LEC FIVE: FINAL VISIT INTRODUCTION TO NMR THEN ON TO SPECTRA INTERPRETATION

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LEC FIVE FINAL VISIT INTRODUCTION TO NMR THEN
ON TO SPECTRA INTERPRETATION

• CHEMICAL SHIFT SHIELDING MECHANISMS
• ?diam prop to 1/r r dist of nucleus to
  valence e-
• ??para prop to ?Q/(?E)(r3)

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STANDARDS
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SAMPLE PREP I
P45
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Advanced Microtubes Matched with CDCl3 Shigemi is
introducing its newest invention, the advanced
NMR Microtube. This 5 mm sample tube has a
thinner glass wall in the sample area than the
existing type (0.40 mm -gt 0.20 mm), increasing
the sample capacity held in the Rf region by a
factor of 1.2. Due to this improved space filling
factor, the signal to noise ratio has been
enhanced by 16. Although the sample volume is
increased, it is to be noted that the artifact
caused by the RF inhomogeneity is very limited.
Therefore, an increased sensitivity is expected.
CLEAR
GREEN
BLUE
SAMPLE PREP II
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HYPHENATED NMR

• CLASSICAL LC-MS
• EMERGING LC-NMR
• NEW WRINKLE SPE --
• LC-NMR-MS

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HYPHENATED NMR 4 METHODS

• LC NMR
• LC-NMR/MS
• SPE-NMR
• CAPILLARY-NMR
• TUBELESS - NMR

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THE CHEMICAL SHIFT PART IPROTON CARBON

• ON TO CHAPTER 3!

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WHAT IS THE CHEMICAL SHIFT?
GLOSSARY ABBREVIATIONS
ORG STRUCTURE ANALYSIS A number that expresses
the resonance frequency of a particular nucleus
relative to a reference. ORG CHEM (BRUICE)
The location of a signal in an NMR spectrum. It
is measured downfield from a reference compound
(most often TMS). ORG CHEM (MCMURRY) The
position on an NMR chart where a nucleus absorbs.
By convention TMS is arbitrarily set at zero and
all other absorptions usually occur down field
(to the left on the chart). Chemical shift units
are expressed in delta units, where 1 delta
equals 1 ppm of the spectrometer operating
frequency.
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IS THE CHEMICAL SHIFT USEFUL?
FIRST DATA USED IN APPLYING NMR TO ORGANIC
STRUCTURE ANALYSIS
WHAT IS THE STRUCTURE OF C6H12? 1H NMR SPECTRUM
BELOW
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IS THE CHEMICAL SHIFT USEFUL?
FIRST DATA USED IN APPLYING NMR TO ORGANIC
STRUCTURE ANALYSIS
WHAT IS THE STRUCTURE OF C6H12? 1H NMR SPECTRUM
BELOW
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EXAMPLES OF CHEMICAL SHIFTS
See 62-63
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EXAMPLES OF CHEMICAL SHIFTS
See 62-63
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EXAMPLES OF CHEMICAL SHIFTS
See the WEB!!
See 62-63
WWW http//www.chem.uni-potsdam.de/tools/
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CHBr3
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CHBr3
CAN CORRELATE ? TO FCN GROUP HOW EASY IS IT TO
MAKE ASSIGNMENTS? HOW EASY IS IT TO DEDUCE
STRUCTURES?
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CORRELATING STRUCTURE WITH CHEM SHIFT
Pg 54
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WHAT FACTORS CAUSE ALIPHATICS NEAR TMS? VINYL,
ARYL FAR FROM TMS? CO FAR FROM TMS?
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P56
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RESOLVING OVERLAPPING SIGNALS
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RESOLVING OVERLAPPING SIGNALS
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Bz-d6
CDCl3
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FACTORS DETERMINING CHEMICAL SHIFTS
USUAL VIEW
13C 0 - 200 PPM 1H 0 10 PPM
INHERENTLY BETTER RESOLUTION AT C IS THIS TRUE?
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FACTORS DETERMINING CHEMICAL SHIFTS

• A KEY EQUATION (pg 58)
• BNET BO BI BO(1-?)

? ?diam ?para ?non-loc ?diam shielding
(review Fig 2.8 p 36) ?para deshielding
no physical model
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FACTORS DETERMINING CHEMICAL SHIFTS

• A KEY EQUATION (pg 58)
• BNET BO BI BO(1-?)

? ?diam ?para ?non-loc ?diam shielding
(review Fig 2.8 p 36) ?para deshielding
no physical model
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FACTORS DETERMINING CHEMICAL SHIFTS

• SHIELDING MECHANISMS (see pg 60 OSA)
• ?diam (1H) prop to 1/r r dist of nucleus
  to valence e-
• ??para (13C) prop to ?Q/(?E)(r3)
• r3 avg of e- orbital radius
• ?E diff in e- excitation energy

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FACTORS DETERMINING CHEMICAL SHIFTS

• SHIELDING MECHANISMS FOR 1H
• ?diamag dominates ?(electroneg important)
• TMM 0.8 ? ? 2.5
• TMS 0.0 ? ? 1.9

TMS
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P59
DATA TO ISOLATE DIRECT ELECTRONEGATIVITY
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P69
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FACTORS DETERMINING CHEMICAL SHIFTS

• SHIELDING MECHANISMS FOR 1H
• ?diamag dominates ?(electroneg important)
• TMM 0.8 ? ? 2.5
• TMS 0.0 ? ? 1.9
• NOTE INCONSISTENCY
• ? C2H6 0.86
• ? C2H4 5.27 INC e- DENSITY
• ? C2H2 2.37

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FACTORS DETERMINING CHEMICAL SHIFTS

• SHIELDING MECHANISMS FOR 1H
• ?diamag dominates ?(electroneg important)
• TMM 0.8 ? ? 2.5
• TMS 0.0 ? ? 1.9
• NOTE INCONSISTENCY
• ? C2H6 0.86
• ? C2H4 5.27 INC e- DENSITY
• ? C2H2 2.37

?non-loc is the answer! ?non-loc ?neigh atom
?ring curr ?elec field ?solvent
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SUMMARY
1. Measurement of ? (a) TMS standard, (b) solvent
important, (c) spectrometer freq key. 2. Chemical
shift correlation tables are useful ubiquitous.
3. Understanding the assignments of p54 will
assist in understanding chemical shift trends (1H
13C). 4. Some important terms (de)shielding,
up filed shift, diamagnetic anisotropy,
paramagnetic shielding, supercon, screening
constant (?). 5. Important chemical shift
mechanisms (a) ?non-loc is the answer for 1H (b)
13C wait until next lecture!