Chem 325 NMR Intro - StFX · Chem 325 NMR Intro 1 Physical properties, chemical properties,...

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Chem 325 NMR Intro 1 Physical properties, chemical properties, formulas Shedding real light on molecular structure: Frequency ν Wavelength Wavelength λ Frequency ν Velocity c = 2.998 × 10 8 ms -1 λ×ν = c Energy of a photon : E = hν = hc/λ h = Planck’s constant = 6.626 × 10 -34 Js The Electromagnetic Spectrum

Transcript of Chem 325 NMR Intro - StFX · Chem 325 NMR Intro 1 Physical properties, chemical properties,...

Chem 325 NMR Intro

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Physical properties, chemical properties, formulas

Shedding real light on molecular structure:

Frequency νννν

Wavelength

Wavelength λλλλ

Frequency νννν

Velocity c = 2.998 ×××× 108 m⋅⋅⋅⋅s-1

λλλλ ×××× νννν = c

Energy of a photon: E = hνννν = hc/λλλλ

h = Planck’s constant = 6.626 ×××× 10-34 J⋅⋅⋅⋅s

The Electromagnetic Spectrum

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Emission and Absorption Spectroscopy

Nuclear Magnetic Resonance

Spectroscopy

Let’s go for a spin!

Electron Spin: A Fourth Quantum Number

The Stern-Gerlach Experiment

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Nucleus: nucleons: protons, neutrons

-these also have ‘spin’ properties: up, down

- spins add together in a complicated way to give total nuclear spin I, characteristic of a given type of nucleus

- some general guidelines….

oddoddevenodd

1H, 13C, 19F, 31P

(I = ½)

35Cl, 37Cl (I = 3/2)

127I (I = 5/2)

½, 3/2, 5/2, …

evenoddoddeven

2H, 14N (I = 1)

10B (I = 2)1, 2, 3, …oddevenoddodd

4He, 12C, 16O, 32S0eveneveneveneven

ExamplesTotal Spin

IZA#p#n

The net nuclear spin gives rise to a number of spin states

#spin states = 2I +1

Spin states characterized by mI or Izvalues:

For a given I value, mI = Iz = +I, I-1, I-2,..,-I+1, -I.

e.g. for I = 0, mI = 0 (only!)

for I = ½, ml = +½, -½

for I = 1, ml = +1, 0, -1

Normally all nuclear spin states are degenerate

→ same energy

-degeneracy can be removed by application of an external magnetic field

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Amount of splitting of the nuclear spin states, ∆∆∆∆E,

is directly proportional to the applied magnetic

field strength B0

is directly proportional to magnetogyric ratio γγγγ of

the particular nucleus type

νγπ

γ hBBh

)2

( E 00 ===∆ h

)2

( 0Bπ

γν =

νννν is the frequency of the EM radiation required for

the transition from the lower to the upper spin states

40.01.00251.719F

75.07.05

50.04.70

10.71.0067.2813C

6.51.0041.12H

300.7.05

200.4.70

42.61.00267.531H

Frequency νννν

(MHz)

Field

strength B0

(Tesla)

γγγγ

(106 rad/Tesla ×××× sec)

Nucleus

• Increasing B0 increases ∆∆∆∆E

• Increasing B0 results in a higher frequency νννν of EM radiation required to produce the transition

• For a given B0, different types of nuclei have different ∆∆∆∆E, thus different νννν values

• EM radiation with νννν values in MHz range are radio waves

• So far: allows us to identify which types of atoms our molecule has by monitoring which EM frequencies are absorbed at a given applied magnetic field strength (but limited to those nuclei with I ≠≠≠≠ 0!)

Precession

Interaction of the nuclear magnetic moment and the

applied field causes the rotational axis to precess about the

field axis (z-axis) (like a toy top)

H B0

ω

Precessional frequency or Larmor frequency ωωωω

For a given field strength B0, nuclei of

different types precess at different

Larmor frequencies according to their

magnetogyric ratio γγγγ values:

ωωωω = γγγγB0

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Mechanism of Absorption

When a photon of, say, νννν = 60 MHz encounters this spinning

charged system the two can couple and change the spin state

of the proton.

H

B0

ω

ν = ω/2ν = ω/2ν = ω/2ν = ω/2ππππ = = = = γγγγΒΒΒΒ0000/2/2/2/2ππππ

H

ω∆Ε∆Ε∆Ε∆ΕThis state is called nuclear magnetic resonance, and the nucleus is said to be in resonance with the incoming radio wave

Mechanism of Absorption

To observe a spectroscopic transition, need a population

difference between the two states involved.

The energy difference corresponding to 60 MHz (∆∆∆∆E = hνννν)

is 2.39 x 10-5 kJ mol-1.

Thermal energy at room temperature (298 K) is sufficient

to appreciably populate both energy levels.

The energy difference is small, so rapid exchange is

occurring between the two populations, but there is

always a net excess of protons in the lower energy state.

Mechanism of Absorption

From the Boltzman distribution equation we can

calculate the population of each energy state:

Nupper/Nlower = e-∆∆∆∆E/kT = e-hνννν/kT

@ 298 K the ratio is 1,000,000 / 1,000,009 !

There is an excess population of 9 nuclei in the lower

energy state!

Transitions

As the applied B0 increases, exchange becomes more

difficult and the excess increases:

In each case, it is these few nuclei that allow us to

observe NMR

96600

48300

32200

16100

1280

960

Excess

nuclei

Frequency

(MHz)

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When radio radiation is applied to a sample both

transitions upward and downward are stimulated.

If too much radiation is applied both states completely

equilibrate – called saturation – no NMR signal can

be observed.

Two mechanisms for relaxation:

1. spin-spin or transverse relaxation, exchange with

other nuclear spins, characterized by time constant T2

2. spin-lattice or longitudinal relaxation, transfer of

energy to surroundings (heat), characterized by time

constant T1

Shielding

• If all protons (1H nuclei!) absorbed the same

amount of energy in a given magnetic field, not

much information could be obtained.

• But protons (1H nuclei!) are surrounded by

electrons that shield them from the external field.

• Circulating electrons create an induced magnetic

field that opposes the external magnetic field.

Electronic Motion

A permanent magnet will induce a current carrying

loop to spin:

Shielding

In the same way, electrons in orbitals will start to

circulate when the molecule is placed in an external

magnetic field. This circulation of electrons creates

another magnetic field that opposes the external field.

B0

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Shielding

Magnetic field ‘felt’ at the nucleus:

BN (= Beff = Blocal) = B0 - σσσσB0 = B0(1-σσσσ)

where σσσσ is the shielding constant

Thus the local field is modulated by the local

electronic or chemical environment of the nucleus.

Known as the chemical shift.

Shielded Nuclei

Magnetic field strength must be increased for a shielded proton to flip at the same frequency.

7.0459 T A small, but NOTICABLE, effect!

Protons in a Molecule

Depending on their chemical environment,

protons in a molecule are shielded by different

amounts.

The Chemical Shift

mos

t des

hiel

ded

leas

t de

shie

lded

high fieldlow field

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NMR Signals

• The number of signals shows how many different

kinds of protons (H atoms!) are present.

• The location of the signals shows how shielded or

deshielded the proton is.

• The intensity of the signal shows the number of

protons (H atoms!) of that type.

The NMR Spectrometer There are two types of NMR

spectrometer, continuous

wave (CW) sweep and

Fourier Transform (FT). CW

instruments have been almost

entirely phased out.

RF (MHz) oscillator

Magnet

RF Detector

Magnetic Field Strength

Field Strength Magnet Type Frequency

1.41T permanent 60 MHz

2.35T electromagnet 100 MHz

4.70T superconducting 200 MHz

7.05T 300 MHz

............ ............

21.2T 900 Mhz

Frequency is that required to observe 1H signals

Modern NMR

Recall the concept of precession of the spinning nuclear magnet in the applied magnetic field:

H B0

ω

µ = magnetic moment

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Behaviour of a collection of spinning nuclei:

Spin population difference

Net moment or magnetization

Application of a second magnetic field

B1 (magnetic field of EM radiation)

matching Larmor frequency

Phase coherence of spins

Transfer of magnetization from

z-axis to x,y-plane

The x,y component of the magnetization is detected

electronically as the ‘resonance’ signal

Rather than show the magnetization vectors

precessing about the z-axis, we will now make the

x- and y-axes rotate about the z-axis with the

Larmor frequency.

Transfer from the stationary laboratory coordinate

system to a rotating coordinate system.

-Initial alignment of net magnetization M along z-axis

-Apply a pulse of B1 along x-axis

-Causes phase coherence of spins and rotation of M by angle

θθθθ, value depends on pulse duration

Magnetization along the y-axis after 90°°°° pulse

Free Induction Decay “FID”

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- the magnetization along the y-axis decays by spin-spin

relaxation, time constant T2

- the magnetization along the z-axis subsequently

reappears by spin-lattice relaxation, time constant T1

-So far, only one type of nucleus, one frequency

-Real sample, different H’s, different frequencies due to

shielding effects

-Coordinate system rotating at one fixed frequency

-Some H magnetizations rotate in the x,y-plane

-The magnetization along the y-axis (detector) oscillates

between + and – values with a cosine dependence on time

-Overall decay still that of spin-spin relaxation, T2

-Horizontal difference between two peaks is inverse of

frequency difference between B1 and the Larmor

frequency

FID

CH3

C

CH3

O

CH3

C

O

OCH3

CH3

CH

CH2

C

OH

O

OH

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• The FID is magnetization as a function of time.

• Need to transform the time-domain data into frequency-domain data.

• The Fourier Transform

• Mathematically simulate the FID with a number of sine waves, distance between the peaks is related to the frequency of the signal

frequency

Fourier Transform

0 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

t1 sec

FT

time

The NMR Graph

Peak positions (x-axis scale):

1. Field strength: 1.500000000 versus 1.500002085 T

cumbersome and depends on resonance frequency!

2. Resonance frequency at constant field strength:

60000000 versus 60000089 Hz

cumbersome and depends on magnetic field strength!

The NMR Graph

Use a reference and quote all field strengths or

frequencies relative to the field or frequency of the

reference peak.

The δδδδ Scale: 6-

ref

sampleref10 1.39

01.50000000

50.00000208

B

B -B δ ×===

6-

6

ref

sampleref10 1.39

Hz 10 63.87

Hz 88.8

- δ ×=

×==

ν

νν

i.e. The sample signal is shifted by 1.39 ppm relative to the reference.

The chemical shift is 1.39 ppm.

Sample signal is at δδδδ = 1.39 relative to the reference (δδδδ = 0).

OR

δδδδ values INDEPENDENT of applied field or frequency

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Tetramethylsilane

“TMS”

• TMS is added to the sample (internal standard).

Soluble in most organic solvents.

• Since silicon is less electronegative than carbon,

TMS protons are highly shielded. Signal defined

as zero.

• Organic protons absorb downfield (to the left) of

the TMS signal.

• All 12 H’s identical, strong signal.

• Also used for 13C spectra.

Si

CH3

CH3

CH3

H3C

Chemical Shift

• Measured in parts per million.

• Ratio of shift downfield from TMS (Hz) to total

spectrometer frequency (Hz).

• Same value for 60, 100, or 300 MHz machine.

• Called the delta (δδδδ) scale.

Delta Scale