This document summarizes information about geminal and vicinal coupling constants in NMR spectroscopy. It discusses how geminal J values are influenced by factors like bond angles, hybridization, and electronegativity. It presents the Karplus relationship between vicinal J and dihedral angle, and notes how additional factors like substituents can impact vicinal J values. The document also briefly discusses long-range couplings seen in allylic and homoallylic systems due to orbital overlap.

1. SEMINAR ON
BY
MISS. NEHA MILIND DHANSEKAR
[ MSc I ANALYTICAL CHEMISTRY]
THE INSTITUTE OF SCIENCE
15, MADAMCAMA RAOD

2. GEMINAL COUPLINGS
• By the Fermi model for geminal coupling (H—C—H) 2J is usually negative
• Geminal coupling (H–C–H) cay be measured directly from the spectrum
when the coupled nuclei are chemically nonequivalent, (the AB or AM part
of an ABX, AMX, ABX3… spectrum).
• If the relationship is 1st order (AM), the coupling may be measured by
inspection. In 2nd order cases (AB), the spectrum must be simulated
computationally, unless the two spins are isolated (a two- spin system)
• When nuclei are chemically equivalent but magnetically nonequivalent (as in
the AA' part of an AA'XX' spectrum) their coupling constant is accessible by
computational methods
2

3. GEMINAL COUPLINGS
• Here, a intervening atom (usually a spin-inactive 12C) communicates spin
information between the two interacting nuclei
• The Fermi model then predicts that the most stable condition between the
these two geminal nuclei must be one in which they are parallel in spin:
3
1H spin 1H spin
12C is spin inactive
13H spin 1H spin
12C is spin inactive

4. GEMINAL COUPLINGS
IMPORTANT
• Splittings are not observed between coupled nuclei when they are
magnetically equivalent, but the coupling constant may be measured by
replacing one of the nuclei with deuterium.
• For example, in CH2Cl2- the geminal H– C– D coupling is seen as the spacing
between the components of the 1: 1: 1 triplet (deuterium has a spin of 1).
• Since coupling constants are proportional to the product of the gyro-
magnetic ratios of the coupled nuclei, J( HCH) may be calculated from
J(HCD):
4

5. GEMINAL COUPLINGS
• As the ∠H-C-H decreases, the amount of electronic interaction between the
two orbitals increases, the electronic spin correlations also increase and J
becomes larger (more negative)
5
H
H
H-C-H 109o
2JHH = -12-18 Hz
H
H
H-C-H 118o
2JHH = -4.3 Hz
H
H
H-C-H 120o
2JHH = +0-3 Hz
In general:
2JHH
90 100 110 120
40
20

6. GEMINAL COUPLINGS
• Variations in J also result from hybridization and ring size
• As ring size decreases, ∠ C-C-C decreases, along with p-character; the
resulting ∠ H-C-H increases, along with the corresponding s -character – J
becomes smaller
6
H
H
H
H
H
H H
H
H
H
C
H
H
2JHH (Hz) = -2 -4 -9 -11 -13 -9 to -15

7. GEMINAL COUPLINGS
• Electron withdrawal by induction tends to make the coupling constant
more positive - for alkanes the negative coupling thus decreases in absolute
value (becoming less negative)
CH4 -12.4 to CH3OH -10.8 Hz
CH3I -9.2 to CH2Br2 -5.5 Hz
• Electron donation makes the coupling more negative
CH4, -12.4 to TMS -14.1 Hz
• Analogous substitution on sp2 carbon changes the coupling profoundly:
7

8. GEMINAL COUPLINGS
• These effects of withdrawal or donation of electrons through the s-bonds
(induction) can be augmented or diminished by p-effects such as
hyperconjugation.
• Lone pairs of electrons can donate electron density and make 2J more
positive, whereas the orbitals of double or triple bonds can withdraw
electrons and make 2J less positive (or more negative).
• The above-mentioned large increase in the geminal coupling of imines or
formaldehyde compared with ethane results from reinforcement of the
effects of s withdrawal and p donation
8

9. GEMINAL COUPLINGS
• These effect of p-withdrawal occurs for carbonyl, nitrile, and aromatic
groups as in the values for acetone (14.9 Hz), acetonitrile (– 16.9 Hz), and
dicyanomethane (– 20.4 Hz).
• The effect is some-what reduced by free rotation in open-chain systems, so
that particularly large effects are created by constraints of rings:
• p-donation by lone pairs makes J more positive. This effect also explains the
difference in the geminal couplings of three-membered rings: cyclopropane
and oxirane
9

10. GEMINAL COUPLINGS
• Remember splittings are not observed for magnetically equivalent nuclei
(like the 4 hydrogens on CH4) ; the 2J values in this table are used as
reference values and generated by observing the 2JHD for the deuterated
analog of these compounds (top page 92 in text).
10

11. GEMINAL COUPLINGS
• Geminal couplings between protons and other nuclei also have been studied.
• The H–C–13C coupling responds to substituents in much the same way as
does the H–C–H coupling; values are smaller, due to the smaller g of 13C.
• Unlike the H–C–H case, the H–C–13C geminal coupling pathway can include a
double or triple bond; such couplings can be useful to determine
stereochemistry:
11

12. GEMINAL COUPLINGS
• The 2JHCN between hydrogen and 15N strongly depends on the presence and
orientation of the nitrogen lone pair.
• 2JHCN is a useful structural diagnostic for syn–anti isomerism in imines,
oximes, and related compounds as the H–C–15N coupling in imines is larger
and negative when the proton is cis to the lone pair but smaller and positive
for a proton trans to the lone pair:
• The cis relationship between the nitrogen lone pair and hydrogen also is
found in heterocycles such as pyridine 2JHCN -10.8.
• In saturated amines with rapid bond rotation values typically are quite small
and negative (CH3NH2 , -1.0).
12

13. GEMINAL COUPLINGS
• 2J between 15N and 13C follow a similar pattern and also can be used for
structural and stereochemical assignments.
• The carbon on the same side as the lone pair (syn) in imines again has a
larger, negative coupling (- 11.6 Hz). The anti-isomer has a 2JCCN of 1.0 Hz.
• Likewise, the two indicated carbons in quinoline have couplings
differentiated by their geometry - as one is syn and the other anti to the
nitrogen lone pair.
13

14. GEMINAL COUPLINGS
• 2JHCP between 31P and hydrogen also have been exploited stereochemically.
• The maximum positive value of 2JHCP is observed when the H—C bond and
the phosphorus lone pair are eclipsed (syn), and the maximum negative
value when they are orthogonal or anti.
• The situation is similar to that for couplings between hydrogen and 15N, but
signs are reversed as a result of the opposite signs of the gyromagnetic
ratios of 15N and 31P.
• The coupling also is structurally dependent, as it is larger for P(III) than for
P(V): 27 Hz for (CH3)3P and 13.4 Hz for (CH3)3P=O.
14

15. GEMINAL COUPLINGS
• Geminal H– C– F couplings are usually close to for an sp3 carbon (47.5 Hz for
CH3CH2F) and for an sp2 carbon (84.7 Hz for CH2=CHF).
• Geminal F–C–F couplings are quite large for saturated carbons (240 Hz for
1,1-difluorocyclohexane), but less than 100 Hz for unsaturated carbons
(35.6 Hz for CH2=CF2).
15

16. VICINAL COUPLINGS
• Coupling constants between protons over three bonds have provided the
most important early stereochemical application of NMR spectroscopy -
vicinal coupling
• As with geminal coupling, the overall lowest energy spin state is one where
the 1H nuclei and electron spins are paired (12C is spin inactive)
• Observe the two possible spin interactions:
16

17. VICINAL COUPLINGS
• Observe that the orbitals must overlap for this communication to take place
– weaker J-constants
• To communicate spin information, one additional “flip” must take place, and
the J-values are usually positive
• The magnitude of the interaction, it can readily be observed, is greatest
when the orbitals are at angles of 0o and 180o to one another:
17
0o dihedral angle0o dihedral angle
180o dihedral angle

18. VICINAL COUPLINGS
• In 1961, Karplus derived a mathematical relationship between 3JHCCH and
dihedral ∠ H‒C‒C‒H .
• The cos2 relationship results from strong coupling when orbitals are
parallel. They can overlap at the syn-periplanar or anti-periplanar
geometries.
• When orbitals are staggered or orthogonal , coupling is weak.
• A and C are empirically determined constants; C and C’ usually are
neglected, as they are thought to be less than 0.3 Hz while A and A’ imply
that J is different at the syn and the anti-maximum
18

19. VICINAL COUPLINGS
• Unfortunately, these multiplicative constants vary from system to system in
the range 8–14 Hz and quantitative applications cannot be transferred easily
from one structure to another.
• In general:
19
0 45 90 135 180
3J (Hz)
ao
12
8
6
4
2

20. VICINAL COUPLINGS
• In chair cyclohexane Jaa is large as faa is close to 180°
• Jee (0– 5 Hz) and Jae (1– 6 Hz) are small as fee and fae are close to 60°
• When cyclohexane rings are flipping between two chair forms, Jaa is
averaged with Jee to give a Jtrans in the range 4– 9 Hz, and Jae is averaged with
Jea to give a smaller Jcis, still in the range 1– 6 Hz.
• In conformationally locked systems (no ring flip) the effect can be used to
assign stereochemistry
20

21. VICINAL COUPLINGS
• Further examples:
21
3Jaa = 10-14 Hz
a = 180o
H
H
H H
3Jee = 4-5 Hz
a = 60o
H
H
3Jae = 4-5 Hz
a = 60o

22. VICINAL COUPLINGS
• For alkenes 3Jtrans (f = 180o) is always larger than 3Jcis (f = 0o)
• 3Jtrans > 3Jcis > 2Jgem allows assignment of the vinyl system (AMX, ABX or ABC
spectrum) trivial
22
3Jtrans = 11-18 Hz
a = 180o
3Jcis = 6-15 Hz
a = 0o
H
H
H H

23. VICINAL COUPLINGS
• For cyclic alkenes internal bond angles may affect 3Jcis
23
3Jtrans = 11-18 Hz
a = 180o
3Jcis = 6-15 Hz
a = 0o
H
H
H H 120o angle in H-C-C
bond reduces overlap
H
H
H
H
H
H
H
H
H
H
3Jcis = 0-2 2-4 5-7 8-11 6-15 Hz

24. VICINAL COUPLINGS
• Despite the potentially general application of the Karplus equation to dihedral
angle problems, there are quantitative limitations.
• The 3J H–C–C–H depends on the C–C bond length or bond order, the H–C–C
valence angle, the electronegativity and orientation of substituents on the
carbon atoms in addition to the H–C–C–H dihedral angles.
• A properly controlled calibration series of molecules must be rigid (mono-
conformational) and have unvarying bond lengths and valence angles. Three
approaches have been developed to take the only remaining factor,
substituent electronegativity, into account:
1. Derive the mathematical dependence of 3J on electronegativity.
2. Empirical allowance by the use of chemical shifts that depend on
electronegativity in a similar fashion as 3J.
3. Eliminate the problem through the use of the ratio (the R value) of two 3J
coupling constants that respond to the same or related dihedral angles and that
have the same multiplicative dependence on substituent electronegativity, which
divides out in R.
24

25. VICINAL COUPLINGS
• These more sophisticated versions of the Karplus method have been used
quite successfully to obtain reliable quantitative results.
• The existence of factors other than the dihedral angle results in ranges of
vicinal coupling constants at constant even in structurally analogous
systems.
• Saturated hydro-carbon chains (H–C–C H) exhibit vicinal couplings in the
range 3– 9 Hz, depending on substituent electronegativity and rotamer
mixes and 8.90 Hz for . Higher substituent electronegativity always lowers
the vicinal coupling constant. In small rings, the variation is almost entirely
the result of substituent electronegativity, with cis ranges of 7– 13 Hz and
trans ranges of 4– 10 Hz in cyclopropanes.
25

26. VICINAL COUPLINGS
• These more sophisticated versions of the Karplus method have been used
quite successfully to obtain reliable quantitative results.
• The existence of factors other than the dihedral angle results in ranges of
vicinal coupling constants at constant even in structurally analogous
systems.
• Saturated hydro-carbon chains (H–C–C–H) exhibit vicinal couplings in the
range 3–9 Hz, depending on substituent electronegativity and rotamer
mixes.
• Higher substituent electronegativity always lowers the vicinal coupling
constant. In small rings, the variation is almost entirely the result of
substituent electronegativity, with cis ranges of 7– 13 Hz and trans ranges of
4– 10 Hz in cyclopropanes.
26

27. VICINAL COUPLINGS
• In small rings, the variation is almost entirely the result of substituent
electronegativity, with cis ranges of 7– 13 Hz and trans ranges of 4– 10 Hz in
cyclopropanes.
• Coupling constants in oxiranes ( epoxides) are smaller because of the effect
of the electronegative oxygen atom.
• 3J is proportional to the overall bond order, as in benzene and in
naphthalene. The ortho-coupling in benzene derivatives varies over the
relatively small range of 6.7– 8.5 Hz, depending on the resonance and polar
effects of the substituents.
• The presence of heteroatoms in the ring expands the range at the lower end
down to 2 Hz, because of the effects of electronegativity ( pyridines) and of
smaller rings (furans, pyrroles).
27

28. VICINAL COUPLINGS
28

29. LONG RANGE COUPLINGS
• As can be deduced from the reduced J values for vicinal coupling and the
Karplus relationship, the greater the number of intervening bonds the less
opportunity for orbital overlap over long range (> 3 bond)
• Long- range coupling constants between protons normally are less than 1 Hz
and frequently are unobservably small.
• In at least two structural circumstances, however, such couplings commonly
become significant.
• Allylic and homoallylic coupling
• W-coupling
• In cases where a rigid structural feature preserves these overlaps, however,
long range couplings are observed – especially where C-H s-bonds interact
with adjacent p-systems
29

30. LONG RANGE COUPLINGS
• Allylic systems are the simplest example of a 4J coupling
• Here, if the allyl C-Ha bond is orthogonal to the p system,4J = 0 Hz; if this bond is
parallel to the vinyl C-Ha bond, 4J = 3 Hz
• In acyclic systems, the dihedral angle is averaged over both favorable and
unfavorable arrangements, so an average 4J is found, as in 2- methylacryloin
• Ring constraints can freeze bonds into the favorable arrangement, as in indene or in
an exactly parallel arrangement as in allene (right)
30

31. LONG RANGE COUPLINGS
• When this type of coupling is extended over five bonds, it is referred to as
homoallylic coupling
• Examples include the meta- and para- protons to the observed proton on an
aromatic ring and acetylenic systems:
31
H
H
C C C C
H H
5J = 0-1 4J 1-3 5J 0-1 Hz
H
H

32. LONG RANGE COUPLINGS
• Rigid aliphatic ring systems exhibit a specialized case of long range coupling
– W-coupling – 4JW
• The more heavily strained the ring system, the less “flexing” can occur, and
the ability to transmit spin information is preserved
32
H H O
H
H HH
4J = 0-1 4J = 3 4J = 7 Hz

33. LONG RANGE COUPLINGS
• Although coupling information always is passed via electron-mediated
pathways, in some cases part of the through- bond pathway may be skipped,
and effect known as through-space coupling
• Two nuclei that are within van der Waals contact in space can interchange
spin information if at least one of the nuclei possesses lone pair electrons -
found most commonly, but not exclusively, in H– F and F– F pairs.
• The six- bond H--F coupling is negligible on the left (2.84 Å) but is 8.3 Hz on
the right ( 1.44 Å) ( the sum of the H and F van der Waals radii is 2.55 Å).
• This is likely is important in the geminal F– C– F coupling, which is unusually
large: 2JFCF for sp3 CF2 (~200 Hz, 109.5o) compared to sp2 CF2 (~50 Hz, 120o)
33

34. REFERENCES
 Introduction to spectroscopy
Author : DONALD L. PAVIA
 Wikipedia
34

35. 35