NUCLEAR MAGNATIC RESONANCE SPECTROSCOPY

INTRODUCTION

BASICS

PRINCIPLE

INSTRUMENTATION

SHEILDING AND DESHIELDING

&APPLICATIONS

INTRODUCTION

Nuclear magnetic resonance (NMR) is a physical phenomenon based upon the quantum mechanical magnetic properties of an atom's nucleus.

NMR also commonly refers to a family of scientific methods that exploit nuclear magnetic resonance to study molecules ("NMR spectroscopy").

The method of NMR was first developed by E.M. Purcell and Felix bloch(1946)

Major application of NMR spectroscopy lies in the area of synthetic organic chemistry, inorganic chemistry, bio-organic chemistry, bio-inorganic chemistry,

NMR Historic Review

2002 Nobel prize in Chemistry was awarded to Kurt Wuthrich

Felix Bloch 1905-1983

Edward M. Purcell 1912-1997

Kurt Wuthrich 1938-

Richard R. Ernst 1933-

CW NMR 40MHz

1960

800 MHz

1nm 10 102 103 104 105 106 107

(the wave) X-ray UV/VIS Infrared Microwave Radio Frequency

(the transition) electronic Vibration Rotation Nuclear

(spectrometer) X-ray UV/VIS Infrared/Raman NMR

Fluorescence

NMR Spectroscopy

Where is it?

Before using NMR

What are N, M, and R ?

Properties of the Nucleus

Nuclear spin

Nuclear magnetic moments

The Nucleus in a Magnetic Field

Precession and the Larmor frequency

Nuclear Zeeman effect & Boltzmann distribution

When the Nucleus Meet the right Magnet and radio wave

Nuclear Magnetic Resonance

Nuclear magnetic moments

Magnetic moment is another important parameter for a nuclei

= I (h/2)

I: spin number; h: Plank constant;

: gyromagnetic ratio (property of a nuclei)

1H: I=1/2 , = 267.512 *106 rad T-1S-1

13C: I=1/2 , = 67.264*106

15N: I=1/2 , = 27.107*106

• Subatomic particles (electrons, protons and neutrons) can be imagined as spinning on their axes.

• In many atoms (such as 12C) these spins are paired against each other, such that the nucleus of the atom has no overall spin.

• However, in some atoms (such as 1H and 13C) the nucleus does possess an overall spin. The rules for determining the net spin of a nucleus are as follows;

PRINCIPLE

1. If the number of neutrons and the number of protons are both even, then the nucleus has NO spin.

2 . If the number of neutrons plus the number of protons is odd, then the nucleus has a half-integer spin (i.e. 1/2, 3/2, 5/2)

3.If the number of neutrons and the number of protons are both odd, then the nucleus has an integer spin (i.e. 1, 2, 3)

IsotopeNatural %Abundance

Spin (I)

MagneticMoment (μ)*

MagnetogyricRatio (γ)†

1H 99.9844 1/2 2.7927 26.7532H 0.0156 1 0.8574 4,107

11B 81.17 3/2 2.6880 --13C 1.108 1/2 0.7022 6,72817O 0.037 5/2 -1.8930 -3,62819F 100.0 1/2 2.6273 25,179

29Si 4.700 1/2 -0.5555 -5,319

31P 100.0 1/2 1.1305 10,840

1. A spinning charge generates a magnetic field.The resulting spin-magnet has a magnetic moment (μ) proportional to the spin.

     

2. In the presence of an external magnetic field (B0), two spin states

exist, +1/2 and -1/2.

3.The magnetic moment of the lower energy +1/2 state is aligned 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.

                    

        

In an Applied Magnetic Field•Nuclei with 2 allowed spin states can align either with or against the field, with slight excess of nuclei aligned with the field

•The nuclei precess about an axis parallel to the applied magnetic field, with a frequency called the Larmor Frequency

Larmor Frequency is Proportional to the Applied Magnetic Field

Slow precession in smallmagnetic field

Faster precession in largermagnetic field

Nuclear Zeeman effect

• Zeeman effect: when an atom is placed in an external magnetic field, the energy levels of the atom are split into several states.

• The energy of a give spin sate (Ei) is directly proportional to the value of mI

and the magnetic field strength B0

Spin State Energy EI=- . B0 =-mIB0 r(h/2p)

•For a nucleus with I=1/2, the energy difference between two states is

 ΔE=E-1/2-E+1/2 = B0 r(h/2p)

m=1/2

m=-1/2

The Zeeman splitting is proportional to the strength of the magnetic field

Boltzmann distribution

Quantum mechanics tells us that, for net absorption of radiation to occur, there must be more particles in the lower-energy state than in the higher one.

If no net absorption is possible, a condition called saturation.

When it’s saturated, Boltzmann distribution comes to rescue:

Pm=-1/2 / Pm=+1/2 = e -DE/kT

where P is the fraction of the particle population in each state,

T is the absolute temperature,

k is Boltzmann constant 1.381*10-28 JK-1

 

Anything that increases the population difference will give rise to a more intense NMR signal.

Nuclear Magnetic Resonance SpectrometerHow to generate signals?

B0: magnet

B1: applied small energy

Rf Energy Can Be Absorbed•Precessing nuclei generates an oscillating electric field of the same frequency

•Rf energy with the same frequency as the Larmor frequency can be applied to the system and absorbed by the nuclei

The Nucleus in a Magnetic Field

Precession and the Larmor frequency

• The magnetic moment of a spinning nucleus processes with a characteristic angular frequency called the Larmor frequency w, which is a function of r and B0

 

Remember = I (h/2) ?

Angular momentum dJ/dt= x B0

Larmor frequency w=rB0

 

Linear precession frequency v=w/2p= rB0/2p

  

J

•For a particle to absorb a photon of electromagnetic radiation, the particle must first be in some sort of uniform periodic motion

• If the particle “uniformly periodic moves” (i.e. precession) at precession, and absorb erengy. The energy is E=hvprecession

•For I=1/2 nuclei in B0 field, the energy gap between two spin states:

DE=rhB0/2p

• The radiation frequency must exactly match the precession frequency

Ephoton=hvprecession=hvphoton=DE=rhB0/2p

This is the so called “ Nuclear Magnetic RESONANCE”!!!!!!!!!

v

E =hvphoton

When the Nucleus Meet the Magnet

Nuclear Magnetic Resonance

Magnet B0 and irradiation energy B1

B0 ( the magnet of machine)

(1) Provide energy for the nuclei to spin

Ei=-miB0 (rh/2p)

Larmor frequency w=rB0

(2) Induce energy level separation (Boltzmann distribution)

The stronger the magnetic field B0, the greater separation

between different nuclei in the spectra

Dv =v1-v2=(r1-r2)B0/2p

  (3) The nuclei in both spin states are randomly oriented around the z axis.

M z=M, Mxy=0

  ( where M is the net

nuclear magnetization)

What happen before irradiation

• Before irradiation, the nuclei in both spin states are processing with characteristic frequency, but they are completely out of phase, i.e., randomly oriented around the z axis. The net nuclear magnetization M is aligned statically along the z axis (M=Mz, Mxy=0)

What happen during irradiation

When irradiation begins, all of the individual nuclear magnetic moments become phase coherent, and this phase coherence forces the net magnetization vector M to process around the z axis. As such, M has a component in the x, y plan, Mxy=Msin is the tip angle which is determined by the power and duration

of the electromagnetic irradiation.

Mo

z

x

B1

yo

x

Mxy

y

90 deg pulse deg pulse

B1(the irradiation magnet, current induced)

(1) Induce energy for nuclei to absorb, but still spin at w or vprecession

Ephoton=hvphoton=DE=rhB0/2p=hvprecession

 

And now, the spin jump to the higher energy ( from m=1/2m= – 1/2)

 

(2) All of the individual nuclear magnetic moments become phase coherent, and the net M process around the z axis at a angel

M z=Mcosa

Mxy=Msina.

m= 1/2

m= –1/2

What happen after irradiation ceases

•After irradiation ceases, not only do the population of the states revert to a Boltzmann distribution, but also the individual nuclear magnetic moments begin to lose their phase coherence and return to a random arrangement around the z axis.

(NMR spectroscopy record this process!!)

•This process is called “relaxation process”

•There are two types of relaxation process : T1(spin-lattice relaxation) & T2(spin-spin relaxation)

• Relaxation processes• How do nuclei in the higher energy state return to the lower state?

• Emission of radiation is insignificant because the probability of re- emission of photons varies with the cube of the frequency. At radio frequencies, re-emission is negligible.

• Ideally, the NMR spectroscopist would like relaxation rates to be fast - but not too fast.

• If the relaxation rate is fast, then saturation is reduced. If the relaxation rate is too fast, line-broadening in the resultant NMR spectrum is observed.

• There are two major relaxation processes;

• Spin - lattice (longitudinal) relaxation

• Spin - spin (transverse) relaxation

T1 (the spin lattice relaxation)

• How long after immersion in a external field does it take for a collection of nuclei to reach Boltzmann distribution is controlled by T1, the spin lattice relaxation time.

(major Boltzmann distribution effect)

•Lost of energy in system to surrounding (lattice) as heat

( release extra energy)

•It’s a time dependence exponential decay process of Mz components

dMz/dt=-(Mz-Mz,eq)/T1

T2 (the spin –spin relaxation)

•This process for nuclei begin to lose their phase coherence and return to a random arrangement around the z axis is called spin-spin relaxation.

•The decay of Mxy is at a rate controlled by the spin-spin relaxation time

T2.

dMx/dt=-Mx/T2

dMy/dt=-My/T2

dephasing

NMR Parameters

Chemical Shift• The chemical shift of a nucleus is the difference between the resonance

frequency of the nucleus and a standard, relative to the standard. This quantity is reported in ppm and given the symbol delta,

= (n - nREF) x106 / nREF

• In NMR spectroscopy, this standard is often tetramethylsilane, Si(CH3)4,

abbreviated TMS, or 2,2-dimethyl-2-silapentane-5-sulfonate, DSS, in biomolecular NMR.

• The good thing is that since it is a relative scale, the d for a sample in a 100 MHz magnet (2.35 T) is the same as that obtained in a 600 MHz magnet (14.1 T).

0TMS

ppm

210 7 515

Aliphatic

Alcohols, protons to ketones

Olefins

AromaticsAmidesAcids

Aldehydes Shielded (up field)

Deshielded (low field)

The NMR scale (The NMR scale (, ppm), ppm)

• We can use the frequency scale as it is. The problem is that since Bloc is a lot smaller than Bo, the range is very small (hundreds of Hz) and the absolute value is very big (MHz).

• We use a relative scale, and refer all signals in the spectrum to the signal of a particular compound.

The good thing is that since it is a relative scale, the in a 100 MHz magnet (2.35 T) is the same as that obtained for the same sample in a 600 MHz magnet (14.1 T).

•

H 3C i C H 3

C H 3

C H 3

- ref

= ppm (parts per million) ref

S

Tetramethyl silane (TMS) is used as reference because it is soluble in most organic solvents, inert, volatile, and has 12 equivalent 1Hs and 4 equivalent 13Cs:

Other references can be used, such as the residual solvent peak, dioxane for 13C, etc. What reference we use is not critical, because the instrument (software/hardware) is calibrated internaly. Don’t use them if you don’t need to...

HO-CH2-CH3

o

lowfield

highfield

0=rBeffect

•

Notice that the intensity of peak is proportional to the number of H

J-coupling

•Nuclei which are close to one another could cause an influence on each other's effective magnetic field. If the distance between non-equivalent nuclei is less than or equal to three bond lengths, this effect is observable. This is called spin-spin coupling or J coupling.

13C

1H 1H 1H

one-bond

three-bond

•Each spin now seems to has two energy ‘sub-levels’ depending on the state of the spin it is coupled to:

The magnitude of the separation is called coupling constant (J) and has units of Hz.

I SS

S

I

IJ (Hz)

Single spin:

One neighboring spins: - CH – CH -

Two neighboring spins: - CH2 – CH -

•N neighboring spins: split into N + 1 lines

• From coupling constant (J) information, dihedral angles can be derived ( Karplus equation)

8.1cos6.1cos5.9

8.1)120cos(6.1)120(cos5.9

9.1)60cos(4.1)60(cos4.6

112

23

112

13

23

J

J

JNH

ψ Ψ

Cα

Cβ

Cγ

ωN

χ1

χ2

C’

N

Nuclear Over Hauser Effect (NOE)

•The NOE is one of the ways in which the system (a certain spin) can release energy. Therefore, it is profoundly related to relaxation processes. In particular, the NOE is related to exchange of energy between two spins that are not scalarly coupled (JIS = 0), but have dipolar coupling.

• The NOE is evidenced by enhancement of certain signals in the spectrum when the equilibrium (or populations) of other nearby are altered. For a two spin system, the energy diagram is as following:

W1S

W1S

W1I

W1I

W2IS

W0IS

•W represents a transition probability, or the rate at which certain transition can take place. For example, the system in equilibrium, there would be W1I and W1S transitions, which represents single quantum transitions.

INSTRUMENTATION

1. MAGNET

Permanent magnets Conventional electromagnets and Super conducting magnets

2. SAMPLE PROBE

3. FIELD SWEEP GENARETOR

4. THE RADIO FREQUENCY SOURCE

5. THE SIGNAL DETECTOR&RECORDER SYSTEM

The spectrometerThe spectrometer

1. Sample preparation

Which buffer to choose? Isotopic labeling?

Best temperature?

Sample Position ?

Preparation for NMR Experiment

N S

2. What’s the nucleus or prohead?

A nucleus with an even mass A and even charge Z nuclear spin I is zero

Example: 12C, 16O, 32S No NMR signal

A nucleus with an even mass A and odd charge Z integer value I

Example: 2H, 10B, 14N NMR detectable

A nucleus with odd mass A I=n/2, where n is an odd integer

Example: 1H, 13C, 15N, 31P NMR detectable

3. The best condition for NMR Spectrometer?

Wobble : Tune & Match & Shimming

RCVR

TuneMatch

Absorption

0%

100% Frequency

4. What’s the goal? Which type of experiment you need?

Different experiments will result in different useful information

234 233 232 231 230 229 228 227 226 225 224 223f1 ppm

The FID (free induction decay) is then Fourier transform to frequency domain to obtain vpression ( chemical shift) for each different nuclei.

frequency (Hz)

5. NMR Data Processing

0 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00t1 sec

Time (sec)

roperties of Some Deuterated NMR Solvents

Solvent B.P. °CResidual1H signal (δ)

Residual13C signal (δ)

acetone-d6 55.5 2.05 ppm206 & 29.8 ppm

acetonitrile-d3 80.7 1.95 ppm 118 & 1.3 ppm

benzene-d6 79.1 7.16 ppm 128 ppm

chloroform-d 60.9 7.27 ppm 26.4 ppm

cyclohexane-d12 78.0 1.38 ppm 26.4 ppm

dichloromethane-d2 40.0 5.32 ppm 53.8 ppm

dimethylsulfoxide-d6 190 2.50 ppm 39.5 ppm

nitromethane-d3 100 4.33 ppm 62.8 ppm

pyridine-d5 1147.19, 7.55 & 8.71 ppm

150, 135.5 & 123.5 ppm

tetrahydrofuran-d8 65.01.73 & 3.58 ppm

67.4 & 25.2 ppm

Limitations of nmr spectroscopy

1.Its lack of sensitivity. fairly large numbers are requried.minimum sample size is about0.1ml having minimum concentrations of about on1%

2.Limited number of nuclei which may be usefully studied with this technique.

3.Inmost of the cases ,the technique is limited to liquid samples or to a liquid capable of solutions in a suitable solvents or of melting at a temperature below 260oc

4.In some compounds two different types of hydrogen atoms resonance at similar resonance frequencies .this results in an overlap of spectra .hence the interpretation of spectra becomes difficult.

APPLICATIONS

1.determination of optical purity

2.study of molecular interactions

3.quantative analysis: assay components, surfactant chain length Determination, hydrogen analysis, iodine value, moisture analysis

4.elemental analysis

5.Multicomponentmixture analysis 6.magnetic resonance imaging

7. NMR has also been used in various special fields that includes industrial quality control, biology, engineering and medicine

8.Structure elucidation

• other applications• Molecular conformation in solution

          Quantitative analysis of mixtures containing known compounds           Determining the content and purity of a sample           Through space connectivity (over Hauser effect)           Chemical dynamics (Line shapes, relaxation phenomena)         Solid State NMR is widely popular for the characterization of polymers, rubbers, ceramics,  and molecular sieves.

Thank you