by the nuclei

Hext : external magnetic field

Hlocal: local field induced by the external field

Hlocal: Electrons in a chemical bond are considered to be in motion and are charged. This induces a local magnetic field which can shield (oppose) or deshield (enhance) the magnetic field experienced by the nucleus. Since the precessional frequency of the nucleus is governed by Heff, changes in this field as a result of local fields caused by bonding electrons, the resonance frequency of magnetically and chemically non-equivalent nuclei differ resulting in slightly different values of . This is the origin of the chemical shift. The local magnetic field is induced by the external field and is directly proportional to the external field

Hlocal : the effect of the external magnetic field on the bonding electrons depends on electron density and molecular structure.

Hlocal is directly proportional to Hext

Remember H is a vector. This property has both magnitude and direction

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Typical chemical shifts for protons: 0 –10 ppmIn a 300 MHz instrument, differences in range about 3000 Hz (3000 Hz shifts relative to a total of 300*106 cycles /sec)

Increasing frequency

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Typical chemical shifts for protons: 0 –10 ppm

CH

CH2

CH3

-CH=

aromatic

ppm

04080120160200

Intensity

0

Typical chemical shifts for 13C: 0 to 220 ppm

CR4

CHR3

R2CH2

CH3

aromatic

>C=C<

>C=O

Common terms used in NMR (terms originating from use of CW instruments)

Shielded: the induced local field opposes the external field

Deshielded: the induced local field field augments the external field

Upfield shift: shift toward lower frequency; higher magnetic field, lower energy

Downfield shift: shift toward higher frequency; lower magnetic field

higher energy

Frequency sweep instruments:

Hext = constant; swept 10 ppm

Heff < than Hext must decrease for resonance

lower frequency, lower energy, nucleus is shielded, upfield shift

Hext Hlocal

Hext Hlocal

Heff > than Hext must increase for resonancehigher frequency, higher energy, nucleus is deshielded, downfield shift

Field sweep instruments: At 600 MHzω = constant; Hext swept from

“140000 to 146000 gauss”

Heff < than Hext must decrease for resonance

lower frequency, lower energy, nucleus is shielded, upfield shift

Hext Hlocal

Then resonance would occur at a lower value of Hext

nucleus is deshielded, downfield shift

Hext Hlocal

nucleuselectron cloud

Field due to circulating e-

Hexternal field

Field felt by the nucleus Heff = Hext - Hlocal

For resonance either Hext must be increased or decreased

relative to the situation where Hlocal = 0

All protons have the same precessional frequency in a vacuum

Sigma bonds

H

H

π bonds in acetylenesHext

Hlocal

π bonds in alkenes

and aldehydes

Hext

O

shielding cone

deshielding regionHlocal

H

Field felt by the nucleus Heff = Hext + Hlocal

For resonance either Hext must be decreased or increased

relative to the situation where Hlocal = 0

Hext

Hlocal

π bonds in aromatic compounds

H

HH

-3.0

9.3

CH2

HH

H H 0.3

Hext

An Example of A Simple Spectrum

Area: 9:1:2

Other Factors Influencing Hlocal

Hlocal is influenced by all local fields; the field effect of the bonding electrons results in the chemical shift, a relatively small perturbation Hlocal is induced by the external field and depends on its magnitude

What about the field effects of the local protons?

Suppose we have two identical protons attached to the same carbon.

What are the possible spin states of this system and how do they effect the local magnetic field?

Nomenclature used to describe spin-spin coupling

First Order Spectra: Chemical shift difference ∆ > 10 J

AX ; A2X; A3X; AMX; A3MX; A3M2X; …

J is a measure of the effective magnetic field of neighboring protons. The effect is generally considered to be transmitted through chemical bonds and not through space

Non-first Order Spectra: Chemical shift ∆ < 10 J

AB ; A2B; A3B; ABC; A3CB; A3B2X; A3B2C …

A2 Case, J = 0 H-C-C-C-C-H

Energy or H

Remember: Ne/Ng = e-H/RT 1

A2 Case H-C-H

+J/4

+J/4

+J/4

-3J/4

A

A

No H – H interaction H – H interaction

For positive J

J = 0

A2 Case

-J/4

-J/4

-J/4

+3J/4A

A

No H – H interaction H – H interaction

For negative J

J =0

AX; X > A

A X

Relative ordering of energy levels without AX interactions

Both opposed to magnetic field

Energy

A

J = 0

A

X

X

AX; X > A

A X

Relative ordering of energy levels with AX interactions

Both opposed to magnetic field

A + J/2

A – J/2

X +J/2

X -J/2

+J/4

+J/4

-J/4

-J/4

For positive J

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0X A

JJ

In the absence of coupling, ie J = 0

In the presence of coupling, ie J ≠ 0

AX; X > A

A X

Relative ordering of energy levels with AX interactions

Both opposed to magnetic field

A + J/2

A – J/2

X +J/2X -J/2

+J/4

+J/4

-J/4

-J/4For negative J

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X A

J J

A2X X > A

No AX interaction, JAA ≠ 0

A2 X

A2X X > A

No AX interaction

A2 X

X

A+J/2

A -J/2

0

A +J/2

A -J/2

0

X

X-J/2

X+J/2

For positive JAX

X

A2X X > A

AX interaction

A2 X

A+J/2

A -J/20

A +J/2

A -J/2

0

For positive JJ = 0

A+J/2

A+J/2

A-J/2

A-J/2

Note that the A transitions are twice as intense

X

No A2X coupling

A

A2X coupling

The 2nS +1 Rule

The number of lines observed for a particular nucleus as a result of n “identical” neighbors is 2nS + 1 where S is the spin of the neighboring nucleus. For most nucleus, S = ½, the relationship simplifies to n+1 lines

“identical” in this context refers to nuclei that have the same or very similar coupling constants to the nucleus being observed.

number of “identical neighbors” multiplicity of nucleus observed

1 2 (1:1)

2 3 (1:2:1)

3 4 (1:3:3:1)

4 5 (1:4:6:4:1)

5 6 (1:5:10:10:5:1)

Examples of First Order Spectra

C

CH3 CH3

OHH

CH3CH2OH

What information do you get out of a 1H NMR spectrum?

Chemical Shift?

An indication of the type of proton and its environment

Multiplicity?

An indication of the number of nearest neighbors and their proximity

Area?

A measure of the relative number of hydrogen nuclei in the molecule

The compound has a IR frequency of 1720 cm-1 and a molecular formula of C4H8O. What is its structure?

3

2

3

CCH3

O

CH2 CH3

OC CH2

O

CCOCH2

CH2

H

O

CH3

CH3 CH3

OC CH2

O

CCOCH2

CH2

CH2

O

CH3

CH3

CH3 CH3

geminal 2J

vicinal 3J

4J

5J

Magnitude of the Vicinal Coupling Constant J

Karplus Equation

3JCHCH = 10 cos2(φ) where φ is the dihedral angle

HH

Summary of the Field Dependence of and J

is the local field that is induced by the magnitude of the external field, Ho. is therefore chemical shift dependent.

J is dependent on the magnetic moment of the proton and is

therefore independent of the external field, Ho.

Effect of Magnetic field strength on 1H NMR Spectra

Raccoon

60 MHz, 600 Mz

H1= H2 = H3 1.0 J12 = -10; J13 = -10; J23 = -10

H4 = H5 = 1.5 J14 = 7; J 15 = 7; J4,5 = -12

H1H4

H5 H3

H2

Effect of Magnetic field strength on 1H NMR Spectra

Raccoon

60 MHz, 600 Mz

H1= 8.0 J12 = 8; J13 = 17; J23 = -6

H2 = 8.6 J

H3 = 8.9

CN

H1

H3

H2