NMR Nuclear Magnetic Resonance
NMR for Organometallic compounds
Index NMR-basics H-NMR NMR-Symmetry Heteronuclear-NMR
Dynamic-NMR NMR and Organometallic compounds
NMR in Organometallic compoundsspins 1/2 nuclei
For small molecules having nuclei I=1/2 : Sharp lines are expected
W1/2 (line width at half height) = 0-10 Hz
If the nuclei has very weak interactions with the environment,
Long relaxation time occur (109Ag => T1 up to 1000 s !!!)
This makes the detection quite difficult!
Isotope Nat. Abun-dance %
() 107 rad T-1 s-1
Frequency (MHz)
Rel. Receptivity
1H 99.985 26.7519 100.0 1.003H - 28.535 106.7 --
3He 0.00013 -20.380 76.2 5.8 * 10-7
13C 1.11 6.7283 25.1 1.8 * 10-4
15N 0.37 -2.712 10.1 3.9 * 10-6
19F 100.0 25.181 94.1 8.3 * 10-1
29Si 4.7 -5.3188 19.9 3.7 * 10-4
31P 100.0 10.841 40.5 6.6 * 10-2
57Fe 2.2 0.8661 3.2 7.4 * 10-7
77Se 7.6 5.12 19.1 5.3 * 10-4
89Y 100.0 -1.3155 4.9 1.2 * 10-4
103Rh 100.0 -0.846 3.2 3.2 * 10-5
107Ag 51.8 -1.087 4.0 3.5 * 10-5
109Ag 48.2 -1.250 4.7 4.9 * 10-5
111Cd 12.8 -5.6926 21.2 1.2 * 10-3
113Cd 12.3 -5.9550 22.2 1.3 * 10-3
NMR in Organometallic compoundsNMR properties of some spins 1/2 nuclei
Index
Isotope Nat. Abundance
%
Magnetogyric ratio ()
107 rad T-1 s-1
Relative NMR
frequency (MHz)
Rel. Receptivity
117Sn 7.6 -9.578 35.6 3.5 * 10-3
119Sn 8.6 -10.021 37.3 4.5 * 10-3
125Te 7.0 -8.498 31.5 2.2 * 10-3
129Xe 26.4 -7.441 27.8 5.7 * 10-3
169Tm 100.0 -2.21 8.3 5.7 * 10-4
171Yb 14.3 4.712 17.6 7.8 * 10-4
183W 14.4 1.120 4.2 1.1 * 10-5
187Os 1.6 0.616 2.3 2.0 * 10-7
195Pt 33.8 5.768 21.4 3.4 * 10-3
199Hg 16.8 4.8154 17.9 9.8 * 10-4
203Tl 29.5 15.436 57.1 5.7 * 10-2
205Tl 70.5 15.589 57.6 1.4 * 10-1
207Pb 22.6 5.540 20.9 2.0 * 10-3
Spin 1/2
Multinuclear NMR
• There are at least four other factors we must consider• Isotopic Abundance. Some nuclei such as 19F and 31P are 100% abundant
(1H is 99.985%), but others such as 17O have such a low abundance (0.037%). Consider: 13C is only 1.1% abundant (need more scans than proton).
• Sensitivity goes with the cube of the frequency. 103Rh (100% abundant but only 0.000031 sensitivity): obtaining a spectrum for the nucleus is generally impractical. However, the nucleus can still couple to other spin-active nuclei and provide useful information. In the case of rhodium, 103Rh coupling is easily observed in the 1H and 13C spectra and the JRhX can often be used to assign structures
• Nuclear quadrupole. For spins greater than 1/2, the nuclear quadrupole moment is usually larger and the line widths may become excessively large.
• Relaxation time
NMR in Organometallic compoundsspins > 1/2 nuclei
These nuclei possess a quadrupole moment (deviation from spherical charge distribution) which cause extremely short relaxation time and extremely large linewidth W1/2 (up to 50 KHz)
W1/2 ~ (2I + 3) Q2 q2
zz tc
I2 (2I -1)
Q = quadrupole momentqzz = electric field gradienttc = correlation timeI = spin quantum number
Narrow lines can be obtained for low molecular weight (small tc)and if nuclei are embedded in ligand field of cubic (tetrahedral, octahedral) symmetry (qzz blocked)
NMR properties of some spins quadrupolar nuclei
Isotope Spin Abun-dance %
() 107 rad T-1 s-1
Freq. (MHz)
Rel. Recep-tivity
Quadrupole moment10-28
m2
2H 1 0.015 4.1066 15.4 1.5 * 10-6 2.8 * 10-3
6Li 1 7.4 3.9371 14.7 6.3 * 10-4 -8 * 10-4
7Li 3/2 92.6 10.3975 38.9 2.7 * 10-1 -4 * 10-2
9Be 3/2 100.0 -3.7596 14.1 1.4 * 10-2 5 * 10-2
10B 3 19.6 2.8746 10.7 3.9 * 10-3 8.5 * 10-2
11B 3/2 80.4 8.5843 32.1 1.3 * 10-1 4.1 * 10-2
14N 1 99.6 1.9338 7.2 1.0 * 10-3 1 * 10-2
17O 5/2 0.037 -3.6279 13.6 1.1 * 10-5 -2.6 * 10-2
23Na 3/2 100.0 7.0801 26.5 9.3 * 10-2 1 * 10-1
25Mg 5/2 10.1 -1.639 6.1 2.7 * 10-4 2.2 * 10-1
27Al 5/2 100.0 6.9760 26.1 2.1 * 10-1 1.5 * 10-1
33S 3/2 0.76 2.055 7.7 1.7 * 10-5 -5.5 * 10-2
35Cl 3/2 75.5 2.6240 9.8 3.6 * 10-3 -1 * 10-1
37Cl 3/2 24.5 2.1842 8.2 6.7 * 10-4 -7.9 * 10-2
39K 3/2 93.1 1.2498 4.7 4.8 * 10-4 4.9 * 10-2
47Ti 5/2 7.3 -1.5105 5.6 1.5 * 10-4 2.9 * 10-1
49Ti 7/2 5.5 -1.5109 5.6 2.1 * 10-4 2.4 * 10-1
51V 7/2 99.8 7.0453 26.3 3.8 * 10-1 -5 * 10-2
55Mn 5/2 100.0 6.608 24.7 1.8 * 10-1 4 * 10-1
Quadrupolar nuclei: Oxygen-17
NMR – From Spectra to Structures An Experimental approachSecond edition (2007) Springler-VerlagTerence N. Mitchellm Burkhard Costisella
Notable nuclei• 19F: spin ½, abundance 100%, sensitivity (H=1.0) : 0.83
2JH-F = 45 Hz, 3JH-F trans = 17 Hz, 3JH-F Cis = 6 Hz 2JF-F = 300 Hz, 3JF-F = - 27 Hz
• 29Si: spin ½, abundance 4.7%, sensitivity (H=1.0) : 0.0078The inductive effect of Si typically moves 1H NMR aliphatic resonances upfield to approximately 0 to 0.5 ppm, making assignment of Si-containing groups rather easy. In addition, both carbon and proton spectra display Si satellites comprising 4.7% of the signal intensity.
• 31P: spin ½, abundance 100%, sensitivity (H=1.0) : 0.07 1JH-P = 200 Hz, 2JH-P ~2-20 Hz, 1JP-P = 110 Hz, 2JF-P ~ 1200-1400 Hz, 3JP-P = 1-27 Hzthe chemical shift range is not as diagnostic as with other nuclei, the magnitude of the X-P coupling constants is terrific for the assignment of structuresKarplus angle relationship works quite well
Notable nuclei• 31P: spin ½, abundance 100%, sensitivity (H=1.0) : 0.07
1JH-P = 200 Hz, 2JH-P ~2-20 Hz, 1JP-P = 110 Hz, 2JF-P ~ 1200-1400 Hz, 3JP-P = 1-27 Hzthe chemical shift range is not as diagnostic as with other nuclei, the magnitude of the X-P coupling constants is terrific for the assignment of structuresKarplus angle relationship works quite well
2JH-P is 153.5 Hz for the phosphine trans to the hydride, but only 19.8 Hz to the (chemically equivalent) cis phosphines.
See Selnau, H. E.; Merola, J. S. Organometallics, 1993, 5, 1583-1591.
Notable nuclei• 103Rh: spin ½, abundance 100%, sensitivity (H=1.0) : 0.000031
1JRh-C = 40-100 Hz, 1JRh-C(Cp) = 4 Hz,
For example, in the 13C NMR spectrum of this linked Cp, tricarbonyl Rh dimer at 240K (the dimer undergoes fluxional bridge-terminal exchange at higher temperatures),
the bridging carbonyl is observed at d232.53 and is a triplet with 1JRh-C = 46 Hz. The equivalent terminal carbonyls occur as a doublet at d190.18 with 1JRh-C = 84 Hz:
See Bitterwolf, T. E., Gambaro, A., Gottardi, F., Valle G Organometallics, 1991, 6, 1416-1420.
Chemical shift for organometallic
In molecules, the nuclei are screened by the electrons. So the effective field at the nucleus is:
Beff = B0(1-)Where is the shielding constant.
The shielding constant has 2 terms: d (diamagnetic) and p (paramagnetic)
d - depends on electron distribution in the ground state
p - depends on excited state as well. It is zero for electrons in s-orbital.
This is why the proton shift is dominated by the diamagnetic term. But heavier nuclei are dominated by the paramagnetic term.
Index
Symmetry
Si
ClBr
H H
Cl
PtBr
PPh3
PPh3 PtBr
PPh3
Cl
PPh3
P31 P31H are equivalents are non-equivalent are equivalent
Non-equivalent nuclei could "by accident" have the same shift and this could cause confusion.
Some Non-equivalent group might also become equivalent due to some averaging process that is fast on NMR time scale. (rate of exchange is greater than the chemical shift difference)
e.g. PF5 : Fluorine are equivalent at room temperature (equatorial
and axial positions are exchanging by pseudorotation)
Index
Symmetry in Boron compounds
Proton - NMR Increasing the 1 s orbital density increases the shielding
M = C M = Si M = Ge
MH4 0.1 3.2 3.1
MH3I 2.0 3.4 3.5
MH3Br 2.5 4.2 4.5
MH3Cl 2.8 4.6 5.1
(MH3)2O 3.2 4.6 5.3
MH3F 4.1 4.8 5.7
Shift to low field when the metal is heavier (SnH4 - = 3.9 ppm)
Index
Proton – NMR : Chemical shift
• Further contribution to shielding / deshielding is the anisotropic magnetic susceptibility from neighboring groups (e.g. Alkenes, Aromatic 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 paramagnetic term is important for those nuclei themselves => large high frequency shifts (low field). The protons bound to these will be shielded ( => 0 to -40 ppm) (these resonances are good diagnostic. )
• 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!!
Index
Proton NMR and other nuclei
• The usual range for proton NMR is quite small if we compare to other nuclei:
• 13C => 400 ppm• 19F => 900 ppm• 195Pt => 13,000 ppm !!!
• Advantage of proton NMR : Solvent effects are relatively small
• Disadvantage: peak overlap
Index
Chemical shifts of other element
There is no room to discuss all chemical shifts for all elements in the periodical table. The discussion will be limited to 13C, 19F, 31P *as these are so widely used.
Alkali Organometallics (lithium) will be briefly discuss
For heavier non-metal element we will discuss 77Se and 125Te.
For transition metal, we will discuss 55Mn and 195Pt
Index
Alkali organometallics: Organolithium
For Lithium: we have the choice between 2 nuclei:
6Li : Q=8.0*10-4 a=7.4% I=17Li : Q=4.5*10-2 a=92.6% I=3/2
6Li : Higher resolution 7Li : Higher sensitivity
7Li NMR : larger diversity of bonding compare to Na-Cs (ionic)
• Solvent effects are important (solvating power affects the polarity of Li-C bond and govern degree of association
• d covers a small range: 10 ppm• Covalent compound appear at low field (2 ppm range)• Coupling 1JC-Li between carbon and Lithium indicate covalent bond
Organolithium
Boron NMR
For Boron: we have the choice between 2 nuclei:
10B : Q= 8.5 * 10-2 a=19.6% I=311B : Q= 4.1 * 10-2 a=80.4% I=3/2
11B : Higher sensitivity
Boron NMR
Boron NMR
11B coupling with Fluorine: 19F-NMR
10B : Q= 8.5 * 10-2 a=19.6% n=10.7 I=3
NaBF4 / D2O
19F-NMR
2nI+1 = 7
2nI+1 = 4
11BF4
10BF4
Isotopic shift
11B : Q= 4.1 * 10-2 a=80.4% n=32.1 I=3/2
Boron can couple to other nuclei as shown here on 19F-NMR
JF-10B
JF-11B
= n10B
n11B
JBF=0.5 Hz JBF=1.4 Hz
C13 shifts
Saturated Carbon appear between 0-100 ppm with electronegative substituents increasing the shifts. CH3-X : directly related to the electronegativity of X.
The effects are non-additive: CH2XY cannot be easily predicted Shifts for aromatic compounds appear between 110-170 ppm -bonded metal alkene may be shifted up to 100 ppm: shift
depends on the mode of coordination one extreme shift is CI4 = -293 ppm !!! Metal carbonyls are found between 170-290 ppm. (very long
relaxation time make their detection very difficult) Metal carbene have resonances between 250-370 ppm
Index
F-19 shifts
• electronegativity• Oxidation state of neighbor• Stereochemistry• Effect of more distant group
Wide range: 900 ppm! And are not easy to interpret. The accepted reference is now: CCl3F. With literature chemical shift,
care must be taken to ensure they referenced their shifts properly. Sensitive to:
Index
F-19 shiftsThe wide shift scale allow to observe all the products in the reaction
of : WF6 + WCl6 --> WFnCln-6 (n=1-6)
WF
F
FF
F
FW
FF
FCl
F
FW
FCl
FCl
F
F
WF
F
FCl
F
Cl
WCl
Cl
FCl
F
FW
ClF
FCl
F
ClW
ClCl
ClCl
F
FW
ClCl
FCl
F
ClW
ClCl
FCl
Cl
Cl
Index
Sn shifts
H-NMR of Sn compound
3 isotopes with spin ½ :
Sn-115 a=0.35%
Sn-117 a=7.61%
Sn-119 a=8.58%
2JSN117-H
2JSN119-H = 54.3 Hz
2JSN119-H = 1.046 * 2JSN117-H
(ratio of g of the 2 isotopes)
NMR – From Spectra to Structures An Experimental approachSecond edition (2007) Springler-VerlagTerence N. Mitchellm Burkhard Costisella
Sn-1193 isotopes with spin ½ :
Sn-115 a=0.35%
Sn-117 a=7.61%
Sn-119 a=8.58%
NMR – From Spectra to Structures An Experimental approachSecond edition (2007) Springler-VerlagTerence N. Mitchellm Burkhard Costisella
Sn-119 coupling
Sn-117 a=7.61%Sn-119 a=8.58%
1- molecule containing 1 Sn-119
2- molecule containing Sn119, Sn117 J between Sn-119 and Sn-1173- molecule containing two Sn119 Form an AB spectra (J=684 Hz)4- molecule containing Sn119 and C13 J between Sn119 and C13
Dynamic NMR
p261
C13
Cycloheptatriene
Dynamic NMR
1H-NMR
P-31 Shifts
• - 460 ppm for P4
• +1,362 ppm phosphinidene complexe: tBuP[Cr(CO)5]2
• Interpretation of the shifts is not easy : there seems to be many contributing factors
• PIII covers the whole normal range: strongly substituent dependant
• PV narrower range: - 50 to + 100.• Unknown can be predicted by extrapolation or interpolation• PX2Y or PY3 can be predicted from those for PX3 and PXY2
• The best is to compare with literature values.
The range of shifts is ± 250 ppm from H3PO4 Extremes:
Index
P-31 Shifts
Index
There are many analogies between Phosphorus and Selenium chemistry.
There are also analogies between the chemical shifts of 31P and 77Se but the effect are much larger in Selenium!
For example:Se(SiH3)2 and P(SiH3)3 are very close to the low frequency limit (high field)
The shifts in the series SeR2 and PR3 increase in the order R= Me < Et < Pri < But
There is also a remarkable correlation between 77Se and 125Te. (see picture next slide)
Other nuclei: Selenium, Telurium
Index
Correlation between Tellurium and Selenium Shifts
Index
Manganese-55
Manganese-55 can be easily observed in NMR but due to it’s large quadrupole moment it produces broad lines • 10 Hz for symmetrical environment e.g. MnO4
-
• 10,000 Hz for some carbonyl compounds. • It’s shift range is => 3,000 ppm
• As with other metals, there is a relationship between the oxidation state and chemical shielding
• Reference: MnVII : d = 0 ppm (MnO4-)
• MnI : d –1000 to –1500• Mn-I : d –1500 to -3000
55Mn chemical shifts seems to reflect the total electron density on the metal atom
Index
Pt-195 Shifts
Platinum is a heavy transition element. It has wide chemical shift scale: 13,000 ppm!
The shifts depends strongly on the donor atom but vary little with long range. For example: PtCl2(PR3)2 have very similar shifts with different R
Many platinum complexes have been studied by 1H, 13C and 31P NMR. But products not involving those nuclei can be missed : PtCl4
2-
Major part of Pt NMR studies deals with phosphine ligands as these can be easily studied with P-31 NMR.
Index
Lines are broad (large CSA) large temperature dependence (1 ppm per degree)
I = ½ a=33.8% K2PtCl6 ref set to 0. Scale: -6000 to + 7000 ppm !!
Pt-195 : coupling with protonsCSA relaxation on 195Pt can have unexpected influence on proton satellites. CSA relaxation increases with the square of the field. If the relaxation (time necessary for the spins to changes their spin state) is fast compare to the coupling, the coupling can even disapear!
N+
CO2-
Pt
Cl
Cl
H
H
H
H
CH2=CH2
1H-NMR
a=33.8%
Pt-195 I = ½ a=33.8%
H6 : dd
J5-6 = 6.2 HzJ4-6 = 1.3 Hz
JH6-Pt195 = 26 Hz
NMR – From Spectra to Structures An Experimental approachSecond edition (2007) Springler-VerlagTerence N. Mitchellm Burkhard Costisella
Pople Notation
Si Si
H
H
HH
ClCl
A B3
P
HF
F
A M 2 X
I
FF
FF
F
A B4
Spin > ½ are generally omitted.
Index
Effect of Coupling with exotic nuclei in NMR Natural abundance 100%
1H, 19F, 31P, 103Rh : all have 100% natural abundance.
When these nuclei are present in a molecule, scalar coupling must be present. Giving rise to multiplets of n+1 lines.
One bond coupling can have hundreds or thousands of Hz.
They are an order of magnitude smaller per extra bound between the nuclei involved. Usually coupling occur up to 3-4 bounds.
Example:
P(SiH3)3 + LiMe -> Product : P-31 NMR shows septet ===>
product is then P(SiH3)2-
Index
P-31 Spectrum of PF2H(NH2)2 labeled with 15N
coupling with H (largest coupling : Doublet) then we see triplet with large coupling with fluorine With further Coupling to 2 N produce triplets, further coupled to 4protons => quintets
2 x 3 x 3 x 5 = 90 lines !
t
1JP-F
1JP-F
t1JP-HTriplet 1JP-N
Quintet 2JP-H
Effect of Coupling with exotic nuclei in NMR
• For example: WF6 as 183W has 14% abundance, the fluorine spectra should show satellite signals separated by the coupling constant between fluorine and tungsten. The central signal has 86% intensity and the satellites have 14%. This will produce 1:12:1 pattern
Low abundance nuclei of spin 1/2
13C, 29Si, 117Sn, 119Sn, 183W : should show scalar coupling
=> satellite signals around the major isotope.
Index
Si-29 coupling • 29Si has 5% abundance.
• For H3Si-SiH3 , the chance of finding
• H3-28Si--29Si-H3 is 10%. Interestingly we can see that the two kind of protons are no longer equivalent so homonuclear coupling become observable! The molecule with 2 Si-29 is present with 0.25% intensity and is difficult to observe.
• The second group gives smaller coupling
Index
Coupling with Platinum 195Pt the abundance is 33%.
Platinum specie will give rise to satellite signal with a relative ratio of 1 : 4 : 1. This intensity pattern is diagnostic for the presence of platinum.
If the atom is coupled to 2 Pt, the situation is more complex:
2/3 x 2/3 => no Pt spin (central resonance)
1/3 x 1/3 => two Pt with spin 1/2 => triplet
remaining molecule has 2x (1/3 x 2/3) = 4/9 => one Pt with spin 1/2 => doublet
Adding the various components together we now have 1:8:18:8:1 pattern. The weak outer lines are often missed, leaving what appear to be a triplet 1:2:1 !!!
Index
Carbon-13 in organometallic NMR13C is extremely useful to organometallic NMR
For example:
Palladium complexe has:
• 4 non-equivalent Methyls
• 2 methylenes
• Allyl : 1 methylene, 2 methynyl
• Phenyl: 4 C: mono-subst.
Index
29Si-NMR
Polymeric siloxanes are easily studied by NMR: These have • terminal R3SiO-• Chain R2Si (O-)2
• Branch R-Si(O-)3
• Quaternary Si(O-)4
All these Silicon have different shifts making it possible to study the degree of polymerization and cross-linking
Index
Coupling with Quadrupolar Nuclei (I>1/2)
• 2nI + 1 lines• The observation
of such coupling depends on the relaxation rate of the quadrupolar nuclei (respect to coupling constant)
Index
Coupling with Quadrupolar Nuclei (I>1/2)
Factors contributing to Coupling constant
• Magnetic Moment of one nuclei interact with the field produced by orbital motion of the electrons – which in turn interact with the second nuclei.
• There is a dipole interaction involving the electron spin magnetic moment
• There is also a contribution from spins of electrons which have non-zero probability of being at the nucleus => Fermi contact
Index
1-bound coupling• Depends on s-orbital character of the bound
– Hybridization of the nuclei involved1JCH => 125 (sp3), 160 (sp2), 250 (sp)
• Electronegativity is another factor: increase the coupling– CCl3H => 1JCH = 209 Hz
• Coupling can be used to determine coordination number of PF , PH compounds, and to distinguish axial, equatorial orientation of Fluorines.– 1JPH = 180 (3 coordinate) , 1JPH = 400 (4 coordinate)
• Coupling can also be used to distinguish single double bond– E.g.
P
RR
Se
R
P
Se
RR
R
Index
2-bound coupling
• 2J can give structural information: There is a relationship between 2J and Bond angle
• => coupling range passes through zero. Therefore the sign of the coupling must be determined
Pt2-
P
P
XX Pt2-
P
X
PX
Trans Cis
J (trans) > J (cis)
Index
3-bound coupling
• Depends on Dihedral angle
3JXY = A cos 2f + B cos f + C
A, B, C : empirical constants
Index
Complicated proton spectra : CH3-CH2-S-PF2
Almost quintet
Index
Complicated Fluorine spectra : PF2-S-PF2
Second order spectra: 19FChemically equivalentMagnetically non-equivalent1JPF different from 3JPH
This type of spectra is frequent in transition metal complex:MCl2(PR3)2
Index
Equivalence and non-equivalence
P
O
O
PhO
PF
F P
FF
F are Non-EquivalentThe 2 phosphorus are Pro-chiral: non-equivalent
Index
To identify a compound: PF215NHSiH3
Use as many techniques as possible
Proton nmr spectra is difficult to analyze with so many J’sBut with 19F, 15N and 31P spectra it’s easier (get heteronuclear J)
Index
To identify a compound: PF215NHSiH3
Use as many techniques as possible
Using decoupler : easier analysis
Index
Multinuclear Approach
Proton NMR spectra: 3 groups of peaks integrating for 12:4:1
Resonances due to Methyl and CH2 have coupling with 31PAnd also shows satellites due to mercury coupling (199Hg 16.8%)
While third resonance is broad
In 31P, there is a single signal: Symmetrical compound: that has Mercury satellites
In 199Hg NMR (with proton decoupling): quintet demonstrate the presence of 4 Phosphorus
Index
Heteronuclear NOE
• NOE enhancement can give useful gain in signal-to-noise• It is most efficient when the heteronuclei is bound to proton
NOEMAX = 1 + gH/2gX
• For nuclei having negative g, NOE is negative (for 29Si, max=-1.5)
Index
Exchange : DNMR – Dynamic NMR
NMR is a convenient way to study rate of reactions – provided that the lifetime of participating species are comparable to NMR time scale (10-
5 s)H
H
H
H
H
GeMe3
At low temperature, hydrogens form an A2B2X spin system
At higher temperature germanium hop from one C to the next
Index
Paramagnetic compounds in NMR
Usually paramegnetic compounds are too braod => give ESRIn NMR, Chemical shift is greatly expanded
Paramagnetic shifts are made up of 2 component:
1. Through space Dipolar interaction between the magnetic moment of the electron and of the nucleus
2. Contact Shift: coupling between electron and nucleus. This interaction would give a doublet in NMR but J ~ millions of Hertz!!With such large coupling, intensity of the 2 resonances are not equal => weighted mean position is not midwayWith fast relaxation, collapse of the multiplet may fall thousands Hertz away from expected position => Contact Shift
Contact Shift give a measure of unpaired spin density at resonating Nucleus.Useful for studying spin distribution in organic radical or in ligands in organo metallic complexes
Paramagnetic compounds in NMR4 sets of resonances: 1 symmetrical Fac: the 3 ligand are identical
3 Asymetrical ligand in Mer occur with 3 time the probability.
Index
NMR-basics
H-NMR
NMR-Symmetry
Heteronuclear-NMR
Dynamic-NMR
NMR and Organometallic compounds
Special 1D-NMR