NMR N MR NMR Nuclear Magnetic Resonance NMR for Organometallic compounds Index NMR-basics H-NMR NMR-Symmetry Heteronuclear-NMR Dynamic-NMR NMR and Organometallic

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NMR Nuclear Magnetic ResonanceNMR for Organometallic compounds

IndexNMR-basicsH-NMRNMR-SymmetryHeteronuclear-NMRDynamic-NMRNMR and Organometallic compoundsNMR in Organometallic compoundsspins 1/2 nucleiFor small molecules having nuclei I=1/2 : Sharp lines are expectedW1/2 (line width at half height) = 0-10 HzIf 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

1.00

3H

-

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 nucleiIndexIsotope

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 1/2Multinuclear NMRThere 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 structuresNuclear quadrupole. For spins greater than 1/2, the nuclear quadrupole moment is usually larger and the line widths may become excessively large. Relaxation timeNMR in Organometallic compoundsspins > 1/2 nucleiThese 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 q2zz tcI2 (2I -1)Q = quadrupole momentqzz = electric field gradienttc = correlation timeI = spin quantum numberNarrow 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 Costisella8Notable nuclei19F: spin , 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 Hz29Si: spin , 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 , 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 nuclei31P: spin , 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 nuclei103Rh: spin , 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 organometallicIn 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 statep - 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.

IndexSymmetry

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)IndexSymmetry 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

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