NMR Nuclear Magnetic Resonance Chapter 13

Proton Nuclear Spin States Two states have the same energy in the absence of a magnetic field Magnetic Field

Protons in an external magnetic field Energy difference between the spin-states depends on the strength of the magnetic field.

NMR Spectrometer

Superconducting NMR Magnet

NMR Sample Tubes

NMR Probe Sample goes in here Coils Insulated tube

NMR Signal Proton NMR Spectrum of Cl 3 CH Magnetic Field

H HoHo Field here less than H o

Electrons shield nucleus from external magnetic field. Magnetic field

E =H Energy to change spin state proportional to field strength E =h Measure field strength in FREQUENCY units Cycles/second

Tetramethylsilane TMS Reference compound for NMR spectrum.

NMR signal Magnetic Field Cl 3 CH TMS 437 Hz at 60x10 6 Hz

Chemical Shift = Position of Signal - Position of TMS Spectrometer Frequency x10 6 Units = parts per million = ppm Chemical Shift Scale = 437 Hz - 0 Hz 60x10 6 Hz x 10 6 = 7.28 ppm

Typical Chemical Shifts

Typical Chemical Shifts

TMSRH Proton Chemical Shifts ppm down field from TMS

Area proportional to number of protons

How many lines would you expect in the proton NMR spectrum of this compound?

Measure signal from this proton Field from this proton can be aligned or opposed to applied field Magnetic Field

Spin Combinations - 2 adjacent protons -C-CH 2 - H 1 : 2 : 1

Spin Combinations - 3 Adjacent Protons -C-CH 3 H 1: 3: 3: 1

Adjacent ProtonsLines n + 1

What is the name of this compound? Ethyl Bromide

What is this compound? CH 3 COCH 2 CH 3

C 6 H 5 COCH 3 What is this compound?

C 6 H 5 COOCH 2 CH 3

y x Magnetic field changes across sample