Direct femtosecond observation of the transient intermediate in the α-cleavage reaction of (CH3)2CO to 2CH3+CO: Resolving the issue of concertedness

Kim, Sang Kyu; Pedersen, Søren Møller; Zewail, Ahmed H.

doi:10.1063/1.469614

Cited by 130 publications

(49 citation statements)

References 29 publications

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“…Femtosecond studies of the lowest excited electronic state of acetone have shown the state to have a very short lifetime (<200 fs) leading to a high fragmentation yield [22,23].…”

Section: Photofragmentation Of Metal Cation/ketone Complexes At 266 Amentioning

confidence: 99%

UV photofragmentation of gas phase M and M2 complexes with ketones, where M = Cu and Ag

Puškar

Stace

2005

Molecular Physics

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Results are reported of an experimental study of the UV photofragmentation patterns of singly and doubly charged complexes consisting of either copper or silver in association with a series of ketones. Considerable variation is seen in response to photoexcitation, ranging from extensive chemical reactivity in the case of Cu þ through to no evidence of photoabsorption on the part of Ag 2þ complexes. It is concluded that the close proximity of a charge has a considerable influence on energy of the non-bonding electrons of the carbonyl group in the ketones, and that this, in turn, influences the * n electronic transition promoted at UV wavelengths.

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“…Femtosecond studies of the lowest excited electronic state of acetone have shown the state to have a very short lifetime (<200 fs) leading to a high fragmentation yield [22,23].…”

Section: Photofragmentation Of Metal Cation/ketone Complexes At 266 Amentioning

confidence: 99%

UV photofragmentation of gas phase M and M2 complexes with ketones, where M = Cu and Ag

Puškar

Stace

2005

Molecular Physics

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“…The use of femtosecond transition state spectroscopy to detect the presence of reaction intermediates was introduced by Zewail and coworkers (84). Their time-resolved experiments on the α-cleavage reaction of acetone and on the decarbonylation of cyclopentanone have shown that both reactions proceed by a stepwise (nonconcerted) mechanism (85,86). In the condensed phase, the solvent provides a "cage" that keeps the reagents and products of a chemical reaction in close proximity, making it difficult to determine if a reaction proceeds by a concerted mechanism.…”

Section: Bound-free Transitions: Concerted-elimination Chemical Reactmentioning

confidence: 99%

COHERENTNONLINEARSPECTROSCOPY: From Femtosecond Dynamics to Control

Dantus

2001

Annu. Rev. Phys. Chem.

118

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This review focuses on the study of the dynamics of isolated molecules and their control using coherent nonlinear spectroscopic methods. Emphasis is placed on topics such as bound-to-free excitation and the study of concerted elimination reactions, free-to-bound excitation and the study of bimolecular reactions, and bound-to-bound excitation and the study of intramolecular rovibrational dynamics and coherence relaxation. For each case the detailed time-resolved information reveals possible strategies to control the outcome. Experimental results are shown for each of the reactions discussed. The methods discussed include pump-probe and four-wave mixing processes such as transient grating and photon echo spectroscopy. Off-resonance transient-grating experiments are shown to be ideal for the study of ground state dynamics, molecular structure, and the molecular response to strong field excitation.

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“…The first ␣-cleavage occurs on a time scale (50 fs) shorter than a vibrational period, indicating * promotion is involved and IVR is circumvented. The second cleavage occurs on a longer timescale, suggesting that electronic to vibrational relaxation could take place before this cleavage [61]. Subsequent femtosecond experiments on a series of cyclic ketones have shown only a small dependence of Norrish Type I dissociation rates on ring size.…”

Section: Is the Initial Cleavage Nonergodic?mentioning

confidence: 99%

Pathways of Peptide Ion Fragmentation Induced by Vacuum Ultraviolet Light

Cui

Thompson

Reilly

2005

J. Am. Soc. Mass Spectrom.

119

181

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One Hundred Fifty-Seven nm photodissociation of singly protonated peptides generates unusual distributions of fragment ions. When the charge is localized at the C-terminus of the peptide, spectra are dominated by x-, v-, and w-type fragments. When it is sequestered at the N-terminus, a-and d-type ions are overwhelmingly abundant. Evidence is presented suggesting that the fragmentation occurs via photolytic radical cleavage of the peptide backbone at the bond between the ␣-and carbonyl-carbons followed by radical elimination to form the observed daughter ions. Low-energy fragmentation appears to be well described by the mobile proton model according to which vibrational excitation of the analyte leads to charge mobility [12]. Transfer of the charge proton to either the backbone carbonyl oxygen or amide nitrogen enables charge-induced cleavage of the peptide backbone. This process yields primarily b-and y-type fragment ions according to the standard nomenclature shown below [13,14].Higher energy activation methods involving collisions with gas molecules or surfaces can enable chargeremote fragmentation [15][16][17]. However, even in these cases, apparently, protonated peptides still generate some fragments through charge-directed processes [17]. Immobilization of the charge, either by using metal adducts [18] or charged chemical modifications [19 -22], has been used to inhibit charge-directed fragmentation. In these cases, primarily a-, d-, and w-type fragments are observed. Several mechanisms have been proposed for charge-remote peptide fragmentation, some involving homolytic radical cleavage [14,23].Electron capture dissociation (ECD) [24] and the recently reported electron-transfer dissociation (ETD)[25] appear to be nonthermal processes. These phenomena generate c-and z-type fragment ions upon the addition of an electron to a multiply charged protein or peptide ion. Two mechanisms have been proposed, one involving reactions of a free hydrogen atom generated when an electron is captured by a charged site on the analyte ion [26], and the other involving localization of the ϳ6 eV of energy that is generated upon charge neutralization. This energy then induces fragmentation through an excited electronic state [27]. Implicit in this second mechanism is the suggestion that techniques which excite appropriate electronic states can lead to unique, nonergodic fragmentation even for molecules as large as peptides and proteins [24,27]. This can occur if ions reach a dissociative electronic state and fragment before the energy is redistributed throughout the molecule. Electronic to vibrational relaxation, resulting in nonspecific excitation is an important competing process that often inhibits nonergodic processes, and rates of relaxation are predicted to increase with the size of the molecule [28].Lasers are powerful tools that can also be used to excite molecules to specific energy levels with either multiphoton or single photon processes. Light is not affected by the electric or magnetic fields of mass spectrometers, s...

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Direct femtosecond observation of the transient intermediate in the α-cleavage reaction of (CH3)2CO to 2CH3+CO: Resolving the issue of concertedness

Cited by 130 publications

References 29 publications

UV photofragmentation of gas phase M and M2 complexes with ketones, where M = Cu and Ag

UV photofragmentation of gas phase M and M2 complexes with ketones, where M = Cu and Ag

COHERENTNONLINEARSPECTROSCOPY: From Femtosecond Dynamics to Control

Pathways of Peptide Ion Fragmentation Induced by Vacuum Ultraviolet Light

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