Chem 526Chem 526 NMR for Analytical Chemists · Chem 526Chem 526 NMR for Analytical Chemists...
Transcript of Chem 526Chem 526 NMR for Analytical Chemists · Chem 526Chem 526 NMR for Analytical Chemists...
1
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)
2
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?
3
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
3
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 ?
5
A) (S/N)/(T)
B) (S/N)/(T1/2)
C) (S/N)/(T2)
NMR Hardware
4
Bruker NMR System
Magnet
ProbeConsole
Probe
7From Bruker’s Avance Beginner’s Guide
Block Diagram of NMR Spectrometer (p115)
8
5
Superconducting Magnet (p116)
9
Probe assembly & Tuning (p118)
LTune for 1H & 2H
CT
1/Z = i{1/L – (CT)}= i{ 1 - L CT}/L
CMTune for13C & 15N
10
Tuning is best when the resonant condition 1 - 2LCT = 0
Probe impedance: Z = R + i(L’-1/CM) = 50
= 1/(LCT)1/2
6
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
11
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)
12
7
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
13
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
15
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
9
Step 1. Login & Changing Sample; Read Shim file & Start lock
BSMS Keyboard
20
Lift Sample Lock (Lock Gain, Lock Amplitude) Spin Sample Adjust Shim
10
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
11
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.
12
Lock signal (2D NMR from solvent)
25
Lock & Shimming
You will look at the signal from 2D in CDCl3.
Optimize B0(z) Lock signalstronger
13
Collecting 1H Signal
Read parameters for 1H 1pulse NMR
Acquiring FID
28
“acqu” gives this window. “zg” starts acquisition.“rga” sets a receiver gain. “NS”, “eda”,,
Make sure that you have some signals
14
Xwin-nmr Window
29
“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
16
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).
17
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?
18
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
39
Continuous Fourier Expansion s(t) = d S()e(t)= d S()exp(-it)
)()()}exp()({ Stitsdt )()}exp()({ titedt
19
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)
40
Array of FIDs are taken for different t1 period
{Sxcos(St2)+Sysin(St2)}cos(It1)
Ch4.1 (See p275)
41
20
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( )
42
s(t1, ω2) = A(ω1 -I)A(ω2 -S)
After FT
43
21
Phase correction (ph0)
Ph 1 correct
44
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)}
45
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
22
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
46
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
47
p , p g pby interactively adjusting 0 and 1.
23
Phase correction (ph0)
Ph1 correct
48
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)