NMR2

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INTEGRATION

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Each different type of proton comes at a different place . You can tell how many different types of hydrogen there are in the molecule. NMR Spectrum of Phenylacetone RECALL from last time

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The area under a peak is proportional to the number of hydrogens that generate the peak. Integration = determination of the area under a peak INTEGRATION OF A PEAK Not only does each different type of hydrogen give a distinct peak in the NMR spectrum, but we can also tell the relative numbers of each type of hydrogen by a process called integration.

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55 : 22 : 33 = 5 : 2 : 3 The integral line rises an amount proportional to the number of H in each peak METHOD 1 integral line integral line simplest ratio of the heights Benzyl Acetate

Benzyl Acetate (FT-NMR):

Benzyl Acetate (FT-NMR) assume CH 3 33.929 / 3 = 11.3 33.929 / 11.3 = 3.00 21.215 / 11.3 = 1.90 58.117 / 11.3 = 5.14 Actually : 5 2 3 METHOD 2 digital integration Modern instruments report the integral as a number. Integrals are good to about 10% accuracy.

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DIAMAGNETIC ANISOTROPY SHIELDING BY VALENCE ELECTRONS

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valence electrons shield the nucleus from the full effect of the applied field B induced (opposes B o ) B o applied magnetic field lines The applied field induces circulation of the valence electrons - this generates a magnetic field that opposes the applied field. Diamagnetic Anisotropy fields subtract at nucleus

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All different types of protons in a molecule have a different amounts of shielding. This is why an NMR spectrum contains useful information (different types of protons appear in predictable places). They all respond differently to the applied magnetic field and appear at different places in the spectrum. PROTONS DIFFER IN THEIR SHIELDING UPFIELD DOWNFIELD Highly shielded protons appear here. Less shielded protons appear here. SPECTRUM It takes a higher field to cause resonance.

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CHEMICAL SHIFT

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PEAKS ARE MEASURED RELATIVE TO TMS TMS shift in Hz 0 tetramethylsilane “TMS” reference compound n Rather than measure the exact resonance position of a peak, we measure how far downfield it is shifted from TMS. Highly shielded protons appear way upfield. Chemists originally thought no other compound would come at a higher field than TMS. downfield

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h n = B o g 2 p constants frequency field strength Stronger magnetic fields (B o ) cause the instrument to operate at higher frequencies ( n ). REMEMBER FROM OUR EARLIER DISCUSSION NMR Field Strength 1 H Operating Frequency 60 Mhz 100 MHz 300 MHz 7.05 T 2.35 T 1.41 T n = ( K) B o

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TMS shift in Hz 0 n downfield The shift observed for a given proton in Hz also depends on the frequency of the instrument used. Higher frequencies = larger shifts in Hz. HIGHER FREQUENCIES GIVE LARGER SHIFTS

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chemical shift = d = shift in Hz spectrometer frequency in MHz = ppm This division gives a number independent of the instrument used. parts per million THE CHEMICAL SHIFT The shifts from TMS in Hz are bigger in higher field instruments (300 MHz, 500 MHz) than they are in the lower field instruments (100 MHz, 60 MHz). We can adjust the shift to a field-independent value, the “chemical shift” in the following way: A particular proton in a given molecule will always come at the same chemical shift (constant value).

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0 1 2 3 4 5 6 7 ppm Hz Equivalent of 1 ppm 1 H Operating Frequency 60 Mhz 60 Hz 100 MHz 100 Hz 300 MHz 300 Hz HERZ EQUIVALENCE OF 1 PPM Each ppm unit represents either a 1 ppm change in B o (magnetic field strength, Tesla) or a 1 ppm change in the precessional frequency (MHz). 1 part per million of n MHz is n Hz n MHz = n Hz 1 10 6 ( ) What does a ppm represent?

NMR Correlation Chart:

NMR Correlation Chart 12 11 10 9 8 7 6 5 4 3 2 1 0 -OH -NH CH 2 F CH 2 Cl CH 2 Br CH 2 I CH 2 O CH 2 NO 2 CH 2 Ar CH 2 NR 2 CH 2 S C C-H C=C-CH 2 CH 2 -C- O C-CH-C C C-CH 2 -C C-CH 3 RCOOH RCHO C=C H TMS CHCl 3 , d (ppm) DOWNFIELD UPFIELD DESHIELDED SHIELDED Ranges can be defined for different general types of protons. This chart is general, the next slide is more definite.

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APPROXIMATE CHEMICAL SHIFT RANGES (ppm) FOR SELECTED TYPES OF PROTONS R-C H 3 0.7 - 1.3 R-C=C-C- H 1.6 - 2.6 R-C-C- H 2.1 - 2.4 O O RO-C-C- H 2.1 - 2.5 O HO-C-C- H 2.1 - 2.5 N C-C- H 2.1 - 3.0 R-C C-C- H 2.1 - 3.0 C- H 2.3 - 2.7 R-N-C- H 2.2 - 2.9 R-S-C- H 2.0 - 3.0 I-C- H 2.0 - 4.0 Br-C- H 2.7 - 4.1 Cl-C- H 3.1 - 4.1 RO-C- H 3.2 - 3.8 HO-C- H 3.2 - 3.8 R-C-O-C- H 3.5 - 4.8 O R-C=C- H H 6.5 - 8.0 R-C- H O 9.0 - 10.0 R-C-O- H O 11.0 - 12.0 O 2 N-C- H 4.1 - 4.3 F-C- H 4.2 - 4.8 R 3 C H 1.4 - 1.7 R-C H 2 -R 1.2 - 1.4 4.5 - 6.5 R-N- H 0.5 - 4.0 Ar-N- H 3.0 - 5.0 R-S- H R-O- H 0.5 - 5.0 Ar-O- H 4.0 - 7.0 R-C-N- H O 5.0 - 9.0 1.0 - 4.0 R-C C- H 1.7 - 2.7

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YOU DO NOT NEED TO MEMORIZE THE PREVIOUS CHART IT IS USUALLY SUFFICIENT TO KNOW WHAT TYPES OF HYDROGENS COME IN SELECTED AREAS OF THE NMR CHART aliphatic C-H CH on C next to pi bonds C-H where C is attached to an electronega-tive atom alkene =C-H benzene CH aldehyde CHO acid COOH 2 3 4 6 7 9 10 12 0 X-C-H X=C-C-H MOST SPECTRA CAN BE INTERPRETED WITH A KNOWLEDGE OF WHAT IS SHOWN HERE

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DESHIELDING AND ANISOTROPY Three major factors account for the resonance positions (on the ppm scale) of most protons. 1. Deshielding by electronegative elements. 2. Anisotropic fields usually due to pi-bonded electrons in the molecule. We will discuss these factors in the sections that follow. 3. Deshielding due to hydrogen bonding.

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DESHIELDING BY ELECTRONEGATIVE ELEMENTS

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highly shielded protons appear at high field “deshielded“ protons appear at low field deshielding moves proton resonance to lower field C H Cl Chlorine “deshields” the proton, that is, it takes valence electron density away from carbon, which in turn takes more density from hydrogen deshielding the proton. electronegative element DESHIELDING BY AN ELECTRONEGATIVE ELEMENT NMR CHART d - d+ d - d+

Electronegativity Dependence of Chemical Shift:

Electronegativity Dependence of Chemical Shift Compound CH 3 X Element X Electronegativity of X Chemical shift d CH 3 F CH 3 OH CH 3 Cl CH 3 Br CH 3 I CH 4 (CH 3 ) 4 Si F O Cl Br I H Si 4.0 3.5 3.1 2.8 2.5 2.1 1.8 4.26 3.40 3.05 2.68 2.16 0.23 0 Dependence of the Chemical Shift of CH 3 X on the Element X deshielding increases with the electronegativity of atom X TMS most deshielded

Substitution Effects on Chemical Shift:

Substitution Effects on Chemical Shift C H Cl 3 C H 2 Cl 2 C H 3 Cl 7.27 5.30 3.05 ppm -C H 2 -Br -C H 2 -CH 2 Br -C H 2 -CH 2 CH 2 Br 3.30 1.69 1.25 ppm most deshielded most deshielded The effect decreases with incresing distance. The effect increases with greater numbers of electronegative atoms.

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ANISOTROPIC FIELDS DUE TO THE PRESENCE OF PI BONDS The presence of a nearby pi bond or pi system greatly affects the chemical shift. Benzene rings have the greatest effect.

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fields add together

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C=C H H H H B o ANISOTROPIC FIELD IN AN ALKENE protons are deshielded shifted downfield secondary magnetic (anisotropic) field lines Deshielded fields add

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B o secondary magnetic (anisotropic) field H H C C ANISOTROPIC FIELD FOR AN ALKYNE hydrogens are shielded Shielded fields subtract

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HYDROGEN BONDING

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HYDROGEN BONDING DESHIELDS PROTONS The chemical shift depends on how much hydrogen bonding is taking place. Alcohols vary in chemical shift from 0.5 ppm (free OH) to about 5.0 ppm (lots of H bonding). Hydrogen bonding lengthens the O-H bond and reduces the valence electron density around the proton - it is deshielded and shifted downfield in the NMR spectrum.

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Carboxylic acids have strong hydrogen bonding - they form dimers. With carboxylic acids the O-H absorptions are found between 10 and 12 ppm very far downfield. In methyl salicylate, which has strong internal hydrogen bonding, the NMR absortion for O-H is at about 14 ppm, way, way downfield. Notice that a 6-membered ring is formed. SOME MORE EXTREME EXAMPLES