Coupling  constants  for  1H  and  13C  NMR  

Common  values  for  1-­‐bond  to  long-­‐range  couplings  

Values for the 1J coupling constant between 13C and 1H:! !

! ! 1JCH = 500 Hz –––––––– where n is the hybrization number spn!!!!

•  sp3: 115 – 140 Hz!•  sp2: 150 – 200 Hz •  sp: 240 – 270 Hz

(n + 1) 1

1J values

1J values

2J coupling: correlates with the s-character of the C–H bonding orbital

(the s orbital component increases the overlap between hybrids)

2-bond coupling constants

HH2C

H

H

H

H2C

H

H

109˚ 120˚ 120˚

–12.4 Hz –4.5 Hz + 2.5 Hz

H H

H

H H

NR

HH

H OH

H H H H H H

OO O

HH

H Li

HH

H Cl

HH

H F

–22.3 Hz –14.5 Hz –10.8 Hz ±0 Hz +5.5 Hz

+16.5 Hz+7.1 Hz–1.3 Hz–3.2 Hz

The geminal coupling constant increases with the s-character of the hybrid orbitals:

Examples of the effects of various substituents:

O

HH

+42.2 Hz

H

H H

H

H

H

0˚ (cis)

180˚ (trans)

90˚

H

H

better interaction

good interaction

HH

3J coupling: strongly dependent on the dihedral angle between C–H bonds, which determines the extent of orbital interactions

The HC–CH dihedral angle

HH

Θ

3J coupling: the Karplus equations and correlation curve

The relationship between the dihedral angle and the vicinal coupling constant 3J (as observed from 1H NMR spectra) is given theoretically by the Karplus equations: 3Jab = J 0 cos2Θ – 0.28 (for 0˚ < Θ < 90˚ range) 3Jab = J 180 cos2Θ – 0.28 (for 90˚ < Θ < 180˚ range)

HH

Θ

3J coupling: dependence on electron-density at the carbons of CH2 or CH-groups

8.9 Hz 8.0 Hz 7.6 Hz

H

HH

Li H

HH

F

19.8 Hz

23.9 Hz

4.7 Hz

12.8 Hz

Higher electron density at an sp3 or sp2 hybrid increases the magnitude of the vicinal coupling constant:

Examples of the effects of various substituents:

H3C–CH2–Li H3C–CH2–SiR3 H3C–CH2–C≡N H3C–CH2–Cl H3C–CH2–OEt H3C–CH2–OEt2

7.2 Hz 7.0 Hz 4.7 Hz3J:

H

HH

Cl

7.3 Hz

14.4 Hz

H

HH

H3C

10.0 Hz

16.8 Hz

H

HH

R3Si

14.7 Hz

20.4 Hz

3J cis:3J trans:

3J coupling: examples of in-plane angle dependence

3J cis:

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

4.0 Hz

7.5 Hz

10.3 Hz

8.8 – 11.0 Hz

9.0 – 12.6 Hz

10 – 13 Hz

0.5 – 1.5 Hz

2.5 – 3.7 Hz

5.1 – 7.0 Hz

3J coupling: Application to substituted, saturated six-membered rings

H

3J a,a: 7–12 Hz (φ = 180˚)

3J e,e: 2–5 Hz (φ = 60˚)

3J a,e: 2–5 Hz (φ = 60˚)

XH'e

H'a

Ha

O

H

HOHO

H

H

OHOH

H

CH2OH

123 H

O

H

HOHO

H

H

OHH

OH

CH2OH

H2O

Mutarotation of D-glucose:

α-D-glucose

3J a,e = 3.5 Hz

α-D-glucose

3J e,e = 7.7 Hz

Long-range coupling: >3 bonds

There are two types: 1)  Allylic / propargylic coupling: Mechanism:

HH

4JEA = 1-3 Hz4JZA = 1-3 Hz

4J = 3-4 Hz 4J = 5-7 Hz5J = 3-4 Hz

4J = 1-2 Hz7J = 1-2 Hz4Jav = 3-5 Hz5Jaa' = 7-11 Hz

HAHZ

HE

H

H

CH

H

CH2-R'

R

RR'

RR'

Ha Ha'

R'R

Hv

H

RR'H3C

H

R' HR

Long-range couplings: >3 bonds

2)  W-coupling: Mechanism:

H

H

H

H

H

H

H

H

H

H

H

H

NH

H

H

4J = 8 Hz 4J = 18 Hz 4J = 1-2 Hz

4J = 1-2 Hz5J = 1 Hz4J = 1-3 Hz Hz

H

H(back-lobe interactions)