NUCLEAR MAGNETIC RESONANCE (NMR) · PDF filehence the term NUCLEAR MAGNETIC RESONANCE DE = h B...
Transcript of NUCLEAR MAGNETIC RESONANCE (NMR) · PDF filehence the term NUCLEAR MAGNETIC RESONANCE DE = h B...
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NUCLEAR MAGNETICRESONANCE (NMR)
SPECTROSCOPY
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Characterization of Organic Compounds
O
O
OH
OMe
OMe
2
NMRUVIRMS
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I = 0, Nuclei is NOT NMR ACTIVEI ≠ 0, Nuclei are NMR ACTIVE
Like electrons protons and neutrons have spins and they pair.
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16O,
Number of spin states = 2I + 1
(NOT NMR ACTIVE)
(NMR ACTIVE)
(NMR ACTIVE)
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1H NMR SPECTROSCOPY
m (magnetic dipole)
A very small magnetic field is associated with the nucleii.e. nuclei with spin behave as tiny bar magnets
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Influence of External Magnetic Field (B0)
parallel antiparallel
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Bo
Precessional motion
Precessional orbit
The frequency of this motion is called precessional frequency (n)
Nuclear magnetic dipole
Further Influence of External Magnetic Field
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parallel anti-parallelApplied magnetic field
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DE = hgB0
is the magnetogyric ratio (a proportionality constant, differing for each nucleus which essentially measuresthe strength of nuclear magnets).
B0 = is the strength of the applied magnetic field.h = Plank’s constant
DE = hn
Precessional frequency (n) is directly proportional to Bo
2p
n = gB02p
Bo = 1.4T, n = 60 MHz
Bo = 14T, n = 600 MHz
Bo = 4.7T, v = 200 MHz
= hgB02p
The basic NMRequation
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For transition from the lower state to the upper state, the energy falls within the radio frequency range
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i.e. when the precessional frequency = radio frequency
When the appropriate radio frequency is supplied,
Absorption will occur and the nucleus and the radio frequency are said to be in resonance and hence the term NUCLEAR MAGNETIC RESONANCE
DE = hB0/2p
the frequency is also called resonance frequency
n = B0/2p
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DE = hB0/2p
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NMR is not sensitive method of analysis
A small excess (0.00003%; Bolzmann’s excess) in the lower energy level is responsible for absorption)
SENSITIVITY OF NMR SPECTROSCOPY
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DE = hB0/2p
Sensitivity can be enhanced by the use of strongerMagnetic field
2.35T, 4.7T, 7.05T, 9.4T, 11.74T, 14.10T, 16.2, 18.4T
Increasing order of sensitivity
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• Na (population in lower level) and Nb (population in upper level) approach equality.
• No further net absorption of energy.• Resonance signals will fade out and this is
called saturation of the signal.
When a proton absorb the correct radiofrequency, nuclei in the lower energy state undergo transition to the higher energy state.
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Such energy loss is called relaxation(the process through which the Boltezmann’s excess
is restored)There are two types of relaxation:1) Spin-lattice relaxation:-
The transfer of energy to the lattice (entire framework or aggregate of neighbours i.e. solvent molecules, or other atoms in the molecule).
2) Spin-spin relaxation:-Transfer of energy to a neighbouring nucleus having identical precessional frequency.
High energy nuclei lose energy by radiationless process.
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The secondary magnetic field (B) generated by the electron circulation shields the nucleus from the external magnetic field
d is shielding factor
0
From this equation, can NMR beuseful in structure elucidation?
Thus NMR equation has to be modified as
n = B0/2p
n = B0(1-d)/2p
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n will vary with electron environment
Electron donatingElectron donating groups increase the electron density around nearby hydrogen atoms resulting in increased increased shielding, shielding, Electron withdrawingElectron withdrawing groups decrease the electron density around nearby hydrogen atoms resulting indedecreased creased shieldingshielding, (, (dedeshieldingshielding).).
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CH3F, CH3O, CH3N, CH3C, CH3Si Decreasing order of electronegaitivity
Increasing order of shielding
Tetramethylsilane (TMS) (CH3)4Sihas among the most shielded nuclei and is used as a reference point in NMR
NMR is useful to distinguish nuclei in differentElectronic environment
Why is it necessary to have a reference point in NMR?Because: the magnetic field strength is not constant.The change in field strength will affect both the sampleand the reference compound in the same way.Relative absorption positions are generated in NMR.
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CHEMICAL SHIFTThe difference in the absorption position (in Hz) of a nucleus (eg. 1H) from the absorption position of a Reference (usually TMS).
CH3
Ois 120 Hz at 1.4T (60 MHz)
is 200 Hz at 2.3T (100 MHz)is 1200 Hz at 14T (600 MHz)
n = gB0(1-d)/2pn = gB02p
B0 = 1.4T, n = 60 MHzResonance frequencyin the neighbourhoodof 60 MHz
Chemical shift = nnucleus - nTMS
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Chemical shift (d) = nnucleus - nTMSz
Z operational frequency in MHz
Unit of d will be: HzMHz
Hz106 Hz
= = 1106
= ppm
Chemical shifts are not given in frequency (Hz)Since it is dependent on the field strength
Field independent parameter can be obtained ifChemical shift in frequency is divided by the operational frequency (Z)
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CH3
O is 120 Hz at 1.4T (60 MHz)
is 200 Hz at 2.3T (100 MHz)
is 1200 Hz at 14T (600 MHz)
dCH3 = nsignal-nTMS
z
12060
= 200100
= 1200600
= = 2ppm
Position of resonance signal is in a unitIndependent of the field strength
At 60 MHz, 1ppm = 60 HzAt 100 MHz, 1ppm = 100 HzAt 600 MHz, 1ppm = 600 Hz
Operational frequency has implications on resolution
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High frequency Low frequency
High fieldLow field
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1) Effect of electronegativitySi-(CH3)4 dH = 0.00 ppm
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d-d+
H
XC
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C in X issp or sp2
sp3
XX
sp3sp3
sp3
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2) ANISOTROPYSecondary magnetic field generated as the result ofcirculation of bonding electrons in a p-bond(also in s-bond)
Paramagnetic anisotropy -(deshielded = downfield)Diamagnetic anisotropy -(shielded = upfield)
d 7.15 ppmH
H
HH
H
H
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[18][18]--ANNULENEANNULENE
HH
d -3.00 d 9.30
If aromatic it should have (4n+2)p electrons Is this aromatic?
[18]-Annulene corresponds to (4X4+2)p electrons.
1H NMR evidence for aromaticity
It should be aromatic.
How about[16]-annulene?
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- Deshielding zone => nuclei will be downfield shifted
+ Shielding zone => nuclei will be upfield shifted
Anisotropy in other p-systems
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CH3
CH3
H H
H H
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Single Bond AnisotropyH H
Hax
Heq
1
3
56
2 4
1.1 ppm
1.6 ppm
Deshieleded Occurs in rigid cyclohexane ring or in cyclohexanone
Hax
Heq1
3
56
2
4 Hax
Heq1
3
5
6
24ring
flipThe axial and equatorial protons will have the same value
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HHd 0.22 ppm
CH3
CH3
H H
H H
s-bond anisotropic effectshields the protons
Free rotation
‘aromatric like’ property
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C C
a
X
b
H
X dHa dHbH 5.28 5.28OCH3 6.38 3.85COCH3 5.85 6.68
C C
a
OCH3
b
....
C C
a
O+
CH3
b
-
..
C C
ab
CH3
O..
.. C C
ab
CH3
O..
....
+
dHa by Inductive effect and dHb by resonance effect
3) Substituent effect on p-system (Resonance)
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HH
HH
H
H
d 7.15 ppm
H
HH
OMe..
:H
HH
OMe
-
:+
6.81 ppmH
HH
OMe
-
+:
6.86 ppm
7.17 ppm
H
HH
NO2
8.25 ppm
7.66 ppm7.45 ppm
=
Resonance, is deshielding for ortho and para protonsInductive effect, is deshielding for ortho > meta > para protonsAnisotropy, is deshielding for ortho protons
H
HH
N+O O
H
HH
ON
+ O
+H
HH
ON
+ O
+
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4) Hybridization of the carbon atomChemical shift is dependent on electronegativity,Anisotropy, resonance and hybridization
s electrons are closer tocarbon than p electrons
Carbon has more controlOn s-electrons thanp-electrons
How about in hybrid orbitals sp3, sp2 and sp?
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CH3-CH3
sp3, 109.5º25% S character
CH2=CH2
sp2, 120º33% S character
sp, 180º50% S character
CH CH
d 0.9 ppm d 5.8 ppm d 2.35 ppm
An increase in %S character will result:-1) In an increase in bond angle2) (Order of electronegativity: sp > sp2 > sp3)
s electrons will be closer to C than HHence, H will be de-shielded, higher d value
Electronegativity & Anisotropic effects are:additive (deshielding) in alkenesand subtractive in alkyenes,Anisotropy (shielding), hybridization (deshielding)
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d+d-RO H +
d+d-RO H R
O HR
O Hd+d- d- d+
Hydrogen bonding
5) Hydrogen bonding
Hydrogen atoms involved in hydrogen bondingare even more deshieldedbecause the O-H bond is more polarized with lower electron density at hydrogen atom.R
O H + D3CO D
RO D + D3C
O H
Exchangeable protons arenot observable in ‘protic solvents’
solvent
a) Intermolecular H-bonding
O-H, N-H, S-H De-shielded compared to C-H
RO H
HO H
‘Protic solvent’sample
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These are also temperature dependent,
These are conc. Dependent,higher conc. results in stronger H-bonding,hence, stronger deshielding effect
Higher temp. results in weaker H-bondingHence, lower d values (weaker de-shielding effect)
RO H +
DO D
RO D + D
O H
D2O exchange OH Signal will disappear
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OR
OH
O
RO
H
dimer even atdilute solution
b) Intramolecular H-bonding
OOH
O-Hydroxyacetophenone
d 10 – 14 ppm
R R
O O
b-DiketoneR R
O OH
d 11 – 16 ppm
Concentration andtemperature have little effect on d value
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O Oconcentration andtemperature have NO effect on d value
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6) Solvent effectsCDCl3, (CD3)2O , CD3OD, (CD3)2S=O, D2O
Change of solvent results in some change in d values
Some Typical 1H Chemical Shifts (δ values) in Selected Solvents SolventCompound
CDCl3 C6D6CD3COC
D3
CD3SOCD3
CD3C≡N D2O
(CH3)3C–O–CH3C–CH3O–CH3
1.193.22
1.073.04
1.133.13
1.113.03
1.143.13
1.213.22
(CH3)3C–O–HC–CH3O–H
1.261.65
1.051.55
1.183.10
1.114.19
1.162.18
------
C6H5CH3CH3C6H5
2.367.15-7.20
2.117.00-7.10
2.327.10-7.20
2.307.10-7.15
2.337.15-7.30
------
(CH3)2C=O 2.17 1.55 2.09 2.09 2.08 2.22
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R R
OOH
d 10 – 18 ppm
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A
B HAHB
What is the cause of splitting?
SPIN-SPIN COUPLING
Proton-Proton Coupling
Because HA ‘sees’ HB in two spin states and vice versa
Two proton System
Is the splitting of NMR signals into doublet, triplet,quartet etc., as the result of spin-spin interaction,through bonding electrons, between/among neighbouring NMR active nuclei.
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HA in isolation
HB
Reason: The presence of HB makes HA to come to resonance twice
Similarly, the presence of HA makes HB to come to resonance twice
HA in the presence of HB
B0 1 1
Two proton system
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A
BB’
Three proton system
What is the splitting of HA?
HA HBHB’
B0
B B'
B B'
B B' B B'
‘SEES’
HA
HBHB’
Free rotationHB and HB’ are identical,splitting is not observedbetween HB and HB’
1 1
triplet
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Multiplicity = 2nI+1
n = Number of equivalent nuclei causing signal splittingI = spin quantum number of the nuclei causing the splitting
For 1H, I = ½ , multiplicity = n+1
n01
234
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Is the distance between lineswithin a set of NMR signals in Hz
5 lines (pentet), 6 (sextet), 7 (heptet), 8 (octet),
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Integral ratio, 1:1
Integral ratio, 2:1
Integral ratio, 1:2Integral ratio, 1:1
Integral is the relative peak area under a set of NMR signal(s) and is dependent on the numberof nuclei contributing to the signal(s)
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Integral ratio 1:3 Integral ratio 2:3
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1H NMR Spectrum - C4H8O2
2H3H3H
IHD= [(2X4)+2]-8 = 12
CH3
CH3
CH2-CH2-CH3
C=O -O- 2
33
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triplet
O
O
Cl H
H
H
H
H
H
HHH
HH
Propionic acid 2-chloro-propyl ester
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J depends mainly on: a) Number of bonds separating the nucleib) Dihedral angle for vicinal coupling (3J)But it is independent of field strength.
H
(10 Hz) (16 Hz)
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4J and above are called long range coupling
(8 Hz) (2 Hz) (less than 1 Hz)
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Long range coupling is observed in: aromatics, olefins, acetylenesand strained aliphatic ring systems
p-electrons are more effective in transmitting coupling information.
CH3
H4J = 0.6-0.9 Hz
broad signal
O CH3
O
sharp signal
Allylic coupling
Homoallylic coupling
HH
HPh
5J = 9 Hz
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H
H O
H
4J = 1.1 Hz
Zigzag arrangement is necessaryi.e. ‘W’ or ‘M’ arrangement of atomsIt is also called ‘W’ coupling
H
H
In strained bicyclic system the coupling constantMay be large
Extensive overlap of the rear lobes is responsible for the coupling interaction
4J = 4 Hz
H
H
H
H
stronger overlap
larger J value
4J = 0 Hz
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Hax
Hax
HeqHeq
13
56
2
4
Rigid system
Φ H6eq
H6ax
C5
H1eq
H1ax
C2
6
Newman projection
Φ is dihedral angle
Dihedral relationship (Φ)
Axial-axial (180°)Axial-equatorial (60°)
Equatorial-equatorial (60°)
Cycloalkanes
J (Hz)
8-122-5
2-5
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Dependence of Vicinal Coupling on Dihedral Angle
H
average value
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Geminal coupling:-
Geminal coupling is common in rigid ring systemHax
Hax
HeqHeq
13
56
2
4
2J = -12 Hz
Coupling between nuclei/protons attached to the same carbon atom (2J) and is negative usually
Vicinal coupling:-Coupling between nuclei/protons attached to adjacent carbon atoms (3J)
HH
XY2J = - 4 Hz
2J = decreases with an increase in bond angle
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Effect of Electronegativity of Substituents
ClCl
Cl
HHH
3J = 6.0 Hz
Electronegative elements directly attached to the same carbon atom as vicinaly coupled protons, reduce coupling constant, and electropositive elements raise it.
For freely rotating chains, the effect is small,In cyclic systems the effect is larger
HCl
H
HHH
3J = 6.5 Hz
HLi
H
HHH
3J = 8.4 Hz
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Hax
Hax
OHHeq
3
56
2
4
Jee = 4.5 Hz Jaa = 10.0 Hz
When an electronegative element is anti-periplanarwith respect to one of the coupling protons, the effect of reducing J value is larger
Rigid Systems OH
Hax
HeqHeq
13
56
2
4
Jae = 4.5 HzJea = 2.5 Hz
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Chemical Equivalence of NucleiTwo or more nuclei are said to be chemically equivalentwhen they are in identical chemical environment,Then, they will have identical chemical shift values.
Chemically equivalent nuclei are interchangeable by dynamic process or symmetry operation.
Symmetry Elements Symmetry OperationAxis of symmetry (C2) rotationPlane of symmetry (s) reflection
Hax
Heq1
3
56
2
4 Hax
Heq1
3
5
6
24ring
flip
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None
*
stereocentre
Enantiotopic diastereotopic
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Magnetic Equivalence of Nuclei• Magnetically equivalent nuclei are those
chemically equivalent nuclei, which also have the same coupling constant with every other nucleus in the molecule.
CH3 CH3
HH7.2 Hz7.2 Hz
triplet
septet
H, H are magnetically equivalent nucleiCH3, CH3 are magnetically equivalent nuclei
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OHH
HH
H
H 12
34
5
6
J2,3 is different from J3,6 OR J5,6 is different from J2,5
(8 Hz) (2 Hz) (less than 1 Hz)
H2 and H6, H3 and H5are chemically equivalentbut they are not magnetically equivalent
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• In situations when a proton is coupled to neighbouring protons with different coupling constants:
• Multiplicity = [(na+1)(nb+1)(nc+1)…]• Where na, nb, nc .. are sets of magnetically
equivalent protons.
J1,2 = 2 Hz J1,3 = 14 Hz
J2,3 = 10 Hz
H
H1
H3
OO
H2
Multiplicity and Magnetic Equivalence
What will be the multiplicities for H1, H2 and H3?
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Multiplicity = [(na+1)(nb+1)(nc+1)…]
Multiplicity of H1 = [(1+1)(1+1)] = 4
Multiplicity of H2 = [(1+1)(1+1)] = 4
Multiplicity of H3 = [(1+1)(1+1)] = 4
na = 1, nb = 1
H
H1
H3
HO2C
2
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J1,2 = 2 Hz J1,3 = 14 Hz
J2,3 = 10 Hz
H1
‘Coupling tree’
J1,3 = 14 Hz
J1,2 = 2 Hz
Double doublet (dd) for H1H2 and H3 are also double doublets, each
H
H1
H3
HO2C
2
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(dd)
(ddd)
(s)
(d)
H
C-H
H
H
16 Hz
10 Hz
7 Hz
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16 Hz
7 Hz
triplet of a doublet (td)
m of H1= [(na+1)(nb+1)]
m of H1 = [(1+1)(2+1)]
(na=1, nb=2)
m of H1 = 6 lines
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16 Hz
7.2 Hz
quartet of a doublet (qd)
m = [(na+1)(nb+1)]
m = [(1+1)(3+1)]
(na=1, nb=3)
m = 8 lines
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16 Hz
10 Hz
7.2 Hz
triplet of a double doublet (tdd)
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1st and 2nd Order SpectraDn/J >10 [the difference between the absorption positions (in Hz)between coupling protons divided by the coupling constant is more than 10], spectrum is 1st order
Dn/J ≤ 10 the spectrum is 2nd order
1st Order 2nd OrderMultiplicity determined by 2In+1 rule (n+1 for 1H)
The 2In+1 rule is not valid. Increased multiplicity
Relative intensity determined by Pascal’s triangle
There is no definitive rule to determine intensity
Spectra are simple Spectra are complex
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Dn(2.8)/J(7) = 0.4
Dn(7)/J(7) =1.0
Dn(28)/J(7) =4
Dn(105)/J(7) =15 1st ORDER
2ND ORDER
105 Hz
J =7 Hz J =7 Hz
28 Hz
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Spin SystemSpin system- a designation which indicatesthe number of chemically and/or magneticallyequivalent protons which are mutually coupled.
AnXm or AnBmA, X and B: relative chemical shift positions(A for highest and X and above for the lowest d)n and m: number of magnetically equivalent protonsdepending on the chemical shift (in Hz) difference
Dn/J >10 the spectrum is 1st order and spin system designation will be AnXm
Dn/J ≤10 the spectrum is 2nd order and spin system designation will be AnBm
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Dn/J = 0.4
Dn/J =1.0
Dn/J =4
Dn/J =15 1st Order
2nd order
AX
AB
Roofing effect
Two proton System
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CH3
ClO
H H
A2
Dn (0)/J(7) = 0
The two methylene protons couple giving two doublets.However the outer lines are too small to be detectedand the inner lines have completely merged.
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AX or AB A2X or A2B
AX2 or AB2 A2X2 or A2B2
First or second order depends on1) Difference in electronic environment2) Strength of magnetic field used for measurement
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H
H
H
NC 1
23
AMX (fully first order)
OHH
HI
H
H
AA’XX’ or AA’BB’ABX, AXY(mixture of 1st and 2nd order)
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A b s o r b a n c e
012345678 d, ppm
A b s o r b a n c e
01234 d, ppm
TMS
TMS
60 MHz
100 MHz
60 MHz
100 MHz
Chemical Shift (d, ppm) =Observed chemical shift from TMS (Hz)
Sptectrometer frequencey (MHz) = ppm
Dn/J >10
Dn/J ≤10
1st order at 2.35 T
2nd order at 1.4 T
Resolutionincreased
A2X3 or A2B3
Five Proton System
HH
XH
HH
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Problems set A, Q1
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Problems set A, Q2
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Problems set A, Q3
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Problems set A, Q4
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C5H11Br
Problems set A, Q5
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C8H9O2Br
1H1H 1H
6H
Problems set A, Q6
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Problems set A, Q7