NMR-Spektroskopie Teil 3: Spin-Spin Kopplung 1. Die skalare Spin-Spin Kopplung 2. Multiplizitaet und Intensitaetsverteilung 3. Geminale, vicinale und allylische Kopplungskonstanten 4. Anschauliche Beschreibung der Kopplung
Kopplungs-Mechanismus X A R CH CH R' A NH 2 OH X possible compound Lowest energy (both nuclei are in the same direction as B 0 )
Spin-Spin Kopplung Chemisch aequivalente Kerne koppeln nicht Chemisch nicht-aequivalente Spins koppeln Nur benachbarte Spins fuehren zu grossen Kopplungen Geminale und vicinale Kerne 2,3 J (H,H) >5 Hz Kopplungen ueber Heteratome sind sehr klein Kopplungen ueber Kohlenstoffatome, die kein H tragen, sind sehr klein, und daher auch selten zu beobachten Beachte : die freie Rotation fuehrt zum Mitteln der Kopplungskonstanten/ zu einem Mittelwert von ~6.5 Hz in alkylketten.
Singulett Dublett (Doublett) Triplett Quartett (Quadruplett) Quintett (Pentuplett) Sextett Septett (Septuplett)
Multiple couplings of different size a splitting diagram 1H 3H 1H The coupling constants for this first-order multiplet are doublets of 4 and 12 Hz, and 2 quartets of 7 Hz. The horizontal scale is 10 mm = 5 Hz. Yield information on number of coupled protons Allows to connect coupling partners- as their J will be identical
Notes on Coupling Constants J ist unabhaengig von der Magnetfeldstaerke Die Groesse der Kopplungskonstante haengt ab von : der Elektronendichte am Kern, der Zahlr der Bindungen zwischen den Kerne, und dem Winkel der Bindungsvektoren. J ist positive eine ungerade Zahl von Bindungen, und negative eine gerade Zahl von Bindungen.. (In Spektren, messen wir jedoch nicht das Vorzeichen, sondern nur den Betrag der Kopplung. )
Der Diederwinkel / die Newmanprojektion
In acyclic molecules, rotation around single bonds averages out these differences
1 H NMR eines Zucker Derivates
H > H H H Typically, 3 J trans is 13-18 Hz and 3 J cis is 7-9 Hz J trans is always larger than J cis and corresponds to vicinal dihedral angles of 180 and 0, respectively H A H C H B H D H E H F AB 1.74 AD 0.86 AC 10.17 AF 1.30 CD 10.41 BE 0.89 BC 17.05 AE 0.80 BD 0.83 H B H C H A H D AB 9.42 BC 5.14 AC 1.06 AD 0.91
Long Range Coupling (greater than 3 bonds) Typically found in alkenes, alkynes, aromatics, heteroaromatics, and in strained ring systems Usually 4 J 0-3 Hz Remember rule: J is positive when across odd number of bonds and negative when across an even number of bonds Allylic coupling 4 J allylic = 0 at Θ = 0 and 180, and largest at 90 (σ / π interaction) Usually J cis > J trans H CH 3 H CH 3 H CCH 3 O J cis 1.3 J trans 0.7 H Br J cis 1.4 J trans 0.8 sign of 4 J allylic dependent upon π order of C=C
Multiplettconstructions
Spin-Spin Coupling of 13 C 13 C-H 1 J CH - usually range from 110 to 320 Hz, and this is affected by s and p character of C-H bond (the greater the s character, the greater the J value sp 3 25% ~ 125 Hz sp 2 33% ~ 165 Hz Hybridization 5 x (%s) Hz sp 50% ~ 250 Hz
13 C-C-H 2 J CH - range -5 to 60 Hz Generally much smaller than the direct 1 J CH coupling! Typically only 3 to 6 Hz
13 C-C-C-H 3 J CH - usually are slightly larger than 2 J CH Again, the 3 J coupling is strongly dihedral angle dependent. This dependency can be described by another, specifically parametrized Karplus equation. Range : 0 to 12 Hz, seldom larger There are also 4 J CH and 5 J CH but these are generally very small
How many signals? ->Chemical equivalence In this example, all methyl-protons, and all methylenprotons, respectively, are (chemically) equivalent, and give rise to one signal each
Chemical equivalence Nuclei in identical environments are chemically equivalent Two nuclei in a molecule are chemically equivalent if they can be converted into another by a symmetry operation Two nuclei are chemically equivalent if they can be interconverted by a rapid motion Spinsystem : the group of all connected (coupled) spins
Chemical equivalence of methylene protons Chiral center Identical molecules Enantiomers Diastereomers
Which protons are chemically equivalent, which are not? f j are examples that a chiral center is not needed to create diastereotopic protons.
Pople notation Spin systems notations specifies all coupled protons Protons with same chemical shifts are placed into sets A, B, C - used for protons where the chemical shifts are close Δν J ~10 A, M, X - used for protons where the chemical shifts are well separate Δν J >10 A a X x the number of protons in each set AX (first order) AB (second order) Chemical equivalent nuclei: same letter A2 Magnetic in-equivalent : (same letter, prime = AA )
AX (first order) Only first order spectra allow the easy determination of J-values and chemical shifts (top example) For first order spectra, the chemical shift of coupled protons needs to be large. Δν > 10 x J Second order spectra show J, but the lines are not centered around ν. Note: J does not scale with Bo, but Δν does. Thus, moving to higher magnetic fields changes higly coupled systems towards first order! AB (second order)
Spektren gestrichener Spinsysteme
Repeat coupling and equivalence NMR active nuclei couple via bonds Coupling leads to peak splitting into multipletts The J coupling constant is Bo-field independent n J depends on number of intervening bonds, steric factors and electronic effects Chemically equivalent nuclei do not couple The collection of coupled nuclei is called spin system In coupled systems higher order spectra are observed if the chemical shift difference in Hz is of similar size as the coupling constant 1 J >> 3 J > 2 J >> 4 J