NMR Spektroskopie

NMR Spektroskopie
•
Teil 1: Wiederholung
Was mir wichtig ist:
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Ein wenig physikalischer Hintergrund
Die Chemische Verschiebung
Die Inkrementenregeln zur Berechnung der chemischen
Verschiebung
Magnetischer
Dipol
μ=γ*P
Spinnender
Kern
Wichtige
Eigenschaften verschiedener Kerne
Nucleus
Spin I
1
½
1
0
½
1
½
0
½
H
2
H
12
C
13
C
14
N
15
N
16
0
31
P
Abundance
[%]
99.98
0.016
98.9
1.1
99.63
0.37
99.96
100
γ
[107 rad /Ts]
26.7519
4.1
NMR frequency
(Bo=2.4T)
100
15.351
6.728
1.938
-2.712
25.144
7.224
10.133
10.841
40.481
Empfindlichkeit und Resonanzfrequenz sind proportional zu γ
1)
2)
Magnetische Feldstaerke oft in MHz (1H) genannt (anstatt Tesla)
1.41T=60 MHz, 14 T =600MHz, 21T = 900 MHz (derzeit maximum)
Verschiedene Kerne sind sehr unterschiedlich in ihrer Larmorfrequenz
ΔE= ħ γ Bo = h ν
Die Energieniveaudifferenz ist proportional zur Feldstaerke des
Magnetfeldes
Ansteigendes Bo
Kein Feld
Spins in alle Richtungen
Keine Energiedifferenz
Magnetfeld
Spins ausgerichtet mit oder
gegen das Feld
Numerische Berechnung der EnergieniveauPopulationen
ΔE = γħB0 = hν
ħ= h/2π = 1.05459 x 10
-34
Js
k= 1.38066 x 10-23 J/K
T = 298 K
Numerische Beispiele:
Bo = 1.44 T or 14.4 T
At 60 MHz:= Nβ/Nα = 0.9999904
(or 9.6 x 10-6 excess spins in α)
At 600 MHz:= Nβ/Nα = 0.9999013
(or 98.7 x 10-6 excess spins)
NMR ist eine sehr unempfindliche Spektroskopie!
Μ0 = makroskopische Magnetisierung
Transversale Magnetisierung koennen wir detektieren!
Der FID – fast induction decay
Precession
Relaxation
T2 –Spin-Spin Relaxation
T1 – Spin-Gitter Relaxation
= Ueberlagerung
Integral:
2
1 2
2
2
3
Die Chemische Verschiebung / the Chemical Shift
Resonanz Bedingung
Definition der δ-Skala
Die Resonanzfrequenz relative zur Referenz (TMS) in der ppm Skala wird die
Cehmische Verschiebung genannt. Diese Skala hat den Vorteil, dass die
Werte unabhaengig von der Feld/Magnetstaerke des Spektrometers sind.
1H-NMR
von mono-substitutierten Aromaten
Mesomerieeffekte bewirken starke positions-abhaengige
Verschiebungen im Aromaten
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Die Substituenteneffekte sind in erster
Naeherung additiv,
d.h. man kann diese
Tabelle auch zur
Abschaetzung von
Verschiebungen in di/tri
substituierten Benzolen
verwenden.
Aber: Bei sterischen
Wechselwirkungen
(grosse Substituenten
in o-Position geht die
Additivitaet verloren,
und die errechneten
Shifts werden ungenau.
Diese oder aehnliche Inkrementrechnungen fuer Alkene, Aromaten usw.
finden in jedem Lehrbuch z.B. Hesse Meier Zeeh
13C-Inkrement-Rechnung
• Beispiel Aromat
• Siehe Material auf
der Website
Und
:
• Benutzen Sie die Inkrementenregeln
• INSBESONDERE zur Bestimmung der Kohlenstoffverschiebungen!
• Tabellen im Hesse Meier Zeeh
• Oder auf der Website =Informationsunterlagen zu
diesem Kurs (NMR_shifts.pdf)
Carbon-13 NMR
12C
13C
1H
I = 0 (sum of protons and neutrons is even) NMR inactive
I = 1/2
I = 1/2
13C
is less sensitive than 1H primarily due to two factors
1. 13C has a low natural abundance (1.1 %)
2. the gyromagnetic ratio (γ) of 13C is 1/4 that of 1H
sensitivity = γ5/2 ; (1/4)5/2 = 0.03 less sensitive
Thus, lower natural abundance and smaller magnetogyric ratio
lowers sensitivity to ~ 1:33 for 13C : 1H
To get the same S/N as proton we would have to increase the number of
scans by a factor of 332 = 1100 !!!
Signal-to-noise ratio /Signal-Rausch Verhaeltniss
13C {1H}
¾
13C
NMR
spectra are generally acquired with proton decoupling.
¾ Irradiation on 1H leads to rapid interconversion of the spin states of
the proton, so that averaged over time the effect of the coupling will
be removed. Thus, the 13C multiplett collapses into a single line.
¾ The decoupling may lead to signal enhancement by a dipolar
interaction between carbon and its attached proton. This interaction
is termed NOE (nuclear Overhauser enhancement) , is distance
dependent, and can lead to intensity increases up to 3 fold .
¾ This signal enhancement makes it impossible to quantify carbon
atoms by integration
¾ Typical chemical shift range : 0 – 230 ppm
¾
13C- 13C
coupling is (generally) not observed due to the low natural
abundance
Coupling in 13C-NMR spectra
1D-13C NMR : broadband decoupled
(0-10 ppm)
Increase sensitivity
due to NOE
Decouples resonances
to singulett.
BB - Fully 1H Decoupled
FID (free induction decay)
signals which are detected
13C
Observe with Gated 1H Coupling
Intensity enhancement due to NOE during irradiation time (note: T >> at)
Signal multiplicity (1JCH coupling retained)
DEPT
¾ Polarizationtransfer (PT) The population difference/sensitivity of 1H is
transferred to 13C
¾ Only CH, CH2 and CH3 groups will be detected (Cq will be missing)
¾ The duration of the last proton pulse width determines the intensity and sign
of the resonances
¾ Unambiguous assignment of CH, CH2, and CH3 by comparison of DEPT 90
and DEPT 135
pulse width of last H-pulse
The combination of
• a cpd/broadband decoupled 13C-NMR experiment
• and the DEPT135 experiment
are the most powerful way, to determine both the frequency
of all carbon atoms , and their multiplicity
in two simple, fast, and relatively sensitive 1-dimensional
NMR experiments.
Carbon chemical shift range
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Very large, leading to
Easy group identification
Very sharp lines since J-couplings are small compared to δ-range
No/rarely overlap
One (dominant) coupling = 1J C-H 120 – 160 Hz (can be up to 320 Hz)
Splits signal -> lowers signal to noise
Multiplicity provides invaluable information on number of directly attached protons
13C
¾
Chemical Shifts
Hybridization
150-100 ppm (sp2),
90-60 ppm (sp), 55-10 ppm (sp3)
¾
Substituent effects are additive for alkanes, alkenes, and
aromatics; many empirical additivity rules exist for alkanes,
alkenes, cyloalkanes, etc.
¾
Magnetic anisotropy and ring current effects, which are
important in 1H NMR, are not important in 13C NMR (usually < 2
ppm)
Many formulas to calculate chemical shifts can be found in
textbooks!!
1H
and 13C chemical shifts are largely
correlated