NMR Spektroskopie Teil 1: Wiederholung Was mir wichtig ist: 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 Abundance [%] γ [10 7 rad /Ts] 1 H ½ 99.98 26.7519 100 2 H 1 0.016 4.1 15.351 12 C 0 98.9 13 C ½ 1.1 6.728 25.144 14 N 1 99.63 1.938 7.224 15 N ½ 0.37-2.712 10.133 16 0 0 99.96 31 P ½ 100 10.841 40.481 NMR frequency (Bo=2.4T) Empfindlichkeit und Resonanzfrequenz sind proportional zu γ 1) Magnetische Feldstaerke oft in MHz ( 1 H) genannt (anstatt Tesla) 1.41T=60 MHz, 14 T =600MHz, 21T = 900 MHz (derzeit maximum) 2) 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 Energieniveau- Populationen ΔE = γħb 0 = 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:= N β /N α = 0.9999904 (or 9.6 x 10-6 excess spins in α) At 600 MHz:= N β /N α = 0.9999013 (or 98.7 x 10-6 excess spins) NMR ist eine sehr unempfindliche Spektroskopie!
Μ 0 = makroskopische Magnetisierung
Transversale Magnetisierung koennen wir detektieren!
Der FID fast induction decay Precession Relaxation = Ueberlagerung T2 Spin-Spin Relaxation T1 Spin-Gitter Relaxation
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.
1 H-NMR von mono-substitutierten Aromaten Mesomerieeffekte bewirken starke positions-abhaengige Verschiebungen im Aromaten
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
13 C-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 12 C I = 0 (sum of protons and neutrons is even) NMR inactive 13 C I = 1/2 1 H I = 1/2 13 C is less sensitive than 1 H primarily due to two factors 1. 13 C has a low natural abundance (1.1 %) 2. the gyromagnetic ratio (γ) of 13 C is 1/4 that of 1 H sensitivity = γ 5/2 ; (1/4) 5/2 = 0.03 less sensitive Thus, lower natural abundance and smaller magnetogyric ratio lowers sensitivity to ~ 1:33 for 13 C : 1 H To get the same S/N as proton we would have to increase the number of scans by a factor of 33 2 = 1100!!!
Signal-to-noise ratio /Signal-Rausch Verhaeltniss
13 C { 1 H} NMR 13 C spectra are generally acquired with proton decoupling. Irradiation on 1 H 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 13 C multiplett collapses into a single line. 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. This signal enhancement makes it impossible to quantify carbon atoms by integration Typical chemical shift range : 0 230 ppm 13 C- 13 C coupling is (generally) not observed due to the low natural abundance
Coupling in 13 C-NMR spectra
1D- 13 C NMR : broadband decoupled (0-10 ppm) Increase sensitivity due to NOE Decouples resonances to singulett. FID (free induction decay) signals which are detected BB - Fully 1 H Decoupled
13 C Observe with Gated 1 H Coupling Intensity enhancement due to NOE during irradiation time (note: T >> at) Signal multiplicity ( 1 J CH coupling retained)
DEPT Polarizationtransfer (PT) The population difference/sensitivity of 1 H is transferred to 13 C Only CH, CH 2 and CH 3 groups will be detected (Cq will be missing) The duration of the last proton pulse width determines the intensity and sign of the resonances Unambiguous assignment of CH, CH 2, and CH 3 by comparison of DEPT 90 and DEPT 135
pulse width of last H-pulse
The combination of a cpd/broadband decoupled 13 C-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 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 = 1 J 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
13 C Chemical Shifts Hybridization 150-100 ppm (sp 2 ), 90-60 ppm (sp), 55-10 ppm (sp 3 ) Substituent effects are additive for alkanes, alkenes, and aromatics; many empirical additivity rules exist for alkanes, alkenes, cyloalkanes, etc. Magnetic anisotropy and ring current effects, which are important in 1 H NMR, are not important in 13 C NMR (usually < 2 ppm) Many formulas to calculate chemical shifts can be found in textbooks!!
1 H and 13 C chemical shifts are largely correlated