nuclear magnetic resonance spectroscopy2

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7/6/12

NUCLEAR MAGNETICRESONANCE

SPECTROSCOPY


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Introduction

NMR is the most powerful tool availablefor organic structure determination.

It is used to study a wide variety of

nuclei: 1H

13C

15N

19F

31P


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Nuclear Spin

A nucleus with an odd atomicnumber or an odd mass number hasa nuclear spin.

The spinning charged nucleusgenerates a magnetic field.


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External Magnetic Field

When placed in an external field,spinning protons act like bar magnets.


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Two Energy States

The magnetic fieldsof the spinningnuclei will align

either with theexternal field, oragainstthe field.

A photon with theright amount ofenergy can beabsorbed andcause thespinning proton

to flip.


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E and Magnet Strength

Energy difference is proportionalto the magnetic field strength.

E = h = h B0

2Gyromagnetic ratio, , is a

constant for each nucleus (26,753s-1gauss-1 for H).

In a 14,092 gauss field, a 60 MHzphoton is required to flip a proton.

Low energy, radio frequency.


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Magnetic Shielding

If all protons absorbed the sameamount of energy in a givenmagnetic field, not much

information could be obtained.But protons are surrounded by

electrons that shield them from

the external field.Circulating electrons create an

induced magnetic field that

opposes the external magneticfield. =>


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Shielded Protons

Magnetic field strength must beincreased for a shielded proton toflip at the same frequency.

=>


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NMR Signals

The numberof signals shows howmany different kinds of protons arepresent.

The location of the signals showshow shielded or deshielded theproton is.

The intensityof the signal showsthe number of protons of that type.

Signal splitting shows the number

of protons on adjacent atoms.


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The NMR Spectrometer


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The NMR Graph


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Tetramethylsilane

TMS is added to the sample.Since silicon is less

electronegative than carbon, TMSprotons are highly shielded. Signaldefined as zero.

Organic protons absorb downfield(to the left) of the TMS signal.

Si

CH3

CH3

CH3

H3C


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Chemical Shift

Measured in parts per million.

Ratio of shift downfield from TMS

(Hz) to total spectrometerfrequency (Hz).

Same value for 60, 100, or 300

MHz machine.Called the delta scale.


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Spin-Spin Splitting

Nonequivalent protons on adjacentcarbons have magnetic fields thatmay align with or oppose the external

field.

This magnetic coupling causes theproton to absorb slightly downfield

when the external field is reinforcedand slightly upfield when the externalfield is opposed.

All possibilities exist, so signal is split.


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Coupling Constants

Distance between the peaks ofmultiplet

Measured in HzNot dependent on strength of the

external field

Multiplets with the same couplingconstants may come from adjacentgroups of protons that split each

other.


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Values forCoupling Constants


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Carbon-13

12C has no magnetic spin.

13C has a magnetic spin, but is only

1% of the carbon in a sample.The gyromagnetic ratio of 13C is

one-fourth of that of 1H.

Signals are weak, getting lost innoise.

Hundreds of spectra are taken,

averaged.


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Differences in

13C TechniqueResonance frequency is ~ one-

fourth, 15.1 MHz instead of 60

MHz.Peak areas are not proportional to

number of carbons.

Carbon atoms with morehydrogens absorb more strongly.


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Spin-Spin Splitting

It is unlikely that a 13C would beadjacent to another 13C, so

splitting by carbon is negligible.13C will magnetically couple with

attached protons and adjacentprotons.These complex splitting patterns

are difficult to interpret.


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