Nuclear Magnetic Resonance Spectroscopy

AnIntroduction

Over the past fifty years nuclear magnetic resonance spectroscopy, commonly referred to as NMR, has become the preeminent technique for determining the structure of organic compounds.

Nuclear magnetic resonance (NMR) spectroscopy: A spectroscopic technique that gives us information

about the number and types of nuclei in a molecule.For example, about the number and types of – Hydrogen nuclei using 1H-NMR spectroscopy.– Carbon nuclei using 13C-NMR spectroscopy.– Phosphorus nuclei using 31P-NMR spectroscopy.

Nuclear Magnetic Resonance spectroscopy involves the transition of a nucleus from one spin state to another with the resultant absorption of electromagnetic radiation by spin active nuclei when they are placed in a magnetic field.

The following features lead to the NMR phenomenon:

1. A spinning charge generates a magnetic field, as shown by the animation on the right.

The resulting spin-magnet has a magnetic moment (μ) proportional to the spin.

The nuclei of many elemental isotopes have a characteristic spin (I). Some nuclei have integral spins (e.g. I = 1, 2, 3 ....), some have fractional spins (e.g. I = 1/2, 3/2, 5/2 ....), and a few have no spin, I = 0 (e.g. 12C, 16O, 32S, ....).

Isotopes of particular interest and use to organic chemists are 1H, 13C, 19F and 31P, all of which have I = 1/2. Since the analysis of this spin state is fairly straightforward, our discussion of NMR will be limited to these and other I = 1/2 nuclei.

NUCLEAR SPIN STATES - HYDROGEN NUCLEUS

+ 1/2 - 1/2

The two statesare equivalentin energy in theabsence of amagnetic or anelectric field.

+ +

The spin of the positivelycharged nucleus generates

a magnetic moment vector, m.m

m

TWO SPIN STATES

In the presence of an external magnetic field (B0), two spin states exist, +1/2 and -1/2.

The magnetic moment of the lower energy +1/2 state is alligned with the external field, but that of the higher energy -1/2 spin state is opposed to the external field. Note that the arrow representing the external field points North.

Nuclear Spins in Strong External Magnetic Fields

In a strong magnetic field (Bo) the two spin states differ in energy.

Bo

N

S

-1/2

+1/2

Bo

DE

+ 1/2

- 1/2

= kBo = hndegenerate at Bo = 0

increasing magnetic field strength

THE ENERGY SEPARATION DEPENDS ON Bo

The Larmor Equation!!!

g is a constant which is different for each atomic nucleus (H, C, N, etc)

DE = kBo = hn can be transformed into

gyromagnetic

ratio g

strength of themagnetic field

frequency ofthe incomingradiation thatwill cause atransition

gn = 2p

Bo

• More nucleons will be in the lower energy state aligned with the magnetic field.

• A nucleon can absorb a quantum of energy in the radio frequency range and align against the magnetic field.

• It emits a radio frequency when it drops back to its original position.

Absorption of Energy

Bo

+1/2

-1/2

+1/2

-1/2

DE = hn

DE

quantized

Radiofrequency

AppliedField

Aligned

Opposed

WHEN A SPIN-ACTIVE HYDROGEN ATOM ISPLACED IN A STRONG MAGNETIC FIELD

….. IT BEGINS TO PRECESS

A SECOND EFFECT OF A STRONG MAGNETIC FIELD

OPERATION OF AN NMR SPECTROMETER DEPENDS ON THIS RESULT

If rf energy having a frequency matching the Larmor frequency is introduced at a right angle to the external field (e.g. along the x-axis), the precessing nucleus will absorb energy and the magnetic moment will flip to its I = -1/2 state. This excitation is shown in the following diagram.

Nuclear Magnetic Resonance

• Resonance: In NMR spectroscopy, resonance is due to the absorption of energy by a precessing nucleus and the results in “flip” of its nuclear spin from a lower energy state to a higher energy state.

• The precessing spins induce an oscillating magnetic field that is recorded as a signal by the instrument.– Signal: A recording in an NMR spectrum of a nuclear

magnetic resonance.

Strong magnetic fields are necessary for NMR spectroscopy.

The earth's magnetic field is not constant, but is approximately 10-4 T at ground level. Modern NMR spectrometers use powerful magnets having fields of 1 to 20 T.

Even with these high fields, the energy difference between the two spin states is less than 0.1 cal/mole.

1H 99.98% 1.00 42.6 267.531.41 60.02.35 100.07.05 300.0

2H 0.0156% 1.00 6.5 41.17.05 45.8

13C 1.108% 1.00 10.7 67.282.35 25.07.05 75.0

19F 100.0% 1.00 40.0 251.7

Resonance Frequencies of Selected Nuclei Isotope Abundance Bo (Tesla) Frequency(MHz) g(radians/Tesla)

4:1

For NMR purposes, this small energy difference (ΔE) is usually given as a frequency in units of MHz (106 Hz), ranging from 20 to 900 Mz, depending on the magnetic field strength and the specific nucleus being studied.

Irradiation of a sample with radio frequency (rf) energy corresponding exactly to the spin state separation of a specific set of nuclei will cause excitation of those nuclei in the +1/2 state to the higher -1/2 spin state.

The nucleus of a hydrogen atom (the proton) has a magnetic moment μ = 2.7927, and has been studied more than any other nucleus

NMR SPECTROMETER - INSTRUMENTATION

DIAMAGNETIC ANISOTROPY

SHIELDING BY VALENCE ELECTRONS

valence electronsshield the nucleusfrom the full effectof the applied field

B induced (opposes Bo)

Bo applied

magnetic fieldlines

The applied fieldinduces circulationof the valenceelectrons - thisgenerates amagnetic fieldthat opposes theapplied field.

Anisotropy

fields subtract at nucleus

All different types of protons in a moleculehave a different amounts of shielding.

This is why an NMR spectrum contains useful information(different types of protons appear in predictable places).

They all respond differently to the applied magnetic field and appear at different places in the spectrum.

PROTONS DIFFER IN THEIR SHIELDING

UPFIELDDOWNFIELD

Highly shielded protons appear here.

Less shielded protonsappear here.

SPECTRUM

It takes a higher fieldto cause resonance.

CHEMICAL SHIFT

• To standardise measurements on different NMR instruments, a standard reference sample is used in each experiment. This is tetramethylsilane (TMS).

This is a symmetrical and inert molecule. All H atoms have the same chemical environment and a single peak is produced from this molecule.

• The difference in energy needed to change the spin state in the sample is compared to TMS and is called the CHEMICAL SHIFT.

• The chemical shift of TMS is defined as zero• The symbol d represents chemical shift and is measured in

ppm. The chemical shift scale is measured from right to left on the spectrum.

chemical shift

= d = shift in Hz

spectrometer frequency in MHz= ppm

parts permillion

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).

NMR Correlation Chart

12 11 10 9 8 7 6 5 4 3 2 1 0

-OH -NH

CH2FCH2ClCH2BrCH2ICH2OCH2NO2

CH2ArCH2NR2

CH2SC C-HC=C-CH2

CH2-C-O

C-CH-CC

C-CH2-CC-CH3

RCOOH RCHO C=CH

TMS

HCHCl3 ,

d (ppm)

DOWNFIELD UPFIELDDESHIELDED SHIELDED

Ranges can be defined for different general types of protons.

IT IS USUALLY SUFFICIENT TO KNOW WHAT TYPESOF HYDROGENS COME IN SELECTED AREAS OFTHE NMR CHART

aliphatic C-H

CH on Cnext to pi bonds

C-H where C is attached to anelectronegative atom

alkene=C-H

benzene CH

aldehyde CHO

acidCOOH

2346791012 0

X-C-H X=C-C-H

MOST SPECTRA CAN BE INTERPRETED WITH A KNOWLEDGE OF WHAT IS SHOWN HERE

Factors influencing the Chemical Shift

• Inductive effect by Electronegative groups• Magnetic Anisotropy• Hydrogen Bonding

highly shieldedprotons appearat high field

“deshielded“protons appear at low field

deshielding moves protonresonance to lower field

C HClChlorine “deshields” the proton,that is, it takes valence electron density away from carbon, whichin turn takes more density fromhydrogen deshielding the proton. electronegative

element

DESHIELDING BY AN ELECTRONEGATIVE ELEMENT

NMR CHART

d- +d

d- +d

Electronegativity Dependence of Chemical Shift

Compound CH3X

Element X

Electronegativity of X

Chemical shift d

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

TMSmostdeshielded

ANISOTROPIC FIELDSDUE 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.

Secondary magnetic field

generated by circulating electrons deshields aromaticprotons

Circulating electrons

Ring Current in BenzeneRing Current in Benzene

Bo

Deshielded

H H fields add together

C=CHH

H H

Bo

ANISOTROPIC FIELD IN AN ALKENE

protons aredeshielded

shifteddownfield

secondarymagnetic(anisotropic)field lines

Deshielded

fields add

Bo

secondarymagnetic(anisotropic)field

H

HCC

ANISOTROPIC FIELD FOR AN ALKYNE

hydrogensare shielded

Shielded

fields subtract

HYDROGEN BONDING DESHIELDS PROTONS

O H

R

O R

HHO

RThe chemical shift dependson how much hydrogen bondingis taking place.

Alcohols vary in chemical shiftfrom 0.5 ppm (free OH) to about5.0 ppm (lots of H bonding).

Hydrogen bonding lengthens theO-H bond and reduces the valence electron density around the proton- it is deshielded and shifted downfield in the NMR spectrum.

NMR Spectrum of Acetaldehyde

offset = 2.0 ppm

CCH3

O

H

SPIN-SPIN SPLITTING

Often a group of hydrogens will appear as a multipletrather than as a single peak.

Multiplets are named as follows:

Singlet QuintetDoublet SeptetTriplet OctetQuartet Nonet

This happens because of interaction with neighboring hydrogens and is calledSPIN-SPIN SPLITTING.

C CH

Cl

Cl H

H

Cl

integral = 2

integral = 1

triplet doublet

1,1,2-TrichloroethaneThe two kinds of hydrogens do not appear as single peaks,rather there is a “triplet” and a “doublet”.

The subpeaks are due tospin-spin splitting and are predicted by the n+1 rule.

C C

H H

H

C C

H H

H

two neighborsn+1 = 3triplet

one neighborn+1 = 2doublet

singletdoublettripletquartetquintetsextetseptet

MULTIPLETSthis hydrogen’s peakis split by its two neighbors

these hydrogens aresplit by their singleneighbor

SOME COMMON SPLITTING PATTERNS

CH2 CH2X Y

CH CHX Y

CH2 CH

CH3 CH

CH3 CH2

CH3

CHCH3

( x = y )

( x = y )

INTENSITIES OF MULTIPLET PEAKS

PASCAL’S TRIANGLE

1 2 1

PASCAL’S TRIANGLE

11 1

1 3 3 11 4 6 4 1

1 5 10 10 5 11 6 15 20 15 6 1

1 7 21 35 35 21 7 1

singlet

doublet

triplet

quartet

quintet

sextet

septet

octet

The interiorentries arethe sums ofthe two numbersimmediatelyabove.

Intensities ofmultiplet peaks

C C

H H

C C

H HA A

upfielddownfield

Bo

THE CHEMICAL SHIFT OF PROTON HA IS AFFECTED BY THE SPIN OF ITS NEIGHBORS

50 % ofmolecules

50 % ofmolecules

At any given time about half of the molecules in solution willhave spin +1/2 and the other half will have spin -1/2.

aligned with Bo opposed to Bo

neighbor aligned neighbor opposed

+1/2 -1/2

C C

H H

C C

H H

one neighbor n+1 = 2 doublet

one neighbor n+1 = 2 doublet

SPIN ARRANGEMENTS

yellow spins

blue spins

The resonance positions (splitting) of a given hydrogen is affected by the possible spins of its neighbor.

C C

H H

H

C C

H H

H

two neighbors n+1 = 3 triplet

one neighbor n+1 = 2 doublet

SPIN ARRANGEMENTS

methylene spinsmethine spins

three neighbors n+1 = 4 quartet

two neighbors n+1 = 3 triplet

SPIN ARRANGEMENTS

C C

H H

H

H

H

C C

H H

H

H

H

methyl spinsmethylene spins

J J

J

J J

THE COUPLING CONSTANT

The coupling constant is the distance J (measured in Hz) between the peaks in a multiplet.

J is a measure of the amount of interaction between the two sets of hydrogens creating the multiplet.

C

H

H

C H

H

H

J

APPLICATIONS

• GEOPHYSICAL: Used to determine the water content in the geophysical samples.

• Engineering Applications: It is used to study the Process engineering aspects like the kinetic and equilibrium studies of Formaldehyde – water – methanol systems.

• Non- destructive testing of DNA, proteins etc.• This is very much useful in Data acquisition in

Petroleum Industry. Here NMR probes are developed and Used.

MRI• Magnetic resonance imaging, noninvasive• “Nuclear” is omitted because of public’s fear

that it would be radioactive.• Only protons in one plane can be in resonance

at one time.• Computer puts together “slices” to get 3D.• Tumors readily detected.