6. 1-d multipulse experiments:

6. 1-D Multipulse Experiments:

1. Spin-echo sequence 2. Attached proton test3. Spin energy levels4. Spin population inversion (SPI)5. Polarization transfer6. INEPT7. DEPT8. Summary

06-1D Multipulse Experiments (Dayrit) 2

The NMR challenge

• Nuclear properties are determined by the spin quantum number, I, and the magnetogyric constant, . The proton, 1H, is the most sensitive nucleus; other nuclei, in particular 13C, are insensitive.

• The energy differences between the spin energy levels are small. As a result, the population difference between spin energy levels is small, and the intensity of the NMR signal is low.

• Many NMR-active isotopes have less than 100% natural abundance.

How can one improve the sensitivity of NMR?


The Single-pulse NMR experiment

• Imperfect pulse• Imperfect magnetic field

homogeneity


The NMR solutions• Develop better NMR hardware: higher field magnets,

(higher Bo), better probes, more powerful computers and

improved electronics.

• Develop ingenious NMR experiments Manipulate spin populations using ingeniously designed

multipulse sequences which vary the pulse angle, excitation of specific nuclei, timing, phase, and others.

Multi-pulse sequences as building blocks. Two of the most serious NMR challenges are: loss of phase

coherence and insensitive nuclei . The corresponding NMR solutions are spin echo and polarization transfer .


Problem: Loss of phase coherence• Nature of problem: The individual spins experience

different local magnetic fields due to imperfections of the hardware, sample conditions, etc. This leads to a “spreading out” of the spins = loss of phase coherence loss of transverse magnetization, M

x and M

y.

• NMR solution: spin-echo: allow the spins to spread out in one direction during time , apply a 180°-pulse and allow the spins to refocus for time Repeat as needed.

• Coupled nuclei are split with varying energies, J (Hz = s-1). The spin-echo sequence can differentiate nuclei according to their J values by selecting the appropriate time constant, .


Spin-echo

90x' - ( - 180x' - ) - ( - 180x' - ) - ( - 180x' - ) ... 1st echo 2nd echo

(o)

y'

x'

z'

90ox'

z'

y'

x'

(slowccw)

(fast, cw)

z'

x'

y' 180o

x'y'

x'

z'

(slowccw)

(fast, cw)

z'

x'

y'

1st echo

(fast, cw)

(slowccw)

z'

x'

y'

z'

x'

y'

2nd echo

180ox'

3rd echo

y'

x'

z'

180ox'

(fast, cw)

(slowccw)

z'

x'

y'

3rd echo


Spin-echo

90x' - ( - 180x' - ) - ( - 180x' - ) ( - 180x' - ) ...

• At the top of the 1st echo, we achieve a refocusing of the spin vectors which spread out during time as a result of inhomogeneities and other effects.

• The spin echo sequence can be used for both 1H and 13C.• It is most useful in the manipulation of vectors of coupled

heteronuclear spin systems. Let us consider the spin-echo experiment on the four types of substituted carbon: Cq, CH, CH2, and CH3.

1st echo 2nd echo 3rd echo


13C Vectors in a 13C1H system (no 1H decoupling): after a 90x’ pulse assuming on-resonance, 0: The slow vector is due to 13C1H while the fast vector is due to 13C1H. The frequencies of the two vectors will be:

(13C 1H) = 0 - ½ 1J(C,H) = 0 -

(13C 1H) = 0 + ½ 1J(C,H) = 0 +

(o - )

(o + )

(o)(13C)

z'

x'

y'

MH

MH

MH

MHy'

x'

z'

90ox''

z'

x'

y'

MH MH

For 13C1HCl3 (no 1H decoupling): 1J(C,H) = 209 Hz.

At 100 MHz, 0 = 77.7 or 7770 Hz.

Therefore, (13C 1H) = 7666 Hz and (13C 1H) = 7875 Hz.

where 2 = 1J(C,H)

(AQT)

77.0 ppm(7700 Hz at 100 MHz)

JCH = 209 Hz

7666 Hz7875 Hz


If we allow the two J-coupled 13C vectors to spread out further (i.e., no acquisition yet), after time , each vector will travel:

{2[½ J(C,H)]} or (2 ) and

{2[-½ J(C,H) ]} or [2 (-) ].

The phase angle between the two 13C vectors, , of a 13C1H system can be calculated as:

= 2 (J(C,H) ) or = 2 2

(o - )

(o + )

(o)(13C)

z'

x'

y'

MH

MH

MH

MHy'

x'

z'

90ox''

z'

x'

y'

MH MH


= 2 (J(C,H) ) or = 2 2

Let J = 1 Hz ( = 0.5 Hz): after = 1 s, the fast vector will

advance by , while the slow vector will regress by ; the

phase angle will be 2 (=0), and they will refocus along the -y’

axis.

MHMH

y'

x'

z'

90ox''

z'

x'

y'

MH

MH MH

MHy'

x'

z'

(13C)

For 13C1HCl3 where J = 209 Hz, the vectors will refocus along

the -y’ axis at = 4.8 ms.


Consider two CH groups, with J1=125 and J2=200 Hz, and let both vectors be on-resonance.

• Each pair of vectors will spread out according to J: = 2 (JC,H) . Therefore, each pair of vectors will refocus along the -y’ axis at : 8 and 5 ms, respectively.

By selecting appropriate values for , we can do the following: • We can select the coupling which we wish to be refocused; and • We can select the sign (or phase) of refocusing to be along either y’ axis.

(13C)

z'

x'

y'y'

x'

z'

90ox''

z'

x'

y'


Effect of broadband 1H decoupling during 13C acquisition:

• For a CH coupled system, broadband 1H decoupling collapses the 13C doublet into a singlet.

•The magnitude and phase (sign) of the singlet depends on the position of the 13C doublet during 1H decoupling.

(o - )

(o + )

(o)

z'

x'

y'

MH

MH

{1H}y'

x'

z'

M

(o)

MH

MHy'

x'

z'

{1H} M

z'

x'

y'

This is the basis for the 13C spin-echo pulse sequence.

Spin-echo sequence for a CH group, on-resonance: There are two 13C vectors corresponding to the attached 1H with the 1H and 1H spins.

(b) The vectors fan out at different depending on the interval .

(c) The 180x°-pulse flips the

vectors across x’ axis (-radians).

(d,e) Broadband 1H decoupling collapses the 13C multiplet into a singlet vector. (continued…)

b c d e

a

(a) 90x pulse on 13C.

Spin-echo sequence for a CH group, on-resonance:

(d,e) Since the vector is now a singlet and is on-resonance, it does not spread out during the second -interval.

Acquisition of the echo gives peaks whose phase and magnitude depend on : an interval of 1/J sec allows the vectors to move by radians. (from: Friebolin,

Basic One- and Two-Dimensional NMR.)

b c d e

a

Spin-echo sequence for three CH groups with off-resonance signals: The sign and magnitude of each of the CH groups depends on both their J(CH) values and the off-resonance position.

B: Truncated sequence: 13C: 90x'--{BB 1H}-AQT

Phase (sign) and magnitude of the CH groups differ. (continued)

Spin-echo sequence for three CH groups with off-resonance signals:

C: Complete sequence:13C: 90x'--180x'-{BB 1H}-AQT

BB decoupling collapses the doublets into singlets, which refocus in the -y’-axis during the second -interval. With spin echo (and refocusing), CH groups give a negative singlet. (from: Friebolin, Basic One- and Two-Dimensional NMR.)

The attached proton test (APT): Spin-echo for 13C-{1H} for Cq, CH, CH2, and CH3, on-resonance, =1/J sec. CH and CH3 groups are (+) while the Cq and CH2 groups are (-). (from: Friebolin, Basic One- and Two-Dimensional NMR.)


Attached Proton Test

If the vectors are on-resonance:

• Cq: No coupling (the 13C signal is a singlet). The effect of the

spin echo sequence is to remove inhomogeneities.

• CH: (13C signal is a doublet) The two vectors move away at a rate of J Hz from each other and centered around the y’-axis.

• CH2: (13C signal is a triplet) The middle vector is stationary

along the y’-axis while the two flanking vectors fan out at a rate of J Hz.

• CH3: (13C signal is a quartet) Each of the four vectors is

separated by J Hz and centered on the y’-axis.


The attached proton test (APT) spectrum of a neuraminic acid derivative. Note that the sign of the CH and CH3 groups are (+) while the Cq and CH2 groups are (-). (from: Friebolin, Basic One- and Two-Dimensional NMR.)


Spin-echo sequence: summary

The spin echo sequence - ( - 180x' - ) - is a building block used in many NMR pulse sequences. It can be used for the following:

1. Refocusing of vectors to correct for inhomogeneities.

2. For heteronuclear coupling, e.g. C-Hn: By setting =J-1, it is

possible to distinguish different multiplies of C using the attached proton test (APT) (J-modulation).

• The intensity of the spin-echo peak is sensitive to the value of J and . Some signals will appear weak if at the y’-component of the vector is small. That is, the various spin-echo signals peak at different values of .


APT: anatomy of a multipulse sequence

One-dimensional multiple-pulse experiments generally have three stages: preparation, evolution / mixing, and detection.

• The preparation stage has two functions: to make sure that the starting conditions are at steady state equilibrium and to set-up the spins so that they are in the proper condition for the transfer of spin information.

• Evolution / mixing is implemented with a fixed waiting period, or , where the transfer of information can take place among the spins; one common feature is a spin-echo sequence: (--180x’/ y’--).

• Detection refers to the acquisition of the FID.

APT: ... -PD-90x'--180x'-{BB 1H}-AQT-PD - ...


Problem: insensitive nuclei• Nature of problem: The two elements of most interest in organic chemistry are H and C. 1H is the most sensitive NMR nucleus; however, 13C is insensitive because of its low and % isotopic abundance.

• NMR solution: polarization transfer: develop a pulse sequence that is able to transfer energy from 1H to 13C. The transfer of energy results in the increase in the spin population of the upper spin energy level of 13C, which results in higher sensitivity.


Polarization transfer: Using spin energy levels

Bo

The single spin system

The spin energy levels for a single spin system are populated following the Boltzmann law:

N / N = exp (-E/kBT)

• For a single spin system, there are two spin energy levels corresponding to and spin orientations.

• The energy separation between the and spins depends on Bo.


Spin energy levels for a coupled spin system, AX

• Spin-spin coupling is a localized interaction due to the proximity of nuclei possessing spin quantum number > 0.

• For a coupled two spin system, AX, four spin energy levels are generated, AX, AX, AX, and AX, where the first spin

refers to the orientation of the A nucleus, and the second spin refers to the X nucleus.

AX

AX

AX

AX


Spin population for a coupled spin system, AX

AX

AX

AX

AX

(26%)

(25%) (25%)

(24%)

Assume that the ground state population of the and spins of both nuclei A and X are:

A = X = 51% and A = X = 49%

The populations of the four AX spin energy levels become:

AX = (.51)2 = .26 AX = (.51)(.49) = .25

AX = (.49)(.51) = .25 AX = (.49)2 = .24



AX

AX

AX

AX

(26%)

(25%) (25%)

(24%)

• The energy difference between the spin energy levels, J Hz, is determined by the energy of interaction of the spins, and not by the external magnetic field, Bo. Thus, J is independent of Bo.

• This means that J will have the same value regardless of the strength of the magnetic field.

• The relative energies and spin population differences among the spin levels remain the same.



• The spin population will be distributed according to the Boltzmann distribution, with the relative populations: AX > ~ > . (The thickness of the bars represent the relative spin populations.)

• The magnitude of the energy differences depends on the strength of the coupling.

4

3

2

1

X2

X1

A2

A1

E

E

1,2 3,4 1,3 2,4

AX


E

A1

A2

X1

X2

1

2

3

4


In a spin system, the population is distributed among spin energy levels according to the Boltzmann distribution. For a heteronuclear AX (1H-13C) system, the relative equilibrium populations are:

(1H 13C)

(1H 13C)

(1H 13C)

(1H 13C)

X A

1,2 3,4 1,3 2,4

E

1H:13C:



Consider a AX system such as CHCl3. There will be 6 isotope

combinations. Polarization transfer will take place only between appropriate pairs.

(1H 12C) and (1H 12C): The carbons are not NMR active; the

protons are singlets. Although this group comprises the majority of natural abundance CHCl3 (~99%), they do not contribute to this

experiment.

(1H 13C), (1H 13C), (1H 13C), and (1H 13C):

There are two types of 1H spins and 1H spins which are

determined by the attached 13C. Similarly, there are two types of 13C spins and 13C spins which are determined by the attached 1H.


Selective Population Inversion (SPI)1H, the “sensitive” nucleus, has the larger . By inverting the populations of the energy levels connected by the A transitions, the intensities of the X (13C) transitions are enhanced. This achieves polarization transfer from the more sensitive nucleus (1H) to the less sensitive nucleus (13C). SPI is the general mechanism of polarization transfer.

invert

E

A1

A2

X1

X2

1

2

3

4invert

E

1,2 3,4 1,3 2,4

AX

1H:13C:


Signal Enhancement by SPI

The new intensities obtained by SPI are a function of the ratios of the magnetogyric constants, :

1 + (A/X) and 1 - (A/X)

Since (1H/13C) 4, the intensities of the positive and negative

peaks are increased by about +5x and -3x, respectively. However, SPI is cumbersome since it requires that the 1H irradiation be selective.

• The differences in the populations of the spin energy levels are reflected in the intensity and sign of the Mz vector: at equilibrium, Mz is positive; if the populations are equal, Mz = 0; if the spin populations are inverted, Mz is negative.


Polarization transfer

• The magnetogyric ratio gives the magnitude of the separation between spin energy levels:

E = hBo / 2

which determines the relative spin populations (Boltzmann distribution):

N / N = exp (-E / kB T )

• Among the nuclei in organic compounds, 1H has the highest magnetogyric ratio, , and therefore is the most sensitive.


Polarization transfer

Polarization transfer is a technique by which we can take advantage of the presence of 1H by transferring spin population levels from 1H to nuclei of lower . Polarization transfer takes place through the intervening bonds.

• This phenomenon has given rise to a family of polarization transfer (PT) experiments, most notably INEPT (Insensitive Nuclei Enhancement by Polarization Transfer) and DEPT (Distortionless Enhancement by Polarization Transfer). The polarization transfer technique was developed from the J-modulated spin-echo sequence.

Recall the spin energy level diagram for an AX system ...


Basic polarization transfer sequence:

The simplest pulse sequence for polarization transfer is a 90x’ - - 90x’ building block:

1H: 90x’ - - 90x’ . . . . . . . 13C: . . . . . . . . . . . . .90x’ - AQT

Assume a CH system. Let us set =(2JCH)-1. For a series of sequences where the waiting time is varied as follows - 0, , 2, etc., the 1H vector will alternate between y’(i.e.: +y, -y, +y, etc). The second 1H 90x’-pulse, converts this to alternating z’. Next, we implement a 13C 90x’-pulse. When the waiting times are 0, 2, 4, etc, the 13C peak is enhanced by polarization transfer because the 1H vector shall be along the -z’ direction (i.e., population inversion).


1H: 90x’ - - 90x’ . . . . . . . 13C: . . . . . . . . . . . .90x’ - AQT

The behavior of the 1H vector for = (2J)-1, and waiting time of 0, , and 2. For these values, the 1H will point at either z’. At intermediate waiting times, it is the y’ component only that will contribute to polarization transfer. (from: Sanders and Hunter, Modern NMR Spectroscopy)


(from: S

anders and Hunter, M

odern N

MR

Spectroscopy)


1H: 90x’ - - 90x’ . . . . . . . 13C: . . . . . . . . . . . . .90x’ - AQT

For CHCl3, JCH=210 Hz. Let =(2J)-1 = 2 ms. If we set in

increments of 250 s, the amplitude of the 13C signal will be modulated as shown:


INEPT pulse sequence:

As in SPI, the main objective of INEPT is the inversion of the spin energy levels associated with the 1H transitions and the transfer of this spin polarization to 13C. INEPT was developed to overcome the limitation of SPI of requiring a selective pulse. The INEPT pulse sequence combined spin echo and polarization transfer: a 180 pulse was placed in the middle of the delay period in order to remove any effects from the 1H shifts.

1H: 90x’ - - 180x’ - - 90y’ . . . . . 13C: . . . . . . . . 180x’ - - 90x’ - . . . . AQT

[ spin echo ]

[ polarization transfer ]

INEPT Pulse Sequence.

(a-f) 1H 90°-pulse and spin-echo sequence. The MH

C

(slow vector, ccw) and MH

C (fast vector, cw) fan out during waiting time, = (4 1JCH)-1.

(e) A simultaneous 180° pulse on 13C flips the 13C vectors by 180° and reverses the direction of the attached 1H vectors.

(f) After an interval of = (4 1JCH)-1, the 1H vectors are antiparallel along the ±x-axis. (continued)


(g) The 1H-90°y’-pulse flips the antiparallel 1H vectors to the ±z-axis. This represents the spin population of 1H.

(g’) The spin population of 1H is transferred to the attached 13C yielding antiparallel 13C vectors along the ±z-axis.


(h) The 13C-90°x’-pulse flips the antiparallel 13C vectors to the ±y-axis for acquisition.

The frequencies and heteronuclear J-couplings evolve during acquisition time. This yields positive and negative 13C peaks separated by 1JCH Hz centered on the resonance frequency.



At the end of the INEPT pulse sequence, the MCH and MC

H

vectors with the appropriate value of 1JCH (=(4JCH)-1) are

antiparallel (=180°) along the ±y’ axis after the 13C 90°x’-pulse.

The vectors will relax back to the +z axis.z

y'

x'

MCHMCHT1 relaxation

x'

y'

z z

y'

x'

FT

J13CThe INEPT 13C signal for CHCl3 will have

(+) and (-) peaks, separated by J Hz. The frequency position depends on whether it is on-resonance (=0) or off-resonance.


Relative frequency of attached protons (J-modulation)

CHn MCH Relative frequency

CH MCH 0 - ½ J(C,H)

MCH 0 + ½ J(C,H)

CH2 MCH 0 - J(C,H)

MCH + MC

H 0

MCH 0 + J(C,H)

CH3 MCH 0 - 1½ J(C,H)

MCH + MC

H + MCH 0 - ½ J(C,H)

MCH + MC

H + MCH 0 + ½ J(C,H)

MCH 0 + 1½ J(C,H)


INEPT for CH, CH2 and CH3 groups

The 13C peaks are enhanced by SPI.

• CH group: Doublet with (+) and (-) peaks.

• CH2 group: Triplet: (+), (0), (-). The middle peak is not detected .

• CH3 group: Quartet: (+1), (+3), (-3), (-1).

• Cq: Singlet vector simply relaxes according to T1.

The amplification of the 13C signals depends on the magnitude of the MH vectors during the instant at which polarization transfer

takes place and this differs for the various carbon groups and 1JCH.

Assuming an average (compromise) 1JCH value of 145 Hz:

= (4 1JCH)-1 = 1.7 ms.


Multiplets observed in the 13C NMR spectra for CH, CH2, and CH3 groups.

Left: 13C NMR spectrum without 1H decoupling.

Right. 13C INEPT without 1H decoupling.

(from: Friebolin, Basic One- and Two-Dimensional NMR Spectroscopy.)


Improving INEPT• It is difficult to interpret an INEPT spectrum because of overlaps of (+/-) multiplet peaks and the absence of a central peak for CH2. Two improvements can be made: the peaks

corresponding to each group can be in the same phase, and multiplets can be collapsed to singlets.

• The pulse sequences that have been developed for this purpose are the refocused INEPT and the refocused INEPT with broadband proton decoupling, {BB 1H}.


Refocused INEPT1H: 90x’ - - 180x’ - - 90y’ - - 180x’ - - BB13C: . . . . . . . . . 180 - - 90y’ - - 180x’ - - AQT [ INEPT ] [ spin-echo ]

1. The refocused INEPT is a combination of INEPT + spin-echo.

2. There is no 13C 90 read pulse. It is the spin-echo that is acquired.

3. {BB 1H} collapses the 13C multiplets into singlets. The 13C vectors evolve only according to their frequencies during AQT.

4. Quaternary 13C nuclei are not affected by polarization transfer, and refocused INEPT does not bring the Cq vectors into the y’

axis during acquisition. Therefore, Cq groups are not detected.



5. The spin-echo is the refocusing step. The second fixed delay time, , is set in the same way as the spin-echo sequence. However, this involves some compromise as the different CHn

groups will refocus differently. A compromise value is = 3/(81JC,H) ~ 2.6 ms.

6. The second pair of simultaneous 180-pulses during the spin-echo portion has a similar effect on the MC vectors: the vectors

are flipped across the y’ axis and their directions are reversed.



7. The CH and CH3 groups appear as positive peaks, while the

CH2 groups appear as negative peaks. Cq nuclei are not

detected.

8. Because the polarization transfer process depends on the magnitude of the MH vectors, INEPT sequence should not be

longer than either the T1 or T2 value of 1H.

INEPT spectra of a neuraminic acid derivative.

A: 13C NMR with {BB 1H};

B: Gated decoupling (with multiplets but no NOE enhancement);

C: INEPT spectrum;

D. Refocused INEPT; and

E. Refocused INEPT spectrum with {BB 1H}. Cq nuclei are not detected by this

sequence. (from: Friebolin, Basic One- and Two-Dimensional NMR Spectroscopy)


Selective Enhancement by Polarization Transfer: DEPT

• One of the disadvantages of the refocused INEPT sequence is its inability to distinguish CH from CH3 unambiguously (peaks

from both types are positive).

• Distortionless Enhancement of Polarization Transfer (DEPT) was developed to overcome this shortcoming. It is today the most widely-used 13C experiment to determine carbon multiplicity.



1H: 90x’ - - 180x’ - - y’ - - BB...13C: . . . . . . . . . .90x’ - - 180x’ - - AQT

• DEPT is similar to INEPT in the way that polarization transfer is effected but differs in that all of the 13C signals are in-phase at the start of the acquisition of signal. There is no need for an additional refocusing period, , as in refocused INEPT; this is the sense in which this pulse sequence is “distortionless.”



1H: 90x’ - - 180x’ - - y’ - - BB...13C: . . . . . . . . . .90x’ - - 180x’ - - AQT

• A compromise value of 1JC,H = 145 Hz is used so is set for

(2 1JC,H)-1 ( = 3.45 ms).

• The principal advantage of DEPT is its ability to select for a desired intensity and sign of carbon signals depending on whether it is CH, CH2, or CH3. This is done by setting the

appropriate value of the 1H y’ pulse.

• We can either display all of the types of carbons -- CH, CH2,

and CH3 -- or, by sub-spectral editing, selectively display only

one type of carbon.


DEPT allows one to distinguish CH, CH2, and CH3 groups through the pulse angle y'. The intensity and sign of the signals depend on the value of y'. (from: Friebolin, Basic One- and Two-Dimensional NMR Spectroscopy.)

CH : ICH = [ (1H) / (13C)] sin

CH2 : ICH2 = [ (1H) / (13C)] sin 2

CH3 : ICH3 = [3 (1H) / 4 (13C)] (sin + sin 3)



1H: 90x’ - - 180x’ - - y’ - - BB...13C: . . . . . . . . . .90x’ - - 180x’ - - AQT

The DEPT sequence allows us to selectively display the peaks corresponding to a specific type of carbon through spectral editing. The intensities of the carbon signals depends on the pulse angle as follows:

DEPTCHn DEPT45 DEPT90 DEPT135

CH : 0.67 1.0 0.67CH2 : 1.0 0.0 -1.0

CH3 : 1.0 0.0 1.0


DEPT135 spectrum of a neuraminic acid derivative displays all of the carbon atoms with attached protons. CH3 and CH groups are (+) while

CH2 groups are (-). Peaks marked with arrows are CH groups. (from:

Friebolin, Basic One- and Two-Dimensional NMR Spectroscopy)


DEPT sub-spectra obtained by spectral editing. Sample is neuraminic acid.

A. 13C NMR {BB1H};

B. CH sub-spectrum (DEPT90)

C. CH2 sub-spectrum

(DEPT45 - DEPT135)

D. CH3 sub-spectrum

(DEPT45 + DEPT135 - 0.707 DEPT90).

(from: Friebolin, Basic One- and Two-Dimensional NMR Spectroscopy.)


Summary: Polarization transfer

• Complex pulse sequences can be put together using simpler pulse sequences as “building blocks”. Among the most useful building blocks are:

• polarization transfer: enhancing insensitive nuclei by selective population inversion of sensitive nuclei

• {BB 1H} for converting 13C multiplets into singlets


Summary: Insensitive Nuclei Enhancement by Polarization Transfer (INEPT) 1H: 90x’ - - 180x’ - - 90y’ . . . . . .13C: . . . . . . . . .180 - - 90x’ - AQT

= 3/(8 1JCH)

• Does not need 1H selective irradiation as in SPI.

• Enhancement of 13C by 1 (1H/13C).

• Enhancement by polarization transfer is larger than NOE: maximum heteronuclear NOE (13C-{1H}) is 200% (increase of 3 times) vs. increase from polarization transfer of 5 times.

• No INEPT signal for Cq


Summary: Refocused INEPT with {BB 1H}

1H: 90x’ - - 180x’ - - 90y’ - - 180x’ - - BB13C: . . . . . . . . .180 - - 90y’ - - 180x’ - - AQT

= 3 / (8 J(CH)) = 3 / (8 J(CH))

• Refocusing sequence (--180--) is inserted in INEPT sequence which brings the 13C vectors of a multiplet into the same phase.

• 13C multiplets singlets.

• CH and CH3: positive; CH2: negative.


Summary: Distortionless Enhancement of Polarization Transfer (DEPT) 1H: 90x’ - - 180x’ - - y’ - - BB13C: . . . . . . . . . 90x’ - - 180x’ - - AQT

= (2 JCH)-1

1 = 45; 2 = 90; 3 = 135

• Spectral editing allows one to distinguish the three types of carbon substitution: CH, CH2, and CH3.

• No DEPT signal for Cq.


Looking ahead . . .

• The 1-dimensional multipulse sequences illustrate what Richard Ernst has called “spin wizardry,” the ability to manipulate and select from the large store of information that is contained in the nuclear spin, and display this in the spectrum.

• In a number of 1-dimensional pulse sequences, we have been forced to select compromise values for the waiting time, , in order to allow for a range of J-modulated signals. In the case of double irradiation experiments (J-coupled and NOE), we perform separate experiments for each nucleus of interest.

• This is where 2-dimensional NMR spectroscopy comes in...

6. 1-d multipulse experiments:

Documents

spin vectors

spinecho sequence

spin echo sequence

spinecho experiment

spin energy levels4

designed multipulse

singlepulse nmr

nmr signal