nmr spectroscopy for chemical reaction kinetics · 1 nmr spectroscopy for chemical reaction...
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NMR Spectroscopy for Chemical Reaction Kinetics
Alexander Marchionea, Arvin Moserb, Sergey Golotvinb, Patrick Wheelerb, Breanna Conklina
ACD/Labs NMR Software Symposium ENC 2014
a – DuPont Central Research and Development, Wilmington, DE, USA b – Advanced Chemistry Development, Inc.
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NMR for in situ reaction studies
• Advantages of NMR for in situ study of reaction kinetics (aside from the obvious) – Compatible with many inert materials of construction (borosilicate
glass, quartz, alumina, zirconia, fluoropolymer) • Enables operation at P from vacuum to > 10,000 psi
– Compatible with wide temperature ranges (commercial probes available with ranges -100 – 500 °C)
– Ability to function with mg quantities of reagents in closed vessels permits safe use of highly toxic or reactive species
– Advances in processing software enable facile generation of conc. v. time plots
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Sealed, thermostatted ampule
• A design suitable for careful kinetic studies – 10 mm ampule with 5 mm neck, flame-sealed after charging. Ampule resides below top of probe Dewar, minimizing temperature gradient.
• Well-suited for gas-phase reactions, or liquid-phase at positive pressure.
Figure from Gas-phase NMR for Chemical Reaction Kinetics. A. A. Marchione, D. C. Roe, P. J. Krusic. Encyclopedia of Magnetic Resonance, 2012.
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Common experimental challenges
• Use of deuterated solvent for field-frequency lock is often not possible or convenient, so field drift is observed in spectra.
• Probe electronics often not at thermal equilibrium at start of a reaction, resulting in changing signal phase with time.
time
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Example #1: Decomposition of gaseous di-t-butyl peroxide
• DTBP is a common thermal and photochemical initiator in both liquid and gas phase.
CH3
CH3
CH3CH3
OO
CH3
CH3
CH3
CH3
CH3O
CH3CH3
CH3O
O
CH3 CH3
+ CH3
CH3 CH3
CH3
CH3CH3
CH3O RH OH
CH3CH3
CH3
CH3
RHCH4
Clean, ideal chemistry (for this study!)
Complications
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Example #1: Decomposition of gaseous di-t-butyl peroxide
• Kinetic 1H NMR spectra (400 MHz), 150 °C, 16 h
time
acetone ethane
DTBP
CF3CH3
(internal standard)
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Example #1: Decomposition of gaseous di-t-butyl peroxide
• Phase correction of individual spectra
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Example #1: Decomposition of gaseous di-t-butyl peroxide
• Manual correction of integral ranges for drift
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Example #1: Decomposition of gaseous di-t-butyl peroxide
• Derivation of first-order rate constant
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Example #1: Decomposition of gaseous di-t-butyl peroxide
• Arrhenius parameters: Ea = 35.1 kcal/mol, log A = 14.3
y = -17486x + 32.437 R² = 0.9989
-14
-12
-10
-8
-6
-4
-2
2.15E-03 2.20E-03 2.25E-03 2.30E-03 2.35E-03 2.40E-03 2.45E-03 2.50E-03 2.55E-03
ln k
(s-1
)
T-1
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Example #2: Isomerization of quadricyclane
• Thermally-induced gas-phase isomerization: 172 °C
8 h1H kinetic series, 400 MHz
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Example #2: Isomerization of quadricyclane
• 13C, 1 atm quadricyclane, n.i.a. 172 °C
8 h13C {1H} kinetic series, 100 MHz
cf. 2.40e-4 by 1H
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Example #3: Diels-Alder addition of maleic anhydride to butadiene
• Classic overall 2nd-order reaction: H
H
CH2
CH2 HH
OOO
+H
HO
O
O80 °C
C6D6
• Spectrus accommodates 2A P, or A + B P if [A]0 = [B]0
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Example #3: Diels-Alder addition of maleic anhydride to butadiene
• Kinetic 1H NMR spectra, 400 MHz
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Example #3: Diels-Alder addition of maleic anhydride to butadiene
• Easy export of integral intensity v. time data
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Summary
• In situ studies of reaction kinetics by NMR spectroscopy is a safe and experimentally-flexible means of probing both liquid and gas-phase reaction systems.
• Advances in spectral processing software permit facile phase and frequency drift corrections, reducing time required for data analysis.
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