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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