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NMR Nuclear Magnetic Resonance

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Title: NMR Nuclear Magnetic Resonance


1
NMR Nuclear Magnetic Resonance
  • NMR for Organometallic compounds

Index
NMR-basics
H-NMR
NMR-Symmetry
Heteronuclear-NMR
Dynamic-NMR
NMR and Organometallic compounds
2
NMR in Organometallic compoundsspins 1/2 nuclei
For small molecules having nuclei I1/2 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
gt T1 up to 1000 s !!!) This makes the detection
quite difficult!
3
NMR in Organometallic compoundsNMR properties of
some spins 1/2 nuclei
Index
4
Spin 1/2
5
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

6
NMR in Organometallic compoundsspins gt 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)
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)
7
NMR properties of some spins quadrupolar nuclei
8
Quadrupolar nuclei Oxygen-17
NMR From Spectra to Structures An Experimental
approachSecond edition (2007) Springler-Verlag Te
rence N. Mitchellm Burkhard Costisella
9
Notable nuclei
  • 19F spin ½, abundance 100, sensitivity (H1.0)
    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 (H1.0)
    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 (H1.0)
    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

10
Notable nuclei
  • 31P spin ½, abundance 100, sensitivity (H1.0)
    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.
11
Notable nuclei
  • 103Rh spin ½, abundance 100, sensitivity
    (H1.0) 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.
12
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
13
Symmetry
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
14
Symmetry in Boron compounds
15
Proton - NMR
Increasing the 1 s orbital density increases the
shielding
Shift to low field when the metal is heavier
(SnH4 - ? 3.9 ppm)
Index
16
Proton NMR Chemical shift
  • Further contribution to shielding / deshielding
    is the anisotropic magnetic susceptibility from
    neighboring groups (e.g. Alkenes, Aromatic rings
    -gt 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 gt large high frequency
    shifts (low field). The protons bound to these
    will be shielded (? gt 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
17
Proton NMR and other nuclei
  • The usual range for proton NMR is quite small if
    we compare to other nuclei
  • 13C gt 400 ppm
  • 19F gt 900 ppm
  • 195Pt gt 13,000 ppm !!!
  • Advantage of proton NMR Solvent effects are
    relatively small
  • Disadvantage peak overlap

Index
18
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
19
Alkali organometallics Organolithium
For Lithium we have the choice between 2 nuclei
6Li Q8.010-4 a7.4 I1 7Li
Q4.510-2 a92.6 I3/2
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

20
Organolithium
21
Boron NMR
For Boron we have the choice between 2 nuclei
10B Q 8.5 10-2 a19.6 I3 11B Q 4.1
10-2 a80.4 I3/2
11B Higher sensitivity
22
Boron NMR
23
Boron NMR
24
11B coupling with Fluorine 19F-NMR
10B Q 8.5 10-2 a19.6 n10.7 I3
2nI1 7
11B Q 4.1 10-2 a80.4 n32.1 I3/2
2nI1 4
Boron can couple to other nuclei as shown here on
19F-NMR
Isotopic shift
19F-NMR
11BF4
NaBF4 / D2O
10BF4
JBF0.5 Hz
JBF1.4 Hz
25
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
26
F-19 shifts
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
  • electronegativity
  • Oxidation state of neighbor
  • Stereochemistry
  • Effect of more distant group

Index
27
F-19 shifts
The wide shift scale allow to observe all the
products in the reaction of WF6 WCl6 --gt
WFnCln-6 (n1-6)
Index
28
Sn shifts
29
H-NMR of Sn compound
NMR From Spectra to Structures An Experimental
approachSecond edition (2007) Springler-Verlag Te
rence N. Mitchellm Burkhard Costisella
3 isotopes with spin ½ Sn-115 a0.35 Sn-117
a7.61 Sn-119 a8.58
2JSN119-H 1.046 2JSN117-H
(ratio of g of the 2 isotopes)
2JSN119-H 54.3 Hz
2JSN117-H
30
Sn-119
NMR From Spectra to Structures An Experimental
approachSecond edition (2007) Springler-Verlag Te
rence N. Mitchellm Burkhard Costisella
3 isotopes with spin ½ Sn-115 a0.35 Sn-117
a7.61 Sn-119 a8.58
31
Sn-119 coupling
1- molecule containing 1 Sn-119
2- molecule containing Sn119, Sn117 J
between Sn-119 and Sn-117
3- molecule containing two Sn119 Form an AB
spectra (J684 Hz)
4- molecule containing Sn119 and C13 J between
Sn119 and C13
Sn-117 a7.61 Sn-119 a8.58
32
Dynamic NMR
p261
33
C13
34
Cycloheptatriene
35
Dynamic NMR
36
1H-NMR
37
P-31 Shifts
The range of shifts is 250 ppm from H3PO4
Extremes
  • - 460 ppm for P4
  • 1,362 ppm phosphinidene complexe
    tBuPCr(CO)52
  • 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.

Index
38
P-31 Shifts
Index
39
Other nuclei Selenium, Telurium
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 lt Et lt Pri lt But There is also a
remarkable correlation between 77Se and 125Te.
(see picture next slide)
Index
40
Correlation between Tellurium and Selenium Shifts
Index
41
Manganese-55
  • Manganese-55 can be easily observed in NMR but
    due to its large quadrupole moment it produces
    broad lines
  • 10 Hz for symmetrical environment e.g. MnO4-
  • 10,000 Hz for some carbonyl compounds.
  • Its shift range is gt 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
42
Pt-195 Shifts
I ½ a33.8 K2PtCl6 ref set to 0. Scale
-6000 to 7000 ppm !!
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 PtCl42-
Major part of Pt NMR studies deals with
phosphine ligands as these can be easily studied
with P-31 NMR.
Lines are broad (large CSA) large temperature
dependence (1 ppm per degree)
Index
43
Pt-195 coupling with protons
CSA 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!
1H-NMR
CH2CH2
a33.8
44
Pt-195
NMR From Spectra to Structures An Experimental
approachSecond edition (2007) Springler-Verlag Te
rence N. Mitchellm Burkhard Costisella
I ½ a33.8
H6 dd
J4-6 1.3 Hz
J5-6 6.2 Hz
JH6-Pt195 26 Hz
45
Pople Notation
Spin gt ½ are generally omitted.
Index
46
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 n1 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 -gt Product P-31 NMR
shows septet gt product is then P(SiH3)2-
Index
47
P-31 Spectrum of PF2H(NH2)2 labeled with 15N
t
1JP-H
t
Triplet 1JP-N Quintet 2JP-H
1JP-F
1JP-F
2 x 3 x 3 x 5 90 lines !
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 gt quintets
48
Effect of Coupling with exotic nuclei in NMR
Low abundance nuclei of spin 1/2
13C, 29Si, 117Sn, 119Sn, 183W should show
scalar coupling gt satellite signals around the
major isotope.
  • 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 1121 pattern

Index
49
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
50
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 gt no Pt spin (central
resonance) 1/3 x 1/3 gt two Pt with spin 1/2 gt
triplet remaining molecule has 2x (1/3 x 2/3)
4/9 gt one Pt with spin 1/2 gt doublet  Adding
the various components together we now have
181881 pattern. The weak outer lines are
often missed, leaving what appear to be a triplet
?121 !!!
Index
51
Carbon-13 in organometallic NMR
13C 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
52
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
53
Coupling with Quadrupolar Nuclei (Igt1/2)
  • 2nI 1 lines
  • The observation of such coupling depends on the
    relaxation rate of the quadrupolar nuclei
    (respect to coupling constant)

Index
54
Coupling with Quadrupolar Nuclei (Igt1/2)
55
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 gt Fermi contact

Index
56
1-bound coupling
  • Depends on s-orbital character of the bound
  • Hybridization of the nuclei involved1JCH gt 125
    (sp3), 160 (sp2), 250 (sp)
  • Electronegativity is another factor increase the
    coupling
  • CCl3H gt 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.

Index
57
2-bound coupling
  • 2J can give structural information There is a
    relationship between 2J and Bond angle
  • gt coupling range passes through zero. Therefore
    the sign of the coupling must be determined

Index
58
3-bound coupling
  • Depends on Dihedral angle3JXY A cos 2f B
    cos f CA, B, C empirical constants

Index
59
Complicated proton spectra CH3-CH2-S-PF2
Almost quintet
Index
60
Complicated Fluorine spectra PF2-S-PF2
Second order spectra 19F Chemically
equivalent Magnetically non-equivalent 1JPF
different from 3JPH
This type of spectra is frequent in transition
metal complex MCl2(PR3)2
Index
61
Equivalence and non-equivalence
F are Non-Equivalent The 2 phosphorus are
Pro-chiral non-equivalent
Index
62
To identify a compound PF215NHSiH3
Use as many techniques as possible
Proton nmr spectra is difficult to analyze with
so many Js But with 19F, 15N and 31P spectra
its easier (get heteronuclear J)
Index
63
To identify a compound PF215NHSiH3
Use as many techniques as possible
Using decoupler easier analysis
Index
64
Multinuclear Approach
Proton NMR spectra 3 groups of peaks integrating
for 1241
Resonances due to Methyl and CH2 have coupling
with 31P And 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
65
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
66
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)
At low temperature, hydrogens form an A2B2X spin
system At higher temperature germanium hop from
one C to the next
Index
67
Paramagnetic compounds in NMR
Usually paramegnetic compounds are too braod gt
give ESR In NMR, Chemical shift is greatly
expanded
Paramagnetic shifts are made up of 2 component
  • Through space Dipolar interaction between the
    magnetic moment of the electron and of the
    nucleus
  • 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 gt weighted mean position is not
    midwayWith fast relaxation, collapse of the
    multiplet may fall thousands Hertz away from
    expected position gt 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
68
Paramagnetic compounds in NMR
4 sets of resonances
1 symmetrical Fac the 3 ligand are identical 3
Asymetrical ligand in Mer occur with 3 time the
probability.
69
Index
NMR-basics
H-NMR
NMR-Symmetry
Heteronuclear-NMR
Dynamic-NMR
NMR and Organometallic compounds
Special 1D-NMR
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