Chem 526Chem 526 NMR for Analytical Chemists · Chem 526Chem 526 NMR for Analytical Chemists Lecture 6 ... – 1H Decoupling (3.4.3) – Off-Resonance Effects ... 110906_Lecture6.ppt

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Chem 526Chem 526

NMR for Analytical Chemists

Lecture 6Lecture 6

(Chapter 3)

Announcement 1• We will have a lab next Tuesday. Dr. Dan

McElheny will see you at this class room

Please read the pre lab materials (do nload• Please read the pre-lab materials (download them at the chem526 web) before Tuesday.

• You will have to pass the exam so that you are allowed to use NMR in RRC. (This is a part of the homework for this week)

Pl k i f 2 (W ill h t t l 4• Please make a pair of 2. (We will have total ~4 groups) before Tuesday.

• The preliminary analysis of the unknown (1H & 13C NMR) is Homework 4 (Due 9/22)

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Basic Spectroscopy Question

Q1 From a dilute sample in H2O solution youQ1. From a dilute sample in H2O solution, you obtained a noisy 13C NMR spectrum for a 0.01 % ethylbenzene sample (0.7 mL) with a signal-to-noise ratio (S/N) of 8 after 1 scans with 1 pulse excitation experiment.

(a) How much S/N do you expect in the NMR spectrum if you accumulate 100 scans?

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p y

Homework 3 #1 (Due 9/20): Explain what the following operation or function means. What is the purpose for each item?

(1) Shimming

(2) Lock

(3) Spinning

(4) Window Function

(5) Fourier Transform( )

(6) Phasing

(7) Probe Tuning

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S/N & Sensitivity

• “Sensitivity” is defined as a S/N ratio obtained during a unit experimental time (T)during a unit experimental time (T).

• What is the most appropriated definition for “sensitivity” for one to make a fair comparison of S/N obtained during different experimental time ?

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A) (S/N)/(T)

B) (S/N)/(T1/2)

C) (S/N)/(T2)

NMR Hardware

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Bruker NMR System

Magnet

ProbeConsole

Probe

7From Bruker’s Avance Beginner’s Guide

Block Diagram of NMR Spectrometer (p115)

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Superconducting Magnet (p116)

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Probe assembly & Tuning (p118)

LTune for 1H & 2H

CT

1/Z = i{1/L – (CT)}= i{ 1 - L CT}/L

CMTune for13C & 15N

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Tuning is best when the resonant condition 1 - 2LCT = 0

Probe impedance: Z = R + i(L’-1/CM) = 50

= 1/(LCT)1/2

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Probe tuning

Reflection

“wobb” displays a refection for RF in a range of !(You are actually inputting RF to the probe. So be careful)

1/2

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input

reflection

Q = L/R ~ /1/2

S/N higher for higher Q

Two types of probes available at RRC East

BBO (Broad Band) & TBI (Triple Inverse)

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S/N from an NMR probe

• , [3.3]

)][(/

2/30

2/3

SSSCCC

de

RTRRTRTf

BNNS

N is the number of nuclei e and d are gyromagnetic ratios of the excited and

detected nuclei B0 is a magnetic field Higer B0 S/N up Tc, Ts, Ta are coil, sample, preamp temperatures,

respectively So Called “Cryoprobe”

)][( SSSCaCC RTRRTRTf

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respectively

Rc, Rs are resistance of the coil and that induced by the sample, respectively.

Reduce coil and preamp temperatures S/N up

Lower Rc, Rs S/N up

So Called Cryoprobe

Detection limit of 900 MHz at CSB

Probe 1H EtB 13C ASTM • 900 TCI 7700:1 1950:1(Cryoprobe)

• 900 TXI 2500:1 --

(Non-cryo inverse probe)

Tc & Ta ~ 20 K

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For 0.1 % Ethyl Benzene in 0.7 mL (~1 mg or 10 μmol) S/N 7700. How much is the detection limit?

S/N ~ 8 for 10 nmol or 1 μg for 4 scans

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Operation of NMR at RRCp

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Step 1. Login & Changing Sample; Read Shim file & Start lock

BSMS Keyboard

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Lift Sample Lock (Lock Gain, Lock Amplitude) Spin Sample Adjust Shim

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Lift SampleLift Sample Compressed air lifts sample

Never put a sample without hearing the sound of air. Q. Why?

Put your sample into a rotor & adjust the position

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Step for using NMR at RRC

Shimming Optimize Magnetic Homogeneity

B0(Z) = B0 +aZ + bZ2 +c Z3 + d X + e Y

• Adjusting shimming: Z1, Z2, Z3 shims change the coefficient a, b, c, respectively.

• You typically need to adjust Z1 & Z3.

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Lock signal (2D NMR from solvent)

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Lock & Shimming

You will look at the signal from 2D in CDCl3.

Optimize B0(z) Lock signalstronger

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Collecting 1H Signal

Read parameters for 1H 1pulse NMR

Acquiring FID

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“acqu” gives this window. “zg” starts acquisition.“rga” sets a receiver gain. “NS”, “eda”,,

Make sure that you have some signals

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Xwin-nmr Window

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“ft” gives you FT; “ef” exponential window function & FT“efp” “ef” & phase correction

Phase Correction

30“apk” Auto phase correction About manual phase correction please ask Dan

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13C NMR

Data Collection and Processingg

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3.2.3 Quadrature Detection (p132)

cos(wRFt)cos(RFt)cos(0t)= cos{(RF+ 0)t} /2

+ cos {(RF – 0)t}/2

cos(w0t) Low pass

Low pass

Q. Explain how the experimental scheme for data acquisition &detection (So called quadrature detection) works

sin(wRFt)

sin(RFt)cos(0t)= sin{(RF+ 0)t}/2

+ sin {(RF – 0)t}/2

Summary of 1D• The motion of the magnetic moment is simply summarized as

IZ –[π/2 IY] IX

and

IX –-[t IZ] IXcos(t) + IYsin(t).

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Complex Signal

Non-decaying Signal• X: Real cost• X: Real cost

cost + i sin t • Y: Imag sint = exp(it)

FIDX R l (t) ( t)• X: Real cos(t)exp(-t)

{cost + i sin t}[Q1] • Y: Imag sin(t)exp(-t) = exp(it)[Q2]

Calculation of FT (p139-140)

• s(t) = exp(it)exp(-t) ( =1/T2)

• [3 14]}{)()( idS [3.14]

How do you calculate?

S() = })(exp{0

ttidt

})(exp{1

tti

}exp{)()(0

titsdtS

=

=

=

2222

i

i

1

0

})(exp{ ttii

A()D() Q1. How are these functions

called?

Q2. What is the maximumintensity of the left term?

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Homework 3 #2Q 1. Plot vs A() for = 100 and =10

Q 2. Plot f vs D() for = 100 and =10

)(

S

Q 3. Plot f vs A() for /2 = 200 and =10

Q 4. Plot f vs A() for /2 = 200 and =40

Q 5. Plot f vs D() for = 200 and =40

2222)(

iS

22)(

A

22

)(

D

Meaning of Fourier Transfer & Eigen Functions

ZZYYXX eAeAeAa Expansion by

eigen vectors

Discrete Fourier Expansions(t) = Snen(t)=Snexp{i(2n)t}

nnn

eAa kknn

nk AeeAea )()(

nk

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Continuous Fourier Expansion s(t) = d S()e(t)= d S()exp(-it)

)()()}exp()({ Stitsdt )()}exp()({ titedt

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2D NMRA. 2D Heteronuclear Correlation NMR (HETCOR) The basic idea of 2D HETCOR NMR is to specify a peak with two frequencies I andS. The sequence is given as an extension of polarization The sequence is given as an extension of polarization transfer experiment, but there is the t1 period for indirectly detecting 1H signal.

Sxcos(It1)

+ Iysin(It1)Ixcos(It1)

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Array of FIDs are taken for different t1 period

{Sxcos(St2)+Sysin(St2)}cos(It1)

Ch4.1 (See p275)

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2D FT Processing

• So we obtained

s(t1, t2) = cos(It1)exp(iSt2).

• First step is Complex FT on t2

s(t1, ω2) = cos(It1)A(ω2 -S)

• Second, Cosine FT on t1s(t ) A( )A( )

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s(t1, ω2) = A(ω1 -I)A(ω2 -S)

After FT

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Phase correction (ph0)

Ph 1 correct

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Phasing (p139, p151)If a signal can be expresses ass(t)= exp(iω0t – λ0t)• S(ω) = A(ω) + iD(ω)

This s(t) is the signal at the sample coil. At the receiver, s’(t)= exp(iω0t – λ0t + i Φ)

{A(ω) + iD(ω)} exp(iΦ) Zero-order phase

However, when we have a delay in signal acquisition: ∆ts’’(t) e p{i (t ∆t) λ (t ∆t)}

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s’’(t)= exp{iω0(t+ ∆t) – λ0(t+∆t)}

= exp(iω0∆t – λ0∆t)s(t)

~ exp(iω0∆t)s(t) when decay is slow

{A(ω) + iD(ω)} exp(iω0∆t) First-order phase

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Phasing (p131, p151)• So in general, your spectrum can be given by S(ω) = {A(ω) + iD(ω)} exp(iω0∆t + iΦ) [3.28]

[A( ) ( ∆t Φ) D( ) i ( ∆t Φ)] A & D mixed up

= [A(ω)cos(ω0∆t +Φ) +D(ω)sin(ω0∆t +Φ)] +i[D(ω)cos(ω0∆t +Φ) -A(ω)sin(ω0∆t +Φ)]

What we do is apply appropriate phase correction:exp(-iω0 Φ1 - iΦ0)to the data

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S(ω)’ = S(ω)exp(-iω0Φ1 - iΦ0) = {A(ω) + iD(ω)}

How to adjust phase

exp(-iω0 Φ1 - iΦ0)

Typically,ω0 Φ1 =1(ω – ωpivot)/(2 SW)

Φ0 = 0

On modern spectrometers, phasing can be performed

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p , p g pby interactively adjusting 0 and 1.

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Phase correction (ph0)

Ph1 correct

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Ch 3.4 Pulse Effects

• We will cover – 1H Decoupling (3.4.3)

– Off-Resonance Effects (3.4.1)

– Composite Pulse (3.4.2)

– Selective Pulse (3.4.4)

– Water Suppression (3 4 5)Water Suppression (3.4.5)

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Ch 3.5 1H decoupling

13C spectrum of ethylbenzene

Q. Why are there no 1H-13C J splitting?