Spin-Spin Splitting

Spin-Spin Splitting

At higher resolution, we see that the -CH2 and -CH3 peaks are split:

Why?shielding is also

affected by nuclear spin

orientations of protons

on adjacent carbons

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Let’s look at this in more detail!

More Spin-Spin Splitting CH3 Protons

split by the spins of the 2 adjacent CH2 protons:

↑↑ ↓↑ ↓↑ ↓↓ 1 2 1

3 peaks CH2 Protons

split by the spins of the 3 adjacent CH3 protons:

↑↑↑ ↑↑↓ ↑↓↑ ↓↑↑ ↓↓↑ ↓↑↓ ↑↓↓ ↓↓↓ 1 3 3 1

4 peaksChemistry Department, University of Isfahan

Quantifying Spin-Spin Splitting Spin-spin splitting is quantified by:

J: Coupling Constant J is reported in absolute units (frequency, Hz) and

is typically 1-20 Hz

J depends only on the electronic and steric

relationships between interacting nuclei

Thus, J is independent of spectrometer B0

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More on Spin-Spin More on Spin-Spin SplittingSplitting

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Spin-Spin Coupling Spin-Spin Coupling Nuclei experiencing the same chemical environment or Nuclei experiencing the same chemical environment or

chemical shift are called equivalent. chemical shift are called equivalent. Those nuclei experiencing different environment or Those nuclei experiencing different environment or

having different chemical shifts are nonequivalent. having different chemical shifts are nonequivalent. Nuclei which are close to one another exert an influence Nuclei which are close to one another exert an influence

on each other's effective magnetic field. on each other's effective magnetic field. This effect shows up in the NMR spectrum when the This effect shows up in the NMR spectrum when the

nuclei are nonequivalent. nuclei are nonequivalent. If the distance between non-equivalent nuclei is less If the distance between non-equivalent nuclei is less

than or equal to three bond lengths, this effect is than or equal to three bond lengths, this effect is observable. This effect is called spin-spin coupling or J observable. This effect is called spin-spin coupling or J couplingcoupling. .

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Spin-Spin Coupling Spin-Spin Coupling Consider A and B, which are three bonds away from one Consider A and B, which are three bonds away from one

another in a molecule. another in a molecule. The spin of each nucleus can beThe spin of each nucleus can be

either aligned with the external field either aligned with the external field

the fields are N-S-N-S, called spin upthe fields are N-S-N-S, called spin up

or opposed to the external field or opposed to the external field

The fields are N-N-S-S, called spin down The fields are N-N-S-S, called spin down

The magnetic field at nucleus A will be either greater than B0 or less than B0 by a constant amount due to the influence of nucleus B.

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Spin-Spin Coupling Spin-Spin Coupling The NMR spectrum for nuclei A and B reflects the splittings The NMR spectrum for nuclei A and B reflects the splittings

observed in the energy level diagram. observed in the energy level diagram. The A absorption line is split into 2 absorption lines centered The A absorption line is split into 2 absorption lines centered

on on AA, and the B absorption line is split into 2 lines centered , and the B absorption line is split into 2 lines centered

on on BB. .

The distance between two split absorption lines is called the The distance between two split absorption lines is called the J coupling constant or the spin-spin splitting constant and is J coupling constant or the spin-spin splitting constant and is a measure of the magnetic interaction between two nuclei. a measure of the magnetic interaction between two nuclei.

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Spin-Spin Coupling Spin-Spin Coupling For the next example, consider a molecule with For the next example, consider a molecule with

three spin 1/2 nuclei, one type A and two type B. three spin 1/2 nuclei, one type A and two type B.

The type B nuclei are both three bonds away from The type B nuclei are both three bonds away from the type A nucleus. the type A nucleus.

The magnetic field at the A nucleus has three The magnetic field at the A nucleus has three possible values due to four possible spin possible values due to four possible spin configurations of the two B nuclei. configurations of the two B nuclei.

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Spin-Spin Coupling Spin-Spin Coupling The magnetic field at a B nucleus has two possible values.The magnetic field at a B nucleus has two possible values.

The energy level diagram for this molecule has six states or The energy level diagram for this molecule has six states or levels because there are two sets of levels with the same levels because there are two sets of levels with the same energy.energy.

Energy levels with the same energy are said to be Energy levels with the same energy are said to be degenerate. The vertical lines represent the allowed degenerate. The vertical lines represent the allowed transitions or absorptions of energy. transitions or absorptions of energy.

Note that there are two lines drawn between some levels Note that there are two lines drawn between some levels because of the degeneracy of those levels. because of the degeneracy of those levels.

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Spin-Spin Coupling Spin-Spin Coupling The resultant NMR spectrum is depicted in the animation The resultant NMR spectrum is depicted in the animation

window. window.

Note that the center absorption line of those centered at Note that the center absorption line of those centered at AA is is

twice as high as the either of the outer two. twice as high as the either of the outer two. This is because there were twice as many transitions in the This is because there were twice as many transitions in the

energy level diagram for this transition. The peaks at energy level diagram for this transition. The peaks at BB are are

taller because there are twice as many B type spins than A taller because there are twice as many B type spins than A type spins. type spins.

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Spin-Spin Coupling Spin-Spin Coupling The complexity of the splitting pattern in a spectrum The complexity of the splitting pattern in a spectrum

increases as the number of B nuclei increases. The increases as the number of B nuclei increases. The following table contains a few examples. following table contains a few examples.

Configuration Peak Ratios

A 1

AB 1:1

AB2 1:2:1

AB3 1:3:3:1

AB4 1:4:6:4:1

AB5 1:5:10:10:5:1

AB6 1:6:15:20:15:6:1

This series is called Pascal's triangle and can be calculated from the coefficients of the expansion of the equation:

(x+1)n

where n is the number of B nuclei in the above table.

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Spin-Spin Coupling Spin-Spin Coupling When there are two different types of nuclei three bonds When there are two different types of nuclei three bonds

away there will be two values of away there will be two values of JJ, one for each pair of , one for each pair of nuclei. nuclei.

By now you get the idea of the number of possible By now you get the idea of the number of possible configurations and the energy level diagram for these configurations and the energy level diagram for these configurations, so we can skip to the spectrum. configurations, so we can skip to the spectrum.

In the following example In the following example JJABAB is greater than is greater than JJBCBC..

Effects of Magnetic Field on Spectrum?

J:unaffected

δ:unaffected

Resolution:

(Hz / ppm)

∝ B0

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CH3CH2OHA (  ) + HOHB ( )         CH3CH2OHB (  ) + HOHA (   )

CH3CH2OHA (  ) + HOHB ( )         CH3CH2OHB (  ) + HOHA (   )

CH3CH2OHA (  ) + HOHB ( )         CH3CH2OHB (  ) + HOHA ( )

CH3CH2OHA (  ) + HOHB ( )         CH3CH2OHB (  ) + HOHA ( )

Chemical Exchange

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NMR SolventsAcetone CD3COCD3

Chloroform CDCl3

Dichloromethane CD2Cl2

Benzene C6D6

Water D2O

N,N-Dimethylformamide (DMF) (CD3)2NCDO

Dimethyl Sulfoxide (DMSO) CD3SOCD3

Ethanol CD3CD2OD

Methanol CD3OD

Tetrehydrofuran (THF) C4D8O

Toluene C6D5CD3

Pyridine C5D5N

1313C-NMR SpectroscopyC-NMR Spectroscopy Each nonequivalent Each nonequivalent 1313C gives a different C gives a different

signalsignal A A 1313C is split by the C is split by the 11H bonded to it H bonded to it

according to the (n + 1) rule according to the (n + 1) rule Coupling constants of 100-250 Hz are Coupling constants of 100-250 Hz are

common, which means that there is often common, which means that there is often significant overlap between signals, and significant overlap between signals, and splitting patterns can be very difficult to splitting patterns can be very difficult to determinedetermine

The most common mode of operation of a The most common mode of operation of a 1313C-NMR spectrometer is a hydrogen-C-NMR spectrometer is a hydrogen-decoupled modedecoupled mode

1313C-NMR SpectroscopyC-NMR Spectroscopy In a hydrogen-decoupled mode, a sample is In a hydrogen-decoupled mode, a sample is

irradiated with two different radio irradiated with two different radio frequenciesfrequencies one to excite all one to excite all 1313C nucleiC nuclei a second is a broad spectrum of frequencies that a second is a broad spectrum of frequencies that

causes all hydrogens in the molecule to undergo causes all hydrogens in the molecule to undergo rapid transitions between their nuclear spin rapid transitions between their nuclear spin statesstates

On the time scale of a On the time scale of a 1313C-NMR spectrum, C-NMR spectrum, each hydrogen is in an average or each hydrogen is in an average or effectively constant nuclear spin state, with effectively constant nuclear spin state, with the result that the result that 11H-H-1313C spin-spin interactions C spin-spin interactions are not observed; they are decoupledare not observed; they are decoupled

Carbon – 13 shiftsCarbon – 13 shifts

Continuous-Wave NMRContinuous-Wave NMR

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Continuous-Wave NMRContinuous-Wave NMR““Block diagram of Continuous-Wave Block diagram of Continuous-Wave NMR.”NMR.”

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Instrumentation - CW-Instrumentation - CW-NMRNMR

R-F transmitterR-F transmitter

R-F receiver & detectorR-F receiver & detector

samplesample

magnetmagnet

recorderrecorder

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Instrumentation CW-Instrumentation CW-NMRNMR

R-F transmitterR-F transmitter irradiate sample with resonant radio irradiate sample with resonant radio

frequencyfrequency less than 1 W output but reproducible less than 1 W output but reproducible

to 1 part in 10to 1 part in 1099

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Instrumentation CW-Instrumentation CW-NMRNMR

R-F receiver & detectorR-F receiver & detector detect the radio frequency detect the radio frequency amplify 10amplify 105 5 timestimes integratorintegrator

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Instrumentation CW-Instrumentation CW-NMRNMR

sweep generator => sweep coilsweep generator => sweep coil modify the applied magnetic fieldmodify the applied magnetic field ““scans” through slight (ppm) scans” through slight (ppm)

changes in magnetic field which changes in magnetic field which produces variations in position of produces variations in position of resonant frequencyresonant frequency

modifies magnetic field to give modifies magnetic field to give spectrumspectrum

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Instrumentation CW-Instrumentation CW-NMRNMR

samplesample sample suspended between pole sample suspended between pole

faces of magnetfaces of magnet sample is spunsample is spun

air driven turbine - average out effects air driven turbine - average out effects of in homogeneities in the fieldof in homogeneities in the field

5 mm O.D. tube5 mm O.D. tube

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Instrumentation CW-Instrumentation CW-NMRNMR

magnetmagnet causes the nuclear spin states to causes the nuclear spin states to

splitsplit allows sample to absorb energy allows sample to absorb energy

equal to the difference between spin equal to the difference between spin statesstates

most expensive componentmost expensive component must be reproducible & must be reproducible &

homogeneoushomogeneous

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MagnetsMagnets

type of magnettype of magnet max. field max. field strengthstrength

permanentpermanent 14,092 G 14,092 G

conventional electromagnetconventional electromagnet 23,49023,490

superconducting solenoidssuperconducting solenoids<169,000<169,000

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MagnetsMagnets

type of magnettype of magnet max. field max. field strengthstrength

permanentpermanent 1.41 T 1.41 T

conventional electromagnetconventional electromagnet 2.35 2.35

superconducting solenoidssuperconducting solenoids<16.9 <16.9

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Sample HandlingSample Handling

solutionsolution solvents should be free of hydrogensolvents should be free of hydrogen

deuterated solvents used where necessarydeuterated solvents used where necessary should contain standardshould contain standard

most commonly tetramethyl silane (TMS)most commonly tetramethyl silane (TMS) TMS signal position set at 0 ppmTMS signal position set at 0 ppm other peaks relative to TMSother peaks relative to TMS

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FT-NMRFT-NMR

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Components of FT Components of FT SpectrometersSpectrometers

"Block diagram "Block diagram of a Fourier of a Fourier transform NMR transform NMR spectrometer."spectrometer."

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MagnetMagnet

in the past, electromagnets or in the past, electromagnets or superconducting solenoidssuperconducting solenoids

today, permanent magnets or today, permanent magnets or superconducting solenoidssuperconducting solenoids

Dr. S. M. Condren

Locking the Magnetic Locking the Magnetic FieldField

field/frequency lock systemfield/frequency lock system reference nucleus is continuously reference nucleus is continuously

irradiated and monitored at a frequency irradiated and monitored at a frequency corresponding to its resonance corresponding to its resonance maximum at the rated field strength of maximum at the rated field strength of the magnetthe magnet

provided by deuterium in the solvent, provided by deuterium in the solvent, with second transmitter coil set to the with second transmitter coil set to the frequency for deuteriumfrequency for deuterium

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ShimmingShimming

coils which provide some changes in the coils which provide some changes in the magnetic field by carefully controlling magnetic field by carefully controlling the current flowing through the coilsthe current flowing through the coils

used to correct for inhomogeneities of used to correct for inhomogeneities of magnetmagnet

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Sample SpinningSample Spinning

sample is spun along its longitudinal sample is spun along its longitudinal axis to counteract some of the field axis to counteract some of the field inhomogeneitiesinhomogeneities

"A minor disadvantage of spinning is "A minor disadvantage of spinning is that the magnetic field is modulated at that the magnetic field is modulated at the spinning frequency, which may lead the spinning frequency, which may lead to to sidebandssidebands on each side of absorption on each side of absorption peaks?"peaks?"

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Sample ProbeSample Probe1.1. holds sample in the magnetic holds sample in the magnetic

fieldfield

2.2. spins the samplespins the sample

3.3. houses the coil or coils that houses the coil or coils that permit excitation and detection of permit excitation and detection of the NMR signal and preamp the NMR signal and preamp

4.4. coil for lockingcoil for locking

5.5. coil for decouplingcoil for decoupling

6.6. variable temperature capabilityvariable temperature capability

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Transmitter/Receiver Transmitter/Receiver CoilsCoils

early instruments, separate coils, at early instruments, separate coils, at right angles to each otherright angles to each other

FT-NMRs, normally same coilFT-NMRs, normally same coil

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Pulse GeneratorPulse Generator

1. a continuous crystal oscillator1. a continuous crystal oscillator

2.2. a gate to switch the pulse on a gate to switch the pulse on and offand off

3.3. a power amplifier to amplify a power amplifier to amplify the pulse to perhaps 50 - 100 Wthe pulse to perhaps 50 - 100 W

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Receiver SystemReceiver System

nanovolt - microvolt signal amplified to nanovolt - microvolt signal amplified to 10 V 10 V

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Detector and Data Detector and Data Processing SystemProcessing System

high-frequency radio signal converted high-frequency radio signal converted to audio-frequency signal to digitizedto audio-frequency signal to digitized

carrier signal with superimposed NMR carrier signal with superimposed NMR signal, 10 ppm PMR spectrum at 270 signal, 10 ppm PMR spectrum at 270 MHz would have a frequency range of MHz would have a frequency range of 2700 Hz2700 Hz

subtract the carrier signal from NMR subtract the carrier signal from NMR signalsignal

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Sampling the Audio Sampling the Audio SignalSignal

digitize signal, must sample signal digitize signal, must sample signal at least twice per cycleat least twice per cycle

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Single-Channel Single-Channel DetectionDetection

disadvantage, does not know if disadvantage, does not know if difference between carrier signal difference between carrier signal and NMR signal is positive or and NMR signal is positive or negative, can produce a false negative, can produce a false signalsignal

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Quadrature Detection Quadrature Detection SystemsSystems

NMR signal is split and fed to two NMR signal is split and fed to two identical detectorsidentical detectors in one detector the signal is treated in in one detector the signal is treated in

the normal manner, carrier signal the normal manner, carrier signal subtracted from NMR signal for subtracted from NMR signal for digitizingdigitizing

in the second detector the carrier in the second detector the carrier signal is altered 90signal is altered 9000 before subtraction before subtraction

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Quadrature Detection Quadrature Detection SystemsSystems

the two audio signal are the same the two audio signal are the same except for phase, the first produces a except for phase, the first produces a cosine wave while the second produces cosine wave while the second produces a sine wavea sine wave

the two signals are digitized, the two signals are digitized, transformed separately into frequency transformed separately into frequency domain signals and combined to domain signals and combined to produce the spectrumproduce the spectrum

folded signal disappears because it is folded signal disappears because it is of opposite sign in the cosine and sine of opposite sign in the cosine and sine wave formswave forms