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HYDROGEN-1 NUCLEAR MAGNETICRESONANCE

Also known as Proton NMR

Natural Hydrogen (H) is used in samples

The spectrum is plot of the intensity of the peak

against its chemical shift, measured in parts permillion (ppm)

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1HNMR SPECTRUM

Different features to provide information about acompounds structure

a. Number of signals

b. Position of signalsc. Intensity of signals.

d. Spin-spin splitting of signals

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NUMBEROF SIGNALS

The number of NMR signals equals the numberof different types of protons in a compound.

Protons in different environment give differentNMR signals.

1 NMR SIGNAL2 Types of Hs

2 NMR SIGNALS3Types of Hs

3 NMR SIGNALS

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1 Type of H

1 NMR SIGNAL

3Types of Hs3 NMR SIGNALS

2 Types of Hs

2 NMR SIGNALS

3Types of Hs3 NMR SIGNALS

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3Types of Hs3 NMR SIGNALS

2 Types of Hs

2 NMR SIGNALS1 Type of H

1 NMR SIGNAL

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For cycloalkanes and alkanes, draw all bonds ofhydrogen

All Hs areequivalent1 NMR SIGNAL

3Types of Hs3 NMR SIGNALS

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POSITIONOF SIGNALS

Shielded Nucleus Deshielded Nucleus

Nucleus feels a smaller resultantfield

Nucleus feels a larger resultantfield

A larger magnetic field is induced A smaller magnetic field isinduced

Lower frequency is needed toachieve resonance Higher frequency is needed toachieve resonance

Absorption shifts upfield Absorption shifts downfield

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POSITIONOF SIGNALS

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POSITIONOF SIGNALS

The Hb protons are deshielded because they arecloser to the electronegative Cl atom.

F is more electronegative than Br, thus Hb are moredeshielded than the Ha protons

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CHEMICAL SHIFTS

Protons in a given environment absorb in apredictable region in an NMR spectrum

The Chemical Shift of a C-H bond increases withincreasing alkyl substitution

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CHEMICAL SHIFTS

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CHEMICAL SHIFTS

Shielded protons absorb at lower chem shift (to theright)

Deshielded protons absorb at higher chem shift (tothe left)

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INTENSITYOF SIGNALS

The area under an NMR signal is proportional tothe number of absorbing protons

An NMR spectrometer automatically integrates thearea under the peaks, prints out a stepped curve

Height of each step is proportional to the areaunder the peak, which is proportional to thenumber of absorbing protons

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INTENSITYOF SIGNALS

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SPIN-SPIN SPLITTING

Peaks are often split into multiple peaks due tomagnetic interactions between nonequivalentprotons on adjacent carbons

N + 1 rulesplitting is into one more peak than thenumber of Hs on the adjacent carbon(s).

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SPIN-SPIN SPLITTING

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There are three 2Dspectra which are widelyused for the structuredetermination ofproteins with a mass ofup to 10 kD

1. 2D COSY,

2. 2D NOESY

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2-D NMR SPECTRA-HAVETWOFREQUENCYAXESAND;-ONEINTENSITYAXIS.

1H-1Hshift-correlatedspectroscopy

they identify protonsthat are coupled (i.e.,that split each others

signal)

Structure elements canbe determined

COSE NOESY

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COSY

for determining whichsignals arise fromneighboring protons (usuallyup to four bonds)

COSY will tell you whichprotons are adjacent to oneanother

help unambiguously assign

which proton corresponds towhich signal and all aroundconnectivity

NOESY

allows to correlate nucleithrough space (distancesmaller than 5).

you could look at theNOESY data to determinewhich protons "see" oneanother to determinerelative stereochemistry

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2D COSY 2D NOESY

only signals of protons which are two orthree bonds apart are visible in a COSY

spectrum (red signals)

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COSY uses the dipolar interaction

of spins (the nuclearOverhauser effect, NOE) forcorrelation of protons.

Condition:a. with spin-spin

coupling betweenprotonswith correlation

Cross peaks: pairs of protons

that are coupled-dots that are not on the diagonal

Dia peaks: coupling between

nuclei separated by 2-3 bond

-diagonal of signals (A and B)divides the spectrum in twoequal halves

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Ha and Hb proton arecoupled!

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the two triangles in the figurehave a commonvertex, since the Hb protons arecoupled to both the Ha and Hcprotons

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NOESY

Nuclear OverhauserEffect

-the transfer ofnuclear spinpolarization from onenuclear spin populationto another via cross-relaxation

Origin of the NOEEffect

When a proton isclose in space to

another proton (or anyother nuclei with spin> 0), their magneticdipoles interact(Dipole-Dipole

interaction, DD). distinct from Jcoupling

NOE signals are positive forsmall molecules in low-viscositysolvents and negative for verylarge molecules or very viscoussolvents relative to a negative

diagonal

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NOESY

The correlationbetween H1' and H2' at2.65 and 1.24 ppm ismostly COSY type.

much weaker through-space correlationappears between H1'and H2 at 2.65 and

7.20 ppm.

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NOESY

Aromaric region of the 2D NOESY spectrum of 12,14-ditbutylbenzo[g]chrysene

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NOEs betweenH-30, H-18 andH-19 equatorialH-18 and H-26H-25, H-24 andH-26H-27 and H-11axialH-19 axial, H-12equatorial, H-15axial and H-8H-28 and H-22H-23 and H-24H-10 and H-4