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Multinuclear NMR Spectroscopy

Also:

117Sn

7.7%

Review: Proton-Proton Coupling

General Formula :

2nI+1

For I = 1/2 : n+1 lines

Proton-Proton Coupling - different values of J

Chemical and Magnetic Equivalence (Ch 5.3)

Chemical Equivalence: Nuclei are related by symmetry (mirror plane, rotational axis) Magnetic Equivalence: 1. Nuclei are isochronous, i.e. have identical chemical shifts and 2. Nuclei have equal coupling to all other nuclei in the molecule

The Case of Substituted Ferrocenes

Hb' Hb Ha Fe Ha' R

3J 4J

ca. 2.0- 2.5 Hz ca. 1.0-2.0 Hz

3J(H a

Hb) similar to 4J (Ha Hb')

Q. Predict the 1H NMR Spectra of Bromoferrocene, 1,1'-Dibromoferrocene, 1,2-Dibromoferrocene, and 1,3-Dibromoferrocene

The Karplus Curve (Ch. 5.2)

Example: Coupling in Cyclohexanes

Review: Carbon-Proton Coupling (Ch. 4.3 & 4.4)

H-decoupled

H-coupled

Carbon-Deuterium Coupling (Ch. 4.13)

For I = 1 : 2n+1 lines

Organometallic Example 1 : 1,1'-fc(SnMe3)(SiMe3)

499.9 MHz 1H NMR Spectrum

0.285

"Tin Satellites" vs. "Si Satellites"

0.340 0.338

0.240

0.247

0.232

0.229

4.319 4.313 4.316

4.263 4.258 4.261

4.035

4.074 4.067 4.070 4.056 4.058 4.048 4.042 4.045

1,1'-fc(SnMe3)(SiMe3) - 13C NMR

125.7 MHz 13C NMR Spectrum (Proton Decoupled)

0.079 -8.446 -9.005 -9.089 -7.019 -7.085

-0.367

-0.112

74.520 74.323 74.126

72.918

72.229

71.322 71.170 71.003

69.301

(Proton Decoupled)

(Proton Decoupled)

-3.311

-3.535

Organometallic Example 2 : Isomerism in Square-planar Phosphine Complexes

195Pt

33.8%

162 MHz 31P Spectrum

???-[PtCl2(PnBu3)2]

trans-[Pt(PR3)2X2] JP,Pt ca. 2000-2500 Hz cis-[Pt(PR3)2X2] JP,Pt ca. 3000-3500 Hz

Note: Q.

Satellites are only seen if there are one or more NMR active isotopes of less than 100% natural abundance. Predict the 31P spectrum of trans-[Rh(Cl(PPh3)2L]; (100% 103Rh)

Organometallic Example 3 : Chemical Shifts in Heteronuclear NMR illustrated for 11B NMR

80% 11B, spin 3/2 20% 10B, spin 3

·

p-bonding leads to shielding

(Et vs. Ph vs. Vi); (Cl vs. F vs. NR2)

·

Coordination number 4 leads to shielding (also: solvent effects!)

11B

NMR of Carboranes

32.1 MHz 11B NMR Spectra

H-Coupled

Chemical Shifts:

· Individual signals for basal and apical B centers

Spin coupling:

· · ·

1J

H-Decoupled

terminal BH 100-200 Hz 1J bridging BH 30-60 Hz High symmetry : sharp signals

Organometallic Example 4 : 7Li NMR - Solvent Effects Solvent-Separated Ion Pairs (SSIP) vs. Contact Ion Pairs (CIP)

Organometallic Example 5 : VT 13C NMR of Organolithium Species

Dynamic NMR Spectroscopy (see Ch. 6)

6Li

enriched sample of tBuLi (I = 1)

13C

NMR Spectra of [tBu4Li4] :

Multiplicity = 2nI + 1

At 185 K

At 299 K

Intramolecular Bond Fluctuation

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