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Making NMR Accessible: Benchtop NMR Spectroscopy in the Classroom

Dean Antic, Ph.D. Senior Applications Scientist

Thermo Scientific picoSpin 45 NMR Spectrometer

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Agenda

•Why NMR spectroscopy?

•picoSpin 45 NMR Spectrometer

•Application Examples • Fischer Esterification • Solvent Effects • Enol-Keto Tautomerism • Reaction Monitoring

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Why NMR Spectroscopy?

• Most powerful analytical method in organic chemistry • Identifies chemical groups • Yields quantitative concentration ratios • Reveals structure

picoSpin 45 Ethyl Acetate spectrum

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What NMR Signals Tell Us

• The number of signals shows how many different kinds of protons are present.

• The location of the signals shows how shielded or deshielded the proton is.

• The intensity of the signal shows the number of protons of that type.

• Signal splitting shows the number of protons on adjacent atoms.

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picoSpin 45 Spectrometer Features • 45 MHz pulsed FT NMR spectrometer

• 1H only ; 19F & High Temperature option

• High Resolution: <60 ppb resolution

• High Performance: SNR for water of >500

• Compact: 10.5 lbs total weight

• Replaceable capillary micro-coil cartridge

• 30 µL sample volume

• No cryogens or compressed gas needed

• No special environment

• Solid-state technology, no moving parts

• Ethernet interface; controlled by web browser

• 12 month license to Mnova

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NMR Spectrometer Alternatives

Anasazi 60 MHz

Bruker 400 MHz

picoSpin 45 MHz

Varian 900 MHz

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The miniaturization of NMR

RF preamplifiers

Permanent magnet

Magnet temperature controller

Fluid cartridge and sub-panel

Embedded web server

Data converters, DSP

Pulse sequencer

Rx and Tx amplifiers

Shim coil controller

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Inside the Cartridge

Tx/Rx Microcoil

0.3 mm ID quartz capillary

PTFE transition

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picoSpin 45 spectrometer in the field Chemistry Lab Fume hood

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Is it Robust?

NASA Zero-G Experiments

picoSpin 45

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Is it Mobile?

Grays Peak • Continental Divide • Summit County, CO • Elevation ~14,278 ft

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Chemical Education

Dr. Kevin Blair with students Western Michigan University

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Teaching Applications

• Chemistry education – organic, physical, inorganic, instrumental • Structure elucidation • Reaction monitoring • Chemical kinetics • Chemical environment • Chemical thermodynamics • Learn NMR concepts

• In the lab • In the classroom

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The Organic Curriculum

• Fischer Esterification – Synthesis of Banana Oil • Demonstrates the use of bench-top NMR in a traditional Organic Chemistry

Teaching Lab • Adaptation of a traditional Organic 200-Level lab experiment

• Solvent Effects – Acetone/Methanol solutions • Demonstrates the use of bench-top NMR in an Advanced Organic Chemistry Lab • Study solvent effects and the impact on NMR spectra

• Chemical Thermodynamics – Enol-Keto Tautomerism • Demonstrates the use of bench-top NMR in a traditional Physical Chemistry

Teaching Lab • Explore concepts in chemical thermodynamics by measuring & quantifying

intramolecular enol-keto tautomerism • Reaction monitoring: A Transesterification

Reaction • Demonstrates the use of bench-top NMR in a traditional Organic Chemistry

Teaching Lab • Utilize a traditional Organic 200-Level lab experiment

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Synthesis of 3-Methylbutyl Acetate

Fischer Esterification

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Experimental Procedure

Acetic acid 3-Methylbutanol H2SO4 3-Methylbutyl acetate

25 mL 20 mL 5 mL

0.42 mol 0.18 mol Conc. 80-90% yield

• Acquire spectrum of reactants;

• Set up reflux apparatus;

• Charge 250 mL RBF with reactants & acid catalyst - acquire spectrum;

• Reflux for 1 hr - acquire spectrum;

• Wash: 2×50 mL H2O, 3×25 mL 5% NaHCO3 - acquire spectrum;

• Salt extraction: 2×50 mL sat. NaCl, dry over MgSO4 - acquire spectrum;

• Set up simple distillation apparatus – collect fraction 135-143 ºC;

• Acquire spectrum of product;

Reference: Gokel, H. D.; Durst, G. W. Experimental Organic Chemistry; McGraw-Hill, New York, 1980; pp 342-347.

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Reactant: Acetic Acid (anhyd.)

10 scans T1 = 6s O

H

O

CH 3

TMS

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Reactant: 3-Methylbutanol

-OH

-CH2- -CH-

-(CH3)2

-CH2-

CH

C H 3

CH 2

O HCH 2

CH 3

16 scans T1 = 8s

TMS

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Product: 3-Methylbutyl acetate

O=CCH3

-CH2- -CH-

-(CH3)2

-CH2-

O

C H 3O

CH

C H 3

CH 2

CH 2

CH 3

16 scans T1 = 8s

TMS

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Typical Spectral Set

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Expanded Spectral Set

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Acetone in Methanol

Solution Effects

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Keto-Enol Tautomerism of an Acyclic β-Diketone

Chemical Thermodynamics

Acetylacetone

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• Acquire: 1H spectrum of acetylacetone (AcAc) as a function of temperature

• Quantify: integrate enol & keto proton signals to determine equilibrium constant Keq

• Determine: ∆𝑯, ∆𝑺, and ∆𝑮 • ∆𝐺𝐺 = ∆𝐻𝐺 − 𝑇∆𝑆𝐺

• ∆𝐺𝐺 = −𝑅𝑇 ln𝐾𝑒𝑒 = ∆𝐻𝐺 − 𝑇∆𝑆𝐺; ln𝐾𝑒𝑒 = − ∆𝐻𝐺𝑅𝑅

+ ∆𝑆𝐺𝑅

• Plot: ln𝐾𝑒𝑒 vs. 1𝑅(K)

; slope = - ∆𝐻𝐺𝑅

, intercept = ∆𝑆𝐺𝑅

• Determine: Temperature dependence of Enol concentration

Goals & Procedure

Reference: Drexler, E. J.; Field, K. W. J. Chem. Ed. 1976, 53(6), 393.

• Set magnet temperature = 34 ºC

• Adjust Tx frequency (Larmor slope ≈ -37 kHz/ºC)

• Inject sample – acquire spectrum • Repeat every 2 ºC up to 58 ºC, shim on AcAc at each step

Goals

Procedure

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Temperature Dependence of Enol Concentration

T(ºC) %Enol Keq

34 80 4.94

58 66 2.97

Literature1

33 81 4.3

Reference: 1Burdett, J. L.; Rodgers, M. T. J. Amer. Chem. Soc. 1964, 86, 2108.

%Enol =𝑒 𝑡 − 0.5 𝑘 𝑡

𝑒 𝑡 ∗ 100

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Temperature Dependence of Keq

𝐾𝑒𝑒 =𝑒 𝑡

0.5 𝑘 𝑡

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Transesterification Synthesis of Methyl Acetate

Reaction Monitoring

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Experimental Procedure

Ethyl Acetate Methanol H2SO4 Δ Methyl acetate Ethanol

2 mL 5 mL 0.3 mL RT - -

1 mol 6 mol Conc. 42-52ºC

Reference: http://www.picospin.com/applications/education/transesterification/

Procedure 1 • Charge test tube with a MeOH and EtOAc

• Add acid catalyst; mix

• Draw 40 µL aliquot, inject sample

• Acquire continuously for ~90-120 min:

• 600-700 scans • T1 = 10 s • Save individual scans • Process 12 scan averages

Procedure 2 – in situ • Charge test tube with a MeOH and EtOAc

• Add acid catalyst; heat in hot water bath

• Draw 40 µL aliquot & inject every 5 min

• Acquire individual spectra ~90-120 min:

• 12 Scans per sample • T1 = 10 s • Save averaged scans

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Initial Reaction Mixture - Before

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Reaction Mixture - During

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Reaction Mixture – After

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Summary We explored:

• The picoSpin 45 NMR spectrometer and its capabilities;

• How to use bench-top NMR in the teaching laboratory by exploring different applications in organic, physical and analytical chemistry teaching labs;

• Adapting your curriculum to bench-top NMR applications.

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Contact Information

• Website resources: www.thermoscientific.com/picospin • Video tutorials: www.youtube.com/picoSpinNMR • Information: info@picospin.com • Application support: dean.antic@thermofisher.com • Technical support: support@picospin.com

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Thank you very much!

Questions?