NMRNMR NNuclear M Magnetic RResonance
ProtonProton NMRNMR
Index NMR-basicsNMR-basics
Anisotropy of Aromatic compounds: in plane and aboveAnisotropy of Aromatic compounds: in plane and above
CH3
CH3
H
H
H
H
H
H
H
H
H
H
ringring 8.14-8.64 ppm 8.14-8.64 ppm
MeMe -4.25 ppm -4.25 ppm
H H
ringring 7.27-6.95 ppm 7.27-6.95 ppm
MeMe -0.51 ppm -0.51 ppm
OUTSIDEOUTSIDE 9.28 ppm 9.28 ppm
INSIDEINSIDE -2.99 ppm -2.99 ppm
HH
HH
H
H
H
H H
H
H
H
H
HH
H
H
H
Anisotropy: AromaticAnisotropy: Aromatic
Electronic effectsElectronic effectsCH2
O
CH3
CH2+
O-
CH3
DeshieldedDeshielded
O
O
O
H
H
7.10 ppm7.10 ppm
COOEt
COOEt
H
H
COOEt
H
H
EtOOC
6.83 ppm6.83 ppm6.28 ppm6.28 ppm
O
H
H7.71 ppm7.71 ppm
6.10 ppm6.10 ppm
O
H
H
H
H7.07 ppm7.07 ppm 6.38 ppm6.38 ppm
6.28 ppm6.28 ppm
5.93 ppm5.93 ppm
8 7 6 5 4 3
O12
3
4
5
6
O7
Electronic effects: conjugation with carbonylElectronic effects: conjugation with carbonyl
8 7 6 5 4 3 2
O1
2
34
56
7.75
6.20
Electronic effects: conjugation with carbonylElectronic effects: conjugation with carbonyl
deshieldeddeshielded
Electronic effects: conjugation with heteroatomElectronic effects: conjugation with heteroatom
OH
H
O+
C-
H
H
SH
H
6.06 ppm6.06 ppm
5.48 ppm5.48 ppm
S
H H
5.81 ppm5.81 ppmO
H H5.78 ppm5.78 ppm
OH
H4.82 ppm4.82 ppm
6.22 ppm6.22 ppm
shieldedshielded
6.5 6.0 5.5 5.0 4.5 4.0 3.5
12
34
O5
Electronic effects: no conjugation with heteroatomElectronic effects: no conjugation with heteroatom
7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5
12
34
O5
2.65 2.60 2.55
6.35 6.30 4.95 4.90
shieldedshielded
Electronic effects: conjugation with heteroatomElectronic effects: conjugation with heteroatom
O CH3
8 7 6 5 4 3 2
8.0 7.5
Electronic effects: conjugation with carbonylElectronic effects: conjugation with carbonyl
deshieldeddeshielded
deshieldeddeshielded
oo
pp
mm
7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5
OCH3
7.3 7.2 7.1 7.0 6.9 6.8
Electronic effects: conjugation with heteroatomElectronic effects: conjugation with heteroatom
ShieldedShielded
shieldedshielded
oopp
mm
NHCH3
7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5
7.0 6.5
Electronic effects: conjugation with heteroatomElectronic effects: conjugation with heteroatom
ShieldedShielded
shieldedshielded
oopp
mm
F
7.5 7.4 7.3 7.2 7.1 7.0 6.9
Cl
7.4 7.3 7.2 7.1
Br
7.6 7.5 7.4 7.3 7.2 7.1
Aromatic: inductive effect and resonance effectAromatic: inductive effect and resonance effect
Hydrogen bondHydrogen bond
Protons on HeteroatomsProtons on Heteroatoms
• OHOH, NHNH, SHSH– Exchangeable (with D2O)
– Hydrogen bonding– On NitrogenNitrogen (1414NN), as the spin state of that
nuclei is 11, there can be partial coupling that produce broaden lines. There can be also full coupling that would produce 3 lines of equal 3 lines of equal intensityintensity (I=1I=1 has 3 orientations3 orientations in a magnetic field)
Protons on HeteroatomsProtons on Heteroatoms• OHOH
– AliphaticAliphatic 0.5-4.0 ppm 0.5-4.0 ppm (depend on Concentration)– Intramolecular hydrogen bondingIntramolecular hydrogen bonding deshield OHOH and render it
less sensitive to concentrationless sensitive to concentration• Usually Usually OHOH exchange rapidly (no coupling with (no coupling with
neighborsneighbors• In In DMSODMSO or or AcetoneAcetone, the exchange rate is slower => , the exchange rate is slower =>
there is there is coupling with neighborscoupling with neighbors
• PhenolsPhenols : : 7.5-4.0 ppm 7.5-4.0 ppmIntramolecular bond 12-10 ppm
• Carboxylic AcidsCarboxylic Acids: : Exist as Dimers 13.2-10 ppm
HH22OO signal moves with signal moves with temperaturetemperature
HH22OO
OHOH in DMSO in DMSO
CHCH33--CHCH22--OHOH
OHOH CHCH22
qdqd
(CH(CH33))2 2 --CHCH--OHOH
OHOHCHCH
Protons on HeteroatomsProtons on Heteroatoms
• NH : NH : 1414N: I=1N: I=1 => 2I+1 lines=> 2I+1 lines• NHNH has different rate of exchange• 14N can relax quickly. Depending on relaxation rate,
heteronuclear coupling will be visible or produce broadened peaks.
• R-NHR-NH : Aliphatic amines => rapid exchange– Sharp singlets : no coupling to N: N: ~3-0.5 ppm~3-0.5 ppm
• R-NHR-NH: Amides, Pyrroles, Indoles, Carbamates– NHNH broad– CHCH shows coupling the NHNH
NHNH
AmideAmide
Protonated AminesProtonated Amines
FormamideFormamide
HH-CO-N-CO-NHH22
H-NMR
H{14N}-NMR
NHNH: Amide, Pyrrole Indole: 8.5-5.0 ppm: 8.5-5.0 ppm
In AmidesAmides: Slow rotationSlow rotation can show different isomers
H NH
CH3
O
H NCH3
H
O
In Amine Salt:In Amine Salt:
• Moderate Rate of exchange => broad peaks ~ 8.5-6.0 ppm•CH => show coupling to NH+
Sometimes broad [NH[NHxx++]] consist of 3 broad hump3 broad hump due to 1414N couplingN coupling
11JJNHNH ~ 50 Hz ~ 50 Hz
SHSH• Slow exchangeSlow exchange
SHSH couple to CHCH
• When shaken with D2O, SH DisapearSH Disapear
~ 1.6 – 1.2 ppm Aliphatic SH ~ 1.6 – 1.2 ppm Aliphatic SH ~ 3.6 – 2.8 ppm Aromatic SH ~ 3.6 – 2.8 ppm Aromatic SH
Chemical Shift and Chemical Shift and CouplingCoupling
An example: An example: CC1010HH1212OO22 II = = 1010 + 1 – + 1 – 1212/2 = /2 = 55
Me-C=CMe-C=C
X 4 = 12X 4 = 12
J=8 HzJ=8 HzJ=7 HzJ=7 Hz
J=7 HzJ=7 Hz
2H2H 2H2H
2H2H 3H3H 3H3H
Me-C=Me-C=
O-CHO-CH22-CH-CH33CH3O
OCH3
CH3
O O CH3
OO
CH3
CH3
CH3
CH3
OO
2.5 2.0 1.5
2.70
Scalar coupling: Coupling through bondScalar coupling: Coupling through bond
2nI + 12nI + 1 lines linesn = 0n = 0 11 22 33
445566
111 11 1
1 2 11 2 11 3 3 11 3 3 1
1 4 6 4 11 4 6 4 11 5 10 10 5 11 5 10 10 5 1
1 6 15 20 15 6 11 6 15 20 15 6 1CH3CH3
doubletdoublet
septetseptet
aabb
aa
bb
Scalar coupling: Coupling through bondScalar coupling: Coupling through bond
2nI + 12nI + 1 lines linesn = 0n = 0 11 22 33
445566
111 11 1
1 2 11 2 11 3 3 11 3 3 1
1 4 6 4 11 4 6 4 11 5 10 10 5 11 5 10 10 5 1
1 6 15 20 15 6 11 6 15 20 15 6 1
2 x triplet2 x triplet
7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0
3.40 3.30 3.20 3.10 3.00
Br
7.50 7.40 7.30 7.20 7.10 7.00
aa
aa
bb
bb
oo
oo
mm
mm
pp
pp
Scalar coupling: Coupling through bondScalar coupling: Coupling through bond
2nI + 12nI + 1 lines linesn = 0n = 0 11 22 33
445566
111 11 1
1 2 11 2 11 3 3 11 3 3 1
1 4 6 4 11 4 6 4 11 5 10 10 5 11 5 10 10 5 1
1 6 15 20 15 6 11 6 15 20 15 6 1
2 x triplet2 x triplet1 quintet1 quintet
Br
3.0 2.5 2.0
aabb
cc
aa
bb
cc
CH2CH21.351.35
CH3CH3 CH2CH2 Roof effectRoof effect0.9 ppm0.9 ppm
4.0 3.5 3.0 2.5 2.0 1.5 1.0
Scalar coupling: Coupling through bond Scalar coupling: Coupling through bond C7 H14 O2 I = 7 -14/2 + 1 = 1C7 H14 O2 I = 7 -14/2 + 1 = 1 2nI + 12nI + 1 lines lines
n = 0n = 0 11 22 33
445566
111 11 1
1 2 11 2 11 3 3 11 3 3 1
1 4 6 4 11 4 6 4 11 5 10 10 5 11 5 10 10 5 1
1 6 15 20 15 6 11 6 15 20 15 6 1
2 x triplet2 x triplet Triplet:Triplet:
Quartet:Quartet:
Quintet:Quintet:Sixtet:Sixtet:
3H3H3H3H
(ppm)(ppm) IntInt multmult J (Hz) COMMENTJ (Hz) COMMENT0.90.9 3H3H triplettriplet 7 CH3->(7 CH3->(CH2CH2))1.11.1 3H3H triplettriplet 7 CH3->(7 CH3->(CH2CH2))1.351.35 2H2H sixtetsixtet 7 CH27 CH2 ( (CH3CH3, , CH2CH2))1.551.55 2H2H quintetquintet 7 CH27 CH2 ( (CH2CH2, , CH2CH2))2.32.3 2H2H quartetquartet 7 7 =C =C- - CH2CH2 ( (CH3CH3) ) 4.14.1 2H2H triplettriplet 77 CH2CH2 -O-O ( (CH2CH2))
CH2CH22.32.3 CH3CH3
1.11.1
2H2H 2H2H
2H2H
2H2H
CH2CH2 OO
OO
4.0 3.5 3.0 2.5 2.0 1.5 1.0
Scalar coupling: Coupling through bondScalar coupling: Coupling through bond
2nI + 12nI + 1 lines linesn = 0n = 0 11 22 33
445566
111 11 1
1 2 11 2 11 3 3 11 3 3 1
1 4 6 4 11 4 6 4 11 5 10 10 5 11 5 10 10 5 1
1 6 15 20 15 6 11 6 15 20 15 6 1
2 x triplet2 x triplet6 6 11
CH31 2
3
4O 5
CH36
OTriplet:Triplet:
44Quartet:Quartet:
55 Quintet:Quintet:
33 Sixtet:Sixtet:
22
Common first order spin system Common first order spin system 2nI + 12nI + 1 lines lines
CC CC
HHaa HHbb
CC CC
HHaa HHbb
HHbb
CC CC
HHaa HHbb
CC
HHbb
CC CC
HHaa HHbb
HHbb
HHbb
CC CC
HHaa HHbb
HHbb
HHbb
CC
HHbb
HHbb
HHbb
Common first order spin system Common first order spin system 2nI + 12nI + 1 lines lines
CC CC
HHaa HHbb
HHb’b’
CC CC
HHaa HHbb
CC
HHb’b’
CC CCCC
HHbb
HHaa HHbb
HHbb
HHcc
JJabab = = JJab’ab’
JJabab JJabab
JJab’ab’
CC CC
HHaa HHbb
CC
HHb’b’
HHbb
CC CC
HHaa HHbb
CC
HHc’c’
HHb’b’
tdtd
qdqd
Geminal Geminal CouplingCoupling
Vicinal Vicinal CouplingCoupling
33J => tool 1J => tool 1
33J => Mestrec toolJ => Mestrec tool
33J => PerchJ => Perch
Using Vicinal Using Vicinal Coupling to establish Coupling to establish
isomerisomerJJaabb
JJaadd
HaHa
JJaacc
Long Range Long Range CouplingCoupling
Long Range couplingLong Range coupling
N
H1
H7
H5
H4 44JJH1-H3H1-H3 = 1.07 Hz = 1.07 Hz
55JJH1-H4H1-H4 = 1.21 Hz = 1.21 Hz55JJH1-H5H1-H5 = 0.95 Hz = 0.95 Hz55JJH4-H7H4-H7 = 0.67 Hz = 0.67 Hz
H
H
44JJH-HH-H = 1-2 Hz = 1-2 Hz
H
H
44JJH-HH-H = 1.1 Hz = 1.1 Hz
H C C C H
44JJH-HH-H = 9 Hz = 9 Hz
H C C C H
44JJH-HH-H = 3 Hz = 3 Hz
H C C C C H
55JJH-HH-H = 3 Hz = 3 Hz
H C C C C H
55JJH-HH-H = 3 Hz = 3 Hz
Spin System in Pople notationSpin System in Pople notationStructural UnitStructural Unit Spin systemSpin system Partial spectrumPartial spectrum
-CH-CH22-CH-CH33 AA33XX22
2.5 2.0 1.5
3.0 2.5 2.0 1.5
-CH-CH-CH-CH33 AA33XX
2.0 1.5 1.0
CHCH22-CH-CH22-CH-CH33 AA33MM22XX22
Each chemical shift is represented by a letter (far way letter for very Each chemical shift is represented by a letter (far way letter for very large shift difference – compare with the size of the coupling) large shift difference – compare with the size of the coupling)
Second Order spectra:Second Order spectra:AB instead of AXAB instead of AX
JJ JJJJ
AA andand BB : center of gravity of doublet : center of gravity of doublet
Chemical shiftChemical shift
= = (1-4) * (2-3) (1-4) * (2-3)
5.05.0
4.04.0
3.03.0
2.02.0
1.01.0
0.50.5
1 2 3 41 2 3 4As the difference in shift become smaller- As the difference in shift become smaller- compare with the size of the coupling the compare with the size of the coupling the outer peaks become smaller in intensityouter peaks become smaller in intensity
SpinWorks => load ABSpinWorks => load AB
AB-SpectraAB-Spectra
AMXAMX C6 H4 O5 N2C6 H4 O5 N2I = 6 - 4/2 + 2/2 +1I = 6 - 4/2 + 2/2 +1I = 6I = 6
Phenyl = Phenyl = 4 I4 INO2 = NO2 = 1 I1 I
AA22X and AX and A22BB
SpinWorks => load ASpinWorks => load A22BB
AMXAMX
AMXAMX
JJMetaMetaParaPara
JJOrthoOrthoMetaMeta
JJOrthoOrthoParaPara
H
H
H
7.58 ppm7.58 ppm
7.2 ppm7.2 ppm
6.83 ppm6.83 ppm
Substituants : Substituants : 2 OMe (~ 3.9 ppm)2 OMe (~ 3.9 ppm)CHO (~ 9.8 ppm)CHO (~ 9.8 ppm)
H
H
H
7.58 ppm7.58 ppm
7.2 ppm7.2 ppm
6.83 ppm6.83 ppm
CHOCHO
CHOCHO
OMeOMe
OMeOMe
OMeOMeOMeOMe
8.5 8.0 7.5
Br
N+
O-
O
HA
HB
HC
HD
Calculated shiftsCalculated shiftsHHAA=8.44 =8.44 HHBB=7.82 =7.82 HHCC=7.31 =7.31 HHDD=8.19 =8.19
HHAA
HHBBHHCCHHDD
meta bromo nitro benzene
AFMXAFMX
JJ
C5 H4 N BrC5 H4 N BrI = 5 – 4/2 – 1/2 +1/2 +1I = 5 – 4/2 – 1/2 +1/2 +1I = 4 (aromatic ring)I = 4 (aromatic ring)
Assignment of Assignment of 11H NMRH NMR of: of: cartilaginealcartilagineal
CHOCHO
MeMe
CHO-9CHO-9J = 2.0 HzJ = 2.0 Hz
H8
7
6
5
4
3
2
1
9
O H
H
ClCl
ClMe
H
H
H
H-1(s)H-1(s)
H8
7
R6
H
H
dddd
JJtranstrans=17 Hz=17 Hz
JJciscis=10.5=10.5 H-5 ddH-5 dd33JJ4,54,5 = 8.5 = 8.544JJ3,5 3,5 = 1.0= 1.0
H-4 ddH-4 dd
33JJ3,43,4=15.5=15.533JJ4,54,5 =8.5 =8.5
H-3 dddH-3 ddd33JJ3,43,4=15.5=15.544JJ3,53,5 =1.0 =1.044JJ4,94,9=2.0=2.0
Complicated proton spectra : CHComplicated proton spectra : CH33-CH-CH22-S-PF-S-PF22
Almost quintetAlmost quintet
tttt44JJFHFH
33JJHHHH33JJHHHH
33JJPHPH
Identifying Identifying 3131P P couplingscouplings
P
HH
{{3131P}P}
C C
H
P P
R
dddd
NMR – From Spectra to Structures An Experimental approachNMR – From Spectra to Structures An Experimental approachSecond edition (2007) Springler-VerlagSecond edition (2007) Springler-VerlagTerence N. Mitchellm Burkhard CostisellaTerence N. Mitchellm Burkhard Costisella
Ph, 2HPh, 2H
CHCH33
1H1H1H1H CHCH22
Identifying Identifying 3131P couplings: another exampleP couplings: another example
NMR – From Spectra to Structures An Experimental approachSecond edition (2007) Springler-Verlag
Terence N. Mitchellm Burkhard Costisella
P31 NMRP31 NMR
Identifying Identifying 3131P couplings: another exampleP couplings: another example
H-nmr P31 decoupledH-nmr P31 decoupled
1H1H CHCH22
NMR – From Spectra to Structures An Experimental approachSecond edition (2007) Springler-VerlagTerence N. Mitchellm Burkhard Costisella
To identify a compound: PFTo identify a compound: PF221515NHSiNHSiHH33
Use as many techniques as possible
Proton nmr spectraProton nmr spectra is difficult to analyze with so many J’sis difficult to analyze with so many J’sBut withBut with 1919F, F, 1515NN andand 3131P P spectra it’s easier (get heteronuclear J)spectra it’s easier (get heteronuclear J)
To identify a compound: PFTo identify a compound: PF221515NHSiHNHSiH33
Use as many techniques as possible
Using decoupler : easier analysisUsing decoupler : easier analysis
Another example H{X}Another example H{X}Another example H{X}Another example H{X}
Changing the solventChanging the solvent
CDClCDCl33
CC66DD66
Changing solvent can be used to Changing solvent can be used to improve improve dispersion of chemical shiftsdispersion of chemical shifts
Changing the solventChanging the solvent
CH2
CH2
Me
OH Me=CH=CH22
CH-OHCH-OH
CHCH22
MeMe
CC66DD66
CH-OHCH-OH
=CH=CH22 CHCH22
ABABXXCDClCDCl33
DecouplingDecoupling
CH2
CH2
Me
OH Me=CH=CH22
CH-OHCH-OH
MeMe
CDClCDCl33
CHCH22
ABAB
CHCH22
ABABXX
Spin-Spin Spin-Spin DecouplingDecoupling
dqdq dqdq
dddd
Homo decouplingJJPHPH
JJHHHH
NMR – From Spectra to Structures An Experimental approachNMR – From Spectra to Structures An Experimental approachSecond edition (2007) Springler-VerlagSecond edition (2007) Springler-VerlagTerence N. Mitchellm Burkhard CostisellaTerence N. Mitchellm Burkhard Costisella
Decoupling Decoupling H-1H-1 glucose derivative glucose derivative
H-2H-2
H-1H-1
Several DecouplingSeveral Decoupling
NOENOEnOenOe
NOE: applying NOE: applying BB22 to the to the AA of an of an AAXX spin system spin system
XX11
XX22 AA22
AA11
XX11
XX22 AA22
AA11
{A}{A}
XX11 p = 2p = 2XX22 p = 2p = 2
XX11 p = 2p = 2XX22 p = 2p = 2
Immediately after irradiation, there is Immediately after irradiation, there is NO changeNO change in the in the intensity of Xintensity of XTurning on the Turning on the DecouplerDecoupler do not change population of the do not change population of the X transitionX transition
NOE: relaxation with double quantum pathway WNOE: relaxation with double quantum pathway W22 probability (positive NOE)probability (positive NOE)
XX11
XX22 AA22
AA11
{A}{A}
XX11
XX22 AA22
AA11
XX11 p = 2p = 2XX22 p = 2p = 2
XX11 p = 3p = 3XX22 p = 3p = 3
After After WW22 relaxation relaxation, there is a , there is a net increase in net increase in
the intensity of X (50%)the intensity of X (50%)
XX11
XX22 AA22
AA11
… … TT11
Dec. continueDec. continue
XX11
XX22 AA22
AA11
Relaxation takes time to establish a new equilibrium: TRelaxation takes time to establish a new equilibrium: T11 process process
delaydelay
WW22
NOE: Relaxation with zero quantum pathway WNOE: Relaxation with zero quantum pathway W00 probability (negative NOE)probability (negative NOE)
XX11
XX22 AA22
AA11
{A}{A}
XX11
XX22 AA22
AA11
XX11 p = 2p = 2XX22 p = 2p = 2
XX11 p = 1p = 1XX22 p = 1p = 1
After After WW00 relaxation relaxation, there is a net , there is a net decrease in decrease in
the intensity of X (50%) the intensity of X (50%) negative NOE negative NOE
XX11
XX22 AA22
AA11
… … TT11
Dec. continueDec. continue
XX11
XX22 AA22
AA11
Relaxation takes time to establish a new equilibrium: TRelaxation takes time to establish a new equilibrium: T11 process process
delaydelay
WW00WW00
NOE: summary of relaxation pathwaysNOE: summary of relaxation pathways
WW11: probability of single quantum : probability of single quantum
relaxation do not create nOerelaxation do not create nOe
A new population ditribution is generated A new population ditribution is generated by relaxation through dipole-dipole by relaxation through dipole-dipole
relaxation : double quantum and zero relaxation : double quantum and zero quantum pathway quantum pathway WW22 and and WW00
If If WW2 2 is efficientis efficient ( (small molecule – fast motion small molecule – fast motion large frequency large frequency ))
Level Level increase increase level level increase also with decoupler continuing increase also with decoupler continuing
XX11
XX22 AA22
AA11
WW22WW00
WW22 pathway yield positive nOe pathway yield positive nOe
If If WW0 0 is efficientis efficient ( (large molecule – slow motion large molecule – slow motion small freq. Diff.small freq. Diff.))
Level Level increase increase level level increase also with decoupler continuing increase also with decoupler continuing
WW00 pathway yield negative nOe pathway yield negative nOe
NOE is a kinetic effect: need delay ~ TNOE is a kinetic effect: need delay ~ T11
It take time to developIt take time to develop It takes time to decayIt takes time to decay
NOE difference: nOe-dNOE difference: nOe-d
d1d1 AQAQ
Dec on frqDec on frq
d1d1 AQAQ
Dec off frqDec off frq
CC CCClCl
HH
MeMe
MeMe
irrirr
differencedifference
irrirr controlcontrol
nOenOe
NOENOE
COOH
HCH3
CH3
{Me –cis} => +19%{Me –cis} => +19%
{Me –trans} => -2%{Me –trans} => -2%
{Ha} => +45%{Ha} => +45%
2
3
1
4
6
5
OH
CH3
Ph
H
OMe
H
H
H6H6H5H5
2
3
1
4
6
5
OMe
CH3
Ph
H
OH
H
H
Choosing a structure by nOeChoosing a structure by nOe
H3H3
{OMe}{OMe}
{OH}{OH}
O
O
CH
O
P
CH3
CH3
Cl
Cl
NOEd
NMR – From Spectra to Structures An Experimental approachNMR – From Spectra to Structures An Experimental approachSecond edition (2007) Springler-VerlagSecond edition (2007) Springler-VerlagTerence N. Mitchellm Burkhard CostisellaTerence N. Mitchellm Burkhard Costisella
NOEd exampleNOEd example
Organometallic compoundsOrganometallic compounds Proton - NMRProton - NMR Increasing the 1 s orbital density increases the shieldingIncreasing the 1 s orbital density increases the shielding
M = CM = C M = SiM = Si M = GeM = Ge
MH4 0.10.1 3.23.2 3.13.1
MH3I 2.02.0 3.43.4 3.53.5
MH3Br 2.52.5 4.24.2 4.54.5
MH3Cl 2.82.8 4.64.6 5.15.1
(MH3)2O 3.23.2 4.64.6 5.35.3
MH3F 4.14.1 4.84.8 5.75.7
Shift to low field when the metal is heavier (Shift to low field when the metal is heavier (SnHSnH44 - - = 3.9 ppm = 3.9 ppm))
Proton – NMR : Chemical shiftProton – NMR : Chemical shift
• Further contribution to shielding / deshieldingshielding / deshielding is the anisotropicanisotropic magnetic susceptibility from neighboring groups (e.g. AlkenesAlkenes, Aromatic ringsAromatic rings -> deshielding in the plane of the bound)
• In transition metal complexes there are often low-lying excited electronic states. When magnetic field is applied, it has the effect of mixing these to some extent with the ground state.
• Therefore the Therefore the paramagnetic term is important for those nuclei themselvesparamagnetic term is important for those nuclei themselves => => large high frequency shifts (low field).large high frequency shifts (low field). The protonsprotons bound to these will be bound to these will be shielded (shielded ( => 0 to -40 ppm => 0 to -40 ppm)) (these resonances are good diagnostic. )
• For transition metal hydride this range should be extended to 70 ppm!For transition metal hydride this range should be extended to 70 ppm!
• If paramagnetic species are to be included, the range can go to 1000 ppm!!If paramagnetic species are to be included, the range can go to 1000 ppm!!
Exchange : DNMR – Dynamic NMRExchange : DNMR – Dynamic NMRNMR is a convenient way to study rate of reactions – provided that the lifetime of participating species are comparable to NMR time scale (1010-5-5 s s)
H
H
H
H
H
GeMe3
At low temperature, hydrogens form an A2B2XA2B2X spin system
At higher temperature germaniumgermanium hop from one C to the next
Index NMR-basicsNMR-basicsNMR-basicsNMR-basics NMR-SymmetryNMR-Symmetry