Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules.

Nuclear Magnetic Nuclear Magnetic Resonance (NMR)Resonance (NMR)Nuclear Magnetic Nuclear Magnetic Resonance (NMR)Resonance (NMR)

for beginnersfor beginners

Overview• NMR is a sensitive, non-destructive

method for elucidating the structure of organic molecules

• Information can be gained from the hydrogens (proton NMR, the most common), carbons (13C NMR) or (rarely) other elements

Spin States• All nuclei have a spin state (I )• Hydrogen nuclei have a spin of I =

±½ (like electrons)• Spin number gives number of

ways a particle can be oriented in a magnetic field: 2I + 1

Spin States• In the absence of a magnetic field

the spin states are degenerate• The “spinning” nucleus generates

its own magnetic field

Spin States• In a magnetic field the states have

different energies

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B’B’

Spin states in a magnetic field

• Energy difference linearly depends on field strength

= magnetic moment of H (2.7927N or +14.106067x10-27J/T)


• Even in a very large field (1-20T) the energy difference is small (~0.1cal/mol)


• A small excess of protons will be in the lower energy state

• These can be promoted to the higher state by zapping them with EM radiation of the proper wavelength

• Wavelength falls in the radio/TV band (frequency of 60-500MHz)


• Stronger magnetic field necessitates shorter wavelength (higher frequency)

• After low energy protons are promoted to the higher energy state they relax back to the lower state

Making NMR work• Not all protons absorb at the same

field values• Either magnetic field strength or

radio frequency must be varied • Frequency/field strength at which

the proton absorbs tells something about the proton’s surroundings

Making NMR work

Sample must be spun to average out magnetic field inhomogeneity

NMR data collection• Continuous wave data collection

(CW): – Magnetic field value is varied– Intensity of emitted RF compared to

RF at detector– Absorption is plotted on graph

NMR data collection

CW NMR of isopropanol

NMR data collection• Pulsed Fourier transform data

collection:– Short bursts of RF energy are shot at

sample– Produces a decay pattern– FT done by computer produces

spectrum

Simple FT FID and spectrum

More complex FT FID and spectrum

Even more complex FT FID

FT NMR Spectrum

Pulsed FT NMR of isopropanol

Chemical shift• Protons in different environments

absorb at different field strengths (for the same frequency)

• Different environment = different electron density around the H

Chemical shift positions

High field, shielded

Low field, deshielded

Reference (tetramethylsilane)

PPM of applied field () from reference

Chemical shift positions

NMR reference• Tetramethylsilane ((CH3)4Si)

• Advantages:– Makes one peak– 12 equivalent H, so little is needed– Volatile, inert, soluble in organic solvents– Absorbs upfield of hydrogens in most

organic compounds

Shielding/deshielding• Electron density affects chemical

shift • Electrons generate a magnetic

field opposed to the applied field• H in high electron density absorbs

upfield (toward TMS, 0ppm) from H in low electron density

Shielding/deshielding• Effect of electronegativity:

electronegative atom nearby removes electron density and causes deshielding

• TMS protons are extremely shielded because Si is electropositive compared to C

Shielding/deshielding• Few protons absorb upfield of TMS• Alkyl groups are electron donating,

so alkanes absorb around 0-2ppm ()

• Hydrogens near electronegative atoms are deshielded

• Absorption is around 3-4

Anisotropy• “Anisotropy”: any characteristic

that varies with direction (asymmetric)

• Applied to the shielding/deshielding characteristics of electrons in some systems

Anisotropy• Aromatic hydrogens are in the

deshielding region of the magnetic field generated by circulating electrons

Typical chemical shifts

Spin-spin coupling• Magnetic field felt by a proton is

affected by the spin states of nearby protons – either shielding or deshielding

• Case 1: neighboring single protons• These H can either be the same or

opposite spins – equal probability• Makes doublets of two equal peaks at

both absorptions

NMR spectrum of dichloroacetaldehyde

Coupling constants• Separation between peaks is the

“coupling constant” • Symbol: J• Measured in Hz• It is the same for both coupled

protons

Spin-spin coupling• Case 2: Single proton next to a pair• Single proton splits the pair into a

doublet• Spin state possibilities for pair:

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Equal energy

Integration ratio: 1:2:1

Spin-spin coupling• Single proton is split into a triplet• Any group of n protons will split its

neighbors into n + 1 peaks• Intensity follows Pascal’s triangle

(Fibonacci series)

Spin coupling example• Chloroethane CH3CH2Cl

Protons on Heteroatoms

• Protons on N or O often give broad uncoupled peaks of uncertain chemical shift

• Protons on nitrogen are broad due to coupling with nitrogen nucleus (spin # = 1)

• Chemical shift can depend on concentration

• Peaks will be sharp and coupled if there is no acid or water present

Protons on heteroatoms

Proton on nitrogen: broad due to interaction with nitrogen (spin number = 1)

Split into doublet by NH – reciprocal splitting is not seen

Phenolic Protons and Concentration

Alcoholic protons and coupling

1H NMR spectrum of methanol at various temperatures

Chemical Shift Differences and

Coupling• Equivalent protons do not split each

other• Adjacent protons (“vicinal”) exhibit

simple coupling if their chemical shifts are very different (/J >10)

• Designated an “AaXx” system (“AaMmXx” for three widely separated sets)

• Subscripts designate the number of protons involved


Coupling

• Sets of protons close to each other are “AaBb” or “AaBbCc”

• The closer two sets are the more the peaks are distorted AX system becoming

an AB system


Coupling

AX system with some distortion

Ternary systems• AaMmXx systems exhibit simple

splitting with two coupling constants

Ternary Systems

Ternary systems

Chemical and magnetic equivalence



NMR spectrum of butane


Coupling• AaBbXx systems are approximately

first order (simple splitting)

• AaBbCc systems are complex

Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules.

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