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1. 1. H. H. Coupling Constants (J). Coupling constants are a very important and useful feature of an NMR spectrum Importantly, coupling constants identifies pairs of nuclei that are chemically bonded to each other - PowerPoint PPT Presentation
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Coupling Constants (J)Coupling Constants (J)
• Coupling constants are a very important and useful feature of an NMR spectrum
• Importantly, coupling constants identifies pairs of nuclei that are chemically bonded to each other
• Multiplicity identifies the number of protons (or other nuclei) that are chemical bonded to the other nuclei
• The magnitude of the coupling constants identifies the coupling partner, and
provides information on dihedral angles, hydrogen bonds, the number of intervening bonds, and the type of coupled nuclei (1H, 13C, 15N, 19F, etc.)
1H 1H
Coupling Constants (J)Coupling Constants (J)
BBoo
-- spin-spin coupling, scalar coupling or spin-spin coupling, scalar coupling or J-couplingJ-coupling
Random tumbling of molecules averages through-space effect of nuclear magnets to zero
BBoo
random tumbling leads to no interaction between the spin-states despite the small
magnetic fields
Coupling Constants (J)Coupling Constants (J)
-- spin-spin coupling, scalar coupling or spin-spin coupling, scalar coupling or J-couplingJ-coupling
Instead, nuclear spin state is communicated through bonding electrons
Energy of electron spin states are Energy of electron spin states are
degenerate in absence of nuclear spindegenerate in absence of nuclear spin
With a nuclear spin, the electron spin With a nuclear spin, the electron spin
opposite to nuclear spin is lower energyopposite to nuclear spin is lower energy
Number of possible energy states of nuclear-Number of possible energy states of nuclear-electron spin pairs increases with the electron spin pairs increases with the
number of nuclear spinsnumber of nuclear spins
Spin state is “sensed” through bonds resulting in higher or lower energy
- aligned or anti-aligned with magnetic field
Coupling ConstantsCoupling Constants
Energy level of a nuclei are affected by covalently-bonded neighbors spin-states
Spin System One Spin System Two
Mixing of Spin Systems One and
Two
Coupling ConstantsCoupling Constants
Mixing of energy levels results in additional transitions – peaks are split
I S
J (Hz)
Spin-States of covalently-bonded nuclei want to be aligned
The magnitude of the separation is called coupling constant (J) and has units of Hz
S
S
I
I
+J/4
-J/4
+J/4
J (Hz)
• Through-bond interaction that results in the splitting of a single peak into multiple peaks of various intensities Spacing in hertz (hz) between the peaks is a constant Independent of magnetic field strength
• Multiple coupling interactions may exist Increase complexity of splitting pattern
• Coupling can range from one-bond to five-bond One, two and three bond coupling are most common Longer range coupling usually occur through aromatic systems
• Coupling can be between heteronuclear and homonuclear spin pairs Both nuclei need to be NMR active i.e. 12C does not cause splitting
Coupling ConstantsCoupling Constants
13C
1H1H 1H
one-bond
three-bond
1H
1H
1H
four-bond
five-bond
Coupling ConstantsCoupling Constants
• Splitting pattern depends on the number of equivalent atoms bonded to the nuclei Determines the number of possible spin-pair combinations and energy levels Each peak intensity in the splitting pattern is determined by the number of spin
pairs of equivalent energy
Coupling ConstantsCoupling Constants
Pascal’s triangle
• Splitting pattern follows Pascal’s triangle Number of peaks and relative peak intensity determined by the number of
attached nuclei Peak separation determined by coupling constant (J) Negative coupling reverse relative energy levels
11 1
1 2 11 3 3 1
1 4 6 4 11 5 10 10 5 1
1 6 15 20 15 6 11 7 21 35 35 21 7 1
3 attached nuclei
Quartet
1 1
3 3
Relative Intensity
JJ
J
singlet doublet triplet quartet pentet 1:1 1:2:1 1:3:3:1 1:4:6:4:1
Common NMR Splitting Patterns
Coupling Rules:1. equivalent nuclei do not interact2. coupling constants decreases with separation ( typically 3 bonds)3. multiplicity given by number of attached equivalent protons (n+1)4. multiple spin systems multiplicity (na+1)(nb+1) 5. Relative peak heights/area follows Pascal’s triangle6. Coupling constant are independent of applied field strength7. Coupling constants can be negative
IMPORTANT: Coupling constant pattern allow for the identification of bonded nuclei.
Coupling ConstantsCoupling Constants
Coupling ConstantsCoupling Constants
Common NMR Splitting Patterns
Coupling ConstantsCoupling Constants
• Coupling only occurs between non-equivalent nuclei Chemical shift equivalence Magnetic equivalence For no coupling to occur, nuclei has to be BOTH chemical shift and magnetic
equivalent
c
H1
H3
H2
Cl
Ha
Hb
The CH3 protons (H1, H2, H3) are in identical environments, are equivalent, and are
not coupled to one another
The Ha and Hb protons are in different environments (proximity to Cl), are
not equivalent, and are coupled
Coupling ConstantsCoupling Constants
Rules for Chemical Shift Equivalence:• Nuclei are interchangeable by symmetry operation
i. Rotation about symmetric axis (Cn)
ii. Inversion at a center of symmetry (i)
iii. reflection at a plane of symmetry ()
iv. Higher orders of rotation about an axis followed by reflection in a plane normal to this axis (Sn)
v. Symmetry element (axis, center or plane) must be symmetry element for entire molecule
C C C
Ha
Ha
Ha
Hb
Hb
Ha
Ha
Ha
Ha
Ha
Ha
Ha
Ha
Ha
Hc
Ha
Hc
Cl
Hb
Cl
Hc
Hb
Hc
Cl
Ha
Cl
180o
Symmetry planes
Examples of Chemical Shift Equivalent Nuclei
Coupling ConstantsCoupling ConstantsRules for Chemical Shift Equivalence:• Nuclei are interchangeable by a rapid process
i. > once in about 10-3 seconds
ii. Rotation about a bond, interconversion of ring pucker, etc.
Ha
Ha
Ha
Ha
Rapid
exchange
Rapid
exchange
NH2
O
HO H H
H2H2
NH2
O
HO
H H
H2H2
Examples of Chemical Shift Equivalent Nuclei
Coupling ConstantsCoupling ConstantsMagnetic Equivalence:• Nuclei must first be chemical shift equivalent
• Must couple equally to each nucleus in every other set of chemically equivalent nuclei
i. need to examine geometrical relationships
ii. the bond distance and angles from each nucleus to another chemical set must be identical
iii. Nuclei can be interchanged through a reflection plane passing through the nuclei from the other chemical set and a perpendicular to a line joining the chemical shift equivalent nuclei
Examples of Non-magnetically equivalent nuclei
Ha
Cl
Cl
Ha'
Hb'
Hb
Chemical shift equivalent, but not magnetic equivalent
C C
Fa
Fa'
Ha
Ha'
C C
Fa
Ha
Fa'
Ha'
C C
Fa
Ha
Ha'
Fa'3Jab ≠ 3Ja’b
3Jab’ ≠ 3Ja’b’
3JHaFa ≠ 3JHa’Fa
3JHaFa’ ≠ 3JHa’Fa’
Hc
Hb
Hc'
Cl
Ha
Cl
3JHaHc ≠ 3JHaHc’
3JHbHc ≠ 3JHbHc’
3JHaHc ≠ 3JHbHc
3JHaHc’ ≠ 3JHbHc’
Coupling ConstantsCoupling ConstantsMagnetic Equivalence:• Non-magnetically equivalent nuclei may lead to second order effects and very complex
splitting patterns
• Second order effects will be discussed later
i. Due to small chemical shift differences between coupled nuclei ( ~ J)
http://www.chem.wisc.edu/areas/reich/chem605/index.htm
Coupling ConstantsCoupling Constants
Multiple Spin Systemsmultiplicity (na+1)(nb+1)
C C C
Cl
Cl
Hb
H
H
Ha
Ha
HaWhat is the splitting pattern for CH2?
3JHb = 6 Hz
3JHa = 7 Hz
11 1
1 2 11 3 3 1
1 4 6 4 11 5 10 10 5 1
1 6 15 20 15 6 11 7 21 35 35 21 7 1
Coupling to Hb splits the CH2 resonance into a doublet separated by 6 Hz
3JHb = 6 Hz
Coupling to Ha splits each doublet into a quartet separated by 7 Hz
Down-field resonance Down-field resonance split into quartetsplit into quartet
up-field resonance up-field resonance split into quartetsplit into quartet
Coupling ConstantsCoupling Constants
What Happens to Splitting Pattern if J changes?
3JHb = 7 Hz
3JHa = 7 HzLooks like a pentet!
3JHb = 6 Hz
3JHa = 3 Hz
Looks like a sextet!
Occurs because of overlap of peaks within the splitting pattern
Intensities don’t follow Pascal’s triangle (1 4 6 4 1)
Intensities don’t follow Pascal’s triangle (1 5 10 10 5 1)
Coupling ConstantsCoupling Constants
Coupling Constants Provide Connectivity Information– chemical shifts identify what functional groups are present
C C C
Cl
Cl
Hb
H
H
Ha
Ha
HaNMR Peaks for coupled nuclei share the same coupling constants
CH2
CH3
CH
6 Hz 6 Hz 6 Hz
7 Hz 7 Hz
7 Hz6 Hz 7 Hz
Integral: 1 2 3
Coupling ConstantsCoupling Constants
Deconvoluting a spin system– determining the J-values– determining the multiplicities present
J coupling analysis:i. Is the pattern symmetric about the center?ii. Assign integral intensity to each line, outer lines assigned
to 1iii. Are the intensities symmetric about the center?iv. Add up the assigned intensities
– Sum must be 2n, n = number of nuclei– Ex: sum = 16, n = 4
v. Separation of outer most lines is a coupling constant– Relative intensity determines the number of coupled
nuclei– Ex: intensity ratio: 1:2, 2 coupled nuclei– 1st splitting pattern is a triplet (1:2:1)
vi. Draw the first coupling patternvii. Account for all the peaks in the spin pattern by repeatedly
matching the 1st splitting patternviii. Smallest coupling constant has been assigned
Coupling ConstantsCoupling Constants
Deconvoluting a spin system– determining the J-values– determining the multiplicities present
J coupling analysis:
ix. Coupling pattern is reduced to the center lines of the 1st splitting pattern.
x. Repeat process
– Ex: sum = 8, n = 3
– Ex: intensity ratio: 1:1, 1 coupled nuclei
– 2nd splitting pattern is a doublet (1:1)
xi. Repeat until singlet is generated
Coupling ConstantsCoupling Constants
Demo ACD C+H NMR Viewer software– first order coupling constants
CH3CH2FCH3CH2R
Coupling ConstantsCoupling Constants
Description of Spin System
– each unique set of spins is assigned a letter from the alphabet the total number of nuclei in the set are indicated as a subscript
– the relative chemical shift difference is represented by separation in the alphabet sequence
Large chemical shift differences are represented by AX or AMX (AX >> JAX)
Small chemical shift differences are represented by AB (AB < 5JAB)
Can also have mixed systems: ABX magnetically in-equivalent nuclei are differentiated by a single quote: AA’XX’ or brackets [AX]2
CH2ClCHCl2
A2X system A2M2X systemA3X2 system
Ha
Cl
Cl
Ha'
Hx'
Hx
[AX]2 or AA’XX’ system
H
H CN
Cl
AB system
O
styreneoxide
HA
HMHX
A M X
A M XTMS
A M X
J(AM)
J(AX)J(AX)
J(MX)
J(AM) J(AM)
J(MX)
J(AX) J(AX)
J(AM) = 4 Hz
J(AX) = 2.5 Hz
J(MX) = 6 Hz
Observed splitting is a result of this electron-nucleus hyperfine interaction• Coupling is measured in hertz (Hz)
Range from 0.05 Hz to thousands of Hz Can be positive or negative
o 1JC-H and many other one-bond coupling are positiveo 1JA-X is negative if are opposite sign o 2JH-H in sp3 CH2 groups are commonly negativeo 3JH-H is always positive
Coupling Constants (J)Coupling Constants (J)
For an AX system, JAX is negative if the energy of the A state is lower when X has the same spin as A ( or )
The spin states and transitions are swapped
reversed
reversed
reversed
Coupling Constants (J)Coupling Constants (J)
Measure the Relative Sign of Coupling Constants• Multiple experimental approaches (different NMR pulse sequences) or
simulations
E. COSY – two-dimensional NMR experiment
cross peaks identify which chemical shifts are coupled
Coupling Constants (J)Coupling Constants (J)
Measure the Relative Sign of Coupling Constants• The cross-peak patterns identifies the coupling constant sign and
magnitude
Yellow-highlighted regions are expanded
Based on the slopes of the diagonal line drawn through coupling pattern
3JAX and 3JBX have the same sign
3JAB opposite sign of 3JAX and 3JBX
Coupling Constants (J)Coupling Constants (J)
• Magnitude of the splitting is dependent on: Number of bonds
Bond order (single, double triple)
Angles between bonds
H3C CH3 H2C CH2
3JHH 8 Hz 3JHH 11.6 & 19.1 Hz
H
X
Y
H
H
X
H
Y
trans 3JHH ~ 17 Hz cis 3JHH ~10 Hz
HC CH
3JHH 9.1 Hz
X
Y
H
H
geminal 2JHH ~2.5 Hz
HA
HB HC
HD
3JAB 9.4 Hz4JAC 1.1 Hz5JAB 0.9 Hz
Coupling Constants (J)Coupling Constants (J)
• Magnitude of the splitting is dependent on: dihedral angle
− Fixed or average conformation
Coupling Constants (J)Coupling Constants (J)
• Magnitude of the splitting is dependent on: Cyclohexanes dihedral angles
− Fixed or average conformation
3Jaa 9-12 Hz3Jee or 3Jea 3-4 Hz
3Jaa >> 3Jee,3Jea
Dual Karplus curves for the axial and equatorial protons
• Magnitude of the splitting is dependent on: Cyclohexanes dihedral angles
− examples
Coupling Constants (J)Coupling Constants (J)
Coupling Constants (J)Coupling Constants (J)
• Magnitude of the splitting is dependent on: Cyclopentanes dihedral angles
− Fixed or average conformation
Coupling Constants (J)Coupling Constants (J)
• Magnitude of the splitting is dependent on: Comparison between Cyclohexanes and Cyclopentanes
Because of range of cyclopentane conformations, vicinal couplings are variable: Jcis > Jtrans and Jcis > Jtrans
Only in rigid cyclopentanes can a stereochemistry be defined: Jcis > Jtrans
In chair cyclohexane, only one vicinal
coupling can be large (>7 Hz)
In cyclopentane, two or three vicinal
coupling can be large (>7 Hz)
Coupling Constants (J)Coupling Constants (J)
• Magnitude of the splitting is dependent on: Cyclobutanes are flatter than cyclopentanes, so: Jcis > Jtrans
− unless structure features induce strong puckering of the ring or electronegative substituents are present
Cyclopropanes are rigidly fixed, so Jcis > Jtrans is always true
Coupling Constants (J)Coupling Constants (J)
• Magnitude of the splitting is dependent on: Orientation
− unless structure features induce strong puckering of the ring or electronegative substituents are present
− Internal hydrogen bonds may lead to constrained conformations and distinct different coupling constants
Since methyl groups can freely rotate, the observed coupling is the average of the three individual coupling constants
Coupling Constants (J)Coupling Constants (J)
Magnitude of the splitting is dependent on: Electronegativity of Substituents
3JH-H coupling constant decreases as electronegativity increases
3JH-H decreases even more with two electronegative substituents
Coupling Constants (J)Coupling Constants (J)
3JH-H coupling constant decreases as electronegativity of substituents
increases for cycloalkenes
3JH-H coupling constant decreases as electronegativity of substituents
increases for alkenes
Magnitude of the splitting is dependent on: Electronegativity of Substituents
Coupling Constants (J)Coupling Constants (J)
Magnitude of the splitting is dependent on: Ring Size
− Coupling constants decrease as ring size gets smaller
− Coupling constants also decrease as ring is formed and gets smaller
Coupling Constants (J)Coupling Constants (J)
Magnitude of the splitting is dependent on: Bond order
− Coupling constant decreases as bond order decreases
Heterocycles
– Heterocycles have smaller coupling constants compared to hydrocarbons systems
3JH-H = 8.65 x (n bond order) + 1.66
• Magnitude of the splitting is dependent on: Proportional to ab
s character of bonding orbital
– Increases with increasing s-character in C-H bond
Coupling Constants (J)Coupling Constants (J)
H3C CH31JC-H 125 Hz
H3C NH21JN-H 95 Hz Br
F
Cl
F
H
F
2JF-H 48.2 Hz
• Magnitude of the splitting is dependent on: Attenuated as the number of bonds increase
– Usually requires conjugated systems (aromatic, allylic, propargylic, allenic) or favorable geometric alignment (W-coupling)
– Not usually seen over more than 4 to 5 bonds (acetylenes and allenes)
Coupling Constants (J)Coupling Constants (J)
Coupling Constants (J)Coupling Constants (J)
Magnitude of the splitting is dependent on: Geminal protons (H-C-H) fall into two major groups
– Unstrained sp3 CH2 protons: 2JH-H -12 Hz
– Vinyl sp2 CH protons: 2JH-H 2 Hz
Coupling Constants (J)Coupling Constants (J)
Magnitude of the splitting is dependent on: Geminal protons coupling constants are effected by the electronic effects of
substituents
– Based on the interaction between the filled and empty orbitals of the CH2 fragment
Note: opposite trend
Coupling Constants (J)Coupling Constants (J)
Magnitude of the splitting is dependent on electronic effects: In acyclic and unstrained ring systems: 2JH-H ~ -10 to -13 Hz
When CH2 is substituted with a -acceptor, like carbonyl or cyano coupling becomes more negative: 2JH-H ~ -16 to -25 Hz
− Reliable and can help with structure assignments
Conjugated aryl, alkene and alkyne substituents also makes coupling becomes more negative
Coupling Constants (J)Coupling Constants (J)
Magnitude of the splitting is dependent on electronic effects: In unsaturated carbons: 2JH-H ~ 2.5 Hz
Electronegative substituents (F,O) behave as -acceptors with a negative effect with 2JH-H close to zero
Electropositive substituents (Si, Li) behave as -donors with a negative effect with 2JH-H
Oxygen substituents can behave as a strong -acceptor and strong -donor (lone pair), both positive effects leading to a large 2JH-H or as a strong p-acceptor leading to large negative coupling
Coupling Constants (J)Coupling Constants (J)Magnitude of the splitting is dependent on electronic effects:
Summary of effects, and acceptors have opposite effects on coupling, as do and donors
Coupling Constants (J)Coupling Constants (J)
Coupling Constants (J)Coupling Constants (J)
Coupling Constants (J)Coupling Constants (J)
Coupling ConstantsCoupling Constants
Weak coupling or first-order approximation• Up to now, we have assumed the frequency difference (chemical shift) between the
coupled nuclei is large
i. >> J
• Second order effects come into play when this assumption is no longer valid
i. < 5J
• Second order effects lead to very complex splitting patterns that are difficult, if not impossible to interpret manually and leads to incorrect chemical shifts and coupling constants
• Interpreting NMR spectra with second-order effects usually requires software
Coupling Constants (J)Coupling Constants (J)
Second-Order Effects (Strong Coupling)
– occurs when chemical shift differences is similar in magnitude to coupling constants (/J < 5)
chemical shifts and coupling constants have similar energy and intermingle results from mixing of the equivalent and spin states
none of the transitions are purely one nuclei described by quantum mechanical wave functions
AB spin system
Coupling Constants (J)Coupling Constants (J)
Second-Order Effects (Strong Coupling) perturbs peak intensity and position
as chemical shift differences decrease, intensity of outer lines become weaker and internal lines become stronger the multiplet leans towards each other (“roof” effect) which increases as chemical shift difference decreases
AB spin system
Second-Order Effects (Strong Coupling) becomes easier to interpret at higher magnetic field strengths
Coupling Constants (J)Coupling Constants (J)
Higher field increases /J
Coupling Constants (J)Coupling Constants (J)
Second-Order Effects (Strong Coupling)
– hierarchy of coupling constants with increasing second-order effects
1. AX and all other first order systems (AX2, AMX, A3X2, etc.)
2. AB
i. Line intensities start to lean
ii. J can be measured, can be calculated
3. AB2
i. Extra lines
ii. Both J and have to be calculated
4. ABX, ABX2, ABX3
i. JAB can be measured, everything else requires calculation
5. ABC
i. Both J and have to be determined from computer simulation
6. AA’XX’
i. Do not become first order even at high magnetic fields
ii. Both J and have to be determined from computer simulation
7. AA’BB’
8. AA’BB’X
9. Etc.
Coupling Constants (J)Coupling Constants (J)
Second-Order Effects (Strong Coupling)
– general effect of strong couplings on NMR spectra
1. Line intensities are no longer integral ratios, no longer follow Pascal’s triangle
2. Line positions are no longer symmetrically related to chemical shift position
i. Multiplet center may no longer be chemical shift (AB and higher)
3. Some or all coupling constants can no longer be obtained from the line separations (ABX and higher)
4. The signs of coupling constants affect the line positions and intensities (ABX and higher)
5. Additional lines over the number predicted by simple coupling rules appear
i. Peaks with intensities of 2 or more are split into individual components
More lines then the expected triplet for the boxed CH2 pair
Coupling Constants (J)Coupling Constants (J)
Second-Order Effects (Strong Coupling)
– general effect of strong couplings on NMR spectra
6. Coupling between equivalent nuclei (JAA’ or JXX’) affects line count and position
i. Second order effects appear even if /J is large for groups of magnetically non-equivalent protons with identical chemical shifts which are coupled
i. Do not get simpler at higher fields
7. Computer analysis becomes mandatory to extract accurate J and values (ABC and higher)
8. Ultimately spectra become so complex that the only useful information is integration, chemical shift and general appearance.
Coupling Constants (J)Coupling Constants (J)
Second-Order Effects
– as the chemical shifts coalesce intensity of outer lines decrease inner peaks eventually collapse to singlet nuclei become chemically and magnetically equivalent
AB spin system
May be misinterpreted as a quartet
Weaker outer lines may be overlooked and interpreted as a doublet
Coupling Constants (J)Coupling Constants (J)
Second-Order Effects (AB)
– analysis of second-order splitting patterns remember: resonance positions are also perturbed separation between outer lines and inner lines (a-b, c-d) yields coupling constant
JAB = (a-b) = (c-d)
true chemical shift is not the doublet centers center = ½(b+c)
AB = √ (a-d) (b-c)
A = center + ½AB
B = center - ½AB
A B
Coupling Constants (J)Coupling Constants (J)
Coupling Constants (J)Coupling Constants (J)
Second-Order Effects (AB2)
– as the chemical shifts coalesce
line intensities no longer follow simple rules
arithmetic average of the line positions no longer give true chemical shifts
JAB can still be measured directly from spectrum
none of the line separation correspond to JAB
additional lines appear
AB2 spin system
Note: splitting of intense lines
Coupling Constants (J)Coupling Constants (J)
Second-Order Effects (AB2)
– four A lines 1 – 4 and four B lines 5 – 8 and the very weak combination line 9
– calculation of A, B, and JAB is simple:
– how to report an AB2 spin system in a journal manuscript:
report the two chemical shifts as an AB2 multiplete (m):
2.63, 2.69 (AB2m, 3H, JAB = 12.2 Hz)
Coupling Constants (J)Coupling Constants (J)
Second-Order Effects (AB2)
– unique features of second-order splitting pattern for AB2 system
Spectrum depends only on the ratio /J
lines 1 to 4 correspond to the one proton part (A)
lines 5 to 8 correspond to the two-proton part (B2)
line 5 (5) is the most intense line
lines 5 and 6 often do not split up
when /J << 1, the spectrum appears nearly symmetrical
lines 1,2, 8 1, 2 ,8) become very weak
looks like a distorted triplet with 1:10:1 area ratio
JAB and JBB do not affect the spectrum
Second-Order Effects (ABX)
– most complex spin-system that can still be manually analyzed
– ABX has a common appearance AB – unsymmetrical 8-line pattern that integrates to 2 protons AB – 4 doublets with the same separation JAB with strong leaning
X – symmetric 6-line pattern that integrates to 1 proton X – 5th and 6th lines are small and not often seen, apparent doublet of doublet JAB and X are directly measurable from spectrum
JAX, JBX, A and B need to be calculated
Coupling Constants (J)Coupling Constants (J)
XAB
X - center of peaks
Second-Order Effects (ABX)
– Many ABX patterns are sufficiently close to AMX (AB >> JAB)
first-order solution has an excellent chance of being correct
– First, identify the distorted doublet of doublets for both A and B
– Remove the splitting (identify the center of each doublet), which leaves an AB pattern
– Solve AB pattern as before to get JAB, A, and B
large errors when JAX and JBX are very different or AB small compared to JAB
Coupling Constants (J)Coupling Constants (J)
A & B doublet of doubletseparation is JAX & JBX
Center doublets and get AB pattern
Second-Order Effects (ABX)
– Correct analysis of ABX patterns Reverse the order of extracting coupling constants to approximate solution
– First, identify the two AB quartets separation between the four pairs of lines are identical tall inner line associated with shorter outer line (leaning)
Coupling Constants (J)Coupling Constants (J)
Identify the two AB quartetsJab+ = Jab-
Second-Order Effects (ABX)
– Correct choice of ab quartet
– Incorrect choice of ab quartet
Coupling Constants (J)Coupling Constants (J)
Second-Order Effects (ABX)
– Solve the two ab quartets
Treat as normal AB patterns and obtain four chemical shifts (a+,b+,a-,b-)
Don’t know which half is a and which is b - two possible solutions
Coupling Constants (J)Coupling Constants (J)
Second-Order Effects (ABX)
– Solution 1 and Solution 2 – depends on the relative sign of JAX and JBX
Solution 1: JAX and JBX same sign
Solution 2: JAX and JBX different sign
Coupling Constants (J)Coupling Constants (J)
Swap the a & b labels
Second-Order Effects (ABX)
– Which solution is the correct one?
– Several criteria can be used:
1. Magnitude of the couplings – one solution may give dubious (very large or very small) couplings
2. Signs of coupling constants – the signs can sometimes be predicted and rule out a solution all vicinal 3J couplings are positive,
geminal 2J couplings at sp3 carbons are usually negative
CHXCHAHB – JAX and JBX have the same sign
CHACHBHX – JAX and JBX have different signs
• Analysis of the X-part – the intensities of the lines in the X-part are always different – most reliable way to identify the correct solution
Coupling Constants (J)Coupling Constants (J)
Two different X patterns depending on relative sign of
JAX and JBX
Second-Order Effects (ABX)
– Effective of relative sign of JAX and JBX on AB pattern
Coupling Constants (J)Coupling Constants (J)
Solution 1JAX and JBX same sign
Solution 2JAX and JBX different sign
Second-Order Effects (ABX)
– AB pattern from ABX spin
system as a function of
changing AB
Coupling Constants (J)Coupling Constants (J)
Second-Order Effects (ABX)
– AB pattern from ABX spin
system as a function of
the relative sign and
Magnitude of JAX and JBX
Coupling Constants (J)Coupling Constants (J)JAX and JBX same sign JAX and JBX different sign
Coupling Constants (J)Coupling Constants (J)
Demo ACD C+H NMR Viewer software– second order coupling constants