A stepwise mechanism for gas-phase unimolecular ion decompositions. Isotope effects in the fragmentation of tert-butoxide anion

William Tumas, Robert F. Foster, Mark J. Pellerite, John I. Brauman

Research output: Contribution to journalArticle

48 Citations (Scopus)

Abstract

Infrared multiple photon (IRMP) photochemical activation of gas-phase ions trapped in an ion cyclotron resonance (ICR) spectrometer has been used to study the mechanism of a gas-phase negative ion unimolecular decomposition. Upon irradiation with a CO2 laser (both high-power pulsed and low-power continuous wave (CW)), tert-butoxide anion, trapped in a pulsed ICR spectrometer, decomposes to yield acetone enolate anion and methane. The mechanism of this formal 1,2-elimination reaction was probed by measuring competitive hydrogen isotope effects (both primary and secondary) in the IR laser photolysis of 2-methyl-2-propoxide-1,1,1-d3 (1) and 2-methyl-2-propoxide-1,1,1,3,3,3-d6 (2) anions. Unusually large secondary isotope effects (pulsed laser, 1.9 for 1 and 1.7 for 2; cw laser, 8 for 1) and small primary isotope effects (pulsed laser, 1.6 for 1 and 2; cw laser, 2.0 for 1) were observed. These isotope effects, particularly the large difference in energy dependence of the primary and secondary effects, are consistent only with a stepwise mechanism involving initial bond cleavage to an intermediate ion-molecule complex followed by a hydrogen transfer within the intermediate complex. The observed secondary isotope effects have been modelled by using statistical reaction rate (RRKM) theory. The implications of this study for several previously reported unimolecular ion decompositions are also discussed.

Original languageEnglish
Pages (from-to)961-970
Number of pages10
JournalJournal of the American Chemical Society
Volume109
Issue number4
Publication statusPublished - 1987

Fingerprint

Isotopes
Anions
Negative ions
Gases
Ions
Lasers
Decomposition
Cyclotron resonance
Cyclotrons
Pulsed lasers
Spectrometers
Hydrogen
Trapped ions
Methane
High power lasers
Photolysis
Acetone
Gas Lasers
Reaction rates
Photons

ASJC Scopus subject areas

  • Chemistry(all)

Cite this

A stepwise mechanism for gas-phase unimolecular ion decompositions. Isotope effects in the fragmentation of tert-butoxide anion. / Tumas, William; Foster, Robert F.; Pellerite, Mark J.; Brauman, John I.

In: Journal of the American Chemical Society, Vol. 109, No. 4, 1987, p. 961-970.

Research output: Contribution to journalArticle

@article{7d96e7e78e7546f08bdf5ad92de78063,
title = "A stepwise mechanism for gas-phase unimolecular ion decompositions. Isotope effects in the fragmentation of tert-butoxide anion",
abstract = "Infrared multiple photon (IRMP) photochemical activation of gas-phase ions trapped in an ion cyclotron resonance (ICR) spectrometer has been used to study the mechanism of a gas-phase negative ion unimolecular decomposition. Upon irradiation with a CO2 laser (both high-power pulsed and low-power continuous wave (CW)), tert-butoxide anion, trapped in a pulsed ICR spectrometer, decomposes to yield acetone enolate anion and methane. The mechanism of this formal 1,2-elimination reaction was probed by measuring competitive hydrogen isotope effects (both primary and secondary) in the IR laser photolysis of 2-methyl-2-propoxide-1,1,1-d3 (1) and 2-methyl-2-propoxide-1,1,1,3,3,3-d6 (2) anions. Unusually large secondary isotope effects (pulsed laser, 1.9 for 1 and 1.7 for 2; cw laser, 8 for 1) and small primary isotope effects (pulsed laser, 1.6 for 1 and 2; cw laser, 2.0 for 1) were observed. These isotope effects, particularly the large difference in energy dependence of the primary and secondary effects, are consistent only with a stepwise mechanism involving initial bond cleavage to an intermediate ion-molecule complex followed by a hydrogen transfer within the intermediate complex. The observed secondary isotope effects have been modelled by using statistical reaction rate (RRKM) theory. The implications of this study for several previously reported unimolecular ion decompositions are also discussed.",
author = "William Tumas and Foster, {Robert F.} and Pellerite, {Mark J.} and Brauman, {John I.}",
year = "1987",
language = "English",
volume = "109",
pages = "961--970",
journal = "Journal of the American Chemical Society",
issn = "0002-7863",
publisher = "American Chemical Society",
number = "4",

}

TY - JOUR

T1 - A stepwise mechanism for gas-phase unimolecular ion decompositions. Isotope effects in the fragmentation of tert-butoxide anion

AU - Tumas, William

AU - Foster, Robert F.

AU - Pellerite, Mark J.

AU - Brauman, John I.

PY - 1987

Y1 - 1987

N2 - Infrared multiple photon (IRMP) photochemical activation of gas-phase ions trapped in an ion cyclotron resonance (ICR) spectrometer has been used to study the mechanism of a gas-phase negative ion unimolecular decomposition. Upon irradiation with a CO2 laser (both high-power pulsed and low-power continuous wave (CW)), tert-butoxide anion, trapped in a pulsed ICR spectrometer, decomposes to yield acetone enolate anion and methane. The mechanism of this formal 1,2-elimination reaction was probed by measuring competitive hydrogen isotope effects (both primary and secondary) in the IR laser photolysis of 2-methyl-2-propoxide-1,1,1-d3 (1) and 2-methyl-2-propoxide-1,1,1,3,3,3-d6 (2) anions. Unusually large secondary isotope effects (pulsed laser, 1.9 for 1 and 1.7 for 2; cw laser, 8 for 1) and small primary isotope effects (pulsed laser, 1.6 for 1 and 2; cw laser, 2.0 for 1) were observed. These isotope effects, particularly the large difference in energy dependence of the primary and secondary effects, are consistent only with a stepwise mechanism involving initial bond cleavage to an intermediate ion-molecule complex followed by a hydrogen transfer within the intermediate complex. The observed secondary isotope effects have been modelled by using statistical reaction rate (RRKM) theory. The implications of this study for several previously reported unimolecular ion decompositions are also discussed.

AB - Infrared multiple photon (IRMP) photochemical activation of gas-phase ions trapped in an ion cyclotron resonance (ICR) spectrometer has been used to study the mechanism of a gas-phase negative ion unimolecular decomposition. Upon irradiation with a CO2 laser (both high-power pulsed and low-power continuous wave (CW)), tert-butoxide anion, trapped in a pulsed ICR spectrometer, decomposes to yield acetone enolate anion and methane. The mechanism of this formal 1,2-elimination reaction was probed by measuring competitive hydrogen isotope effects (both primary and secondary) in the IR laser photolysis of 2-methyl-2-propoxide-1,1,1-d3 (1) and 2-methyl-2-propoxide-1,1,1,3,3,3-d6 (2) anions. Unusually large secondary isotope effects (pulsed laser, 1.9 for 1 and 1.7 for 2; cw laser, 8 for 1) and small primary isotope effects (pulsed laser, 1.6 for 1 and 2; cw laser, 2.0 for 1) were observed. These isotope effects, particularly the large difference in energy dependence of the primary and secondary effects, are consistent only with a stepwise mechanism involving initial bond cleavage to an intermediate ion-molecule complex followed by a hydrogen transfer within the intermediate complex. The observed secondary isotope effects have been modelled by using statistical reaction rate (RRKM) theory. The implications of this study for several previously reported unimolecular ion decompositions are also discussed.

UR - http://www.scopus.com/inward/record.url?scp=33845281271&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=33845281271&partnerID=8YFLogxK

M3 - Article

AN - SCOPUS:33845281271

VL - 109

SP - 961

EP - 970

JO - Journal of the American Chemical Society

JF - Journal of the American Chemical Society

SN - 0002-7863

IS - 4

ER -