Product excitation in collision‐induced dissociative ionization of acetone: Internal energy distribution

122

The unimolecular reactions of protonated and metalated polyglycols with kiloelectronvolt translational energies have been studied by collisionally activated dissociation and neutralization-reionization mass spectrometry. The former method provides information on the ionic dissociation products, whereas the latter allows for the identification of the complementary neutral losses. Protonated linear polyglycols mainly undergo charge-initiated decompositions that lead to eliminations of smaller oligomers, On the other hand, protonated crown ethers ("cyclic" polyglycols) favor charge-induced reactions that proceed by cleavages of two ethylene oxide units in the form of 1,4-dioxane. Replacement of one O by NH in the crown ether dramatically changes its unimolecular chemistry; now, charge-remote 1,4-eliminations from ring-opened isomers are preferred. Charge-remote reactions are also the major decomposition channels of all metalated precursors studied. The linear polyglycols decompose primarily by 1,4-H2 eliminations and to a lesser extent by homolytic cleavages near chain ends. The reverse is true for metalated crown ethers, which preferentially produce distonic radical cations by the loss of saturated radicals; these reactions are proposed to involve prior rearrangement to open-chain isomers. The nature of the metal ion (Li(+), Na(+), or K(+)) does not greatly affect the unimolecular chemistry of the cationized polyglycol. In general, metalated precursors form many abundant fragment ions over the entire mass range; hence, collisional activation of such ions at high kinetic energy should be particularly useful for structure elucidations.

Section: Resultsmentioning

confidence: 99%

Dissociation characteristics of [M + X]⁺ ions (X = H, Li, Na, K) from linear and cyclic polyglycols

Selby

Wesdemiotis

Lattimer³

1994

122

“…It is quite remarkable that an excited state of the acetone ion could remain stable during its passage from the ion source to the collision center even though it had such a large excess of internal energy. However, a number of other experiments also indicated [23,24] that acetone molecular ion can have excess energy in this range and not decompose. In our earlier paper, we proposed on the basis of similar energetics that this long-lived state is the A state of the acetone ion.…”

Section: Resultsmentioning

confidence: 99%

Fundamentals of tandem mass spectrometry: a dynamics study of simple C−C bond cleavage in collision-activated dissociation of polyatomic ions at low energy

Shukla

Qian

Anderson

et al. 1990

The loss of methyl radical in collision-activated dissociation (CAD) of acetone and propane molecular ions has been studied at low energy using a tandem hybrid mass spectrometer. Although the two processes are very similar chemically and energetically, very different dynamical features are observed. Acetyl ions from acetone ion are predominantly backward-scattered, with intensity maxima lying inside and outside the elastic scattering circle, confirming our previous observation that electronically excited states are important in low-energy acetone CAD. Ethyl ions from propane ion show a forward-scattered peak maximum at a nonzero scattering angle, which is consistent with generally accepted models for vibrational excitation and redistribution of energy before dissociation. Both processes demonstrate that CAD at low energy proceeds via small-impact-parameter collisions with momentum transfer. Comparison of the present results with higher energy CAD dynamics studies and earlier work leads to some tentative general conclusions about energy transfer in these processes.

“…The average deposited internal energy falls from 9 to 7 to 6 eV as the kinetic energy is changed from 8 -> 5 -> 3 keY. It is worth noting that the mean internal energy of neutralizedreionized W(CO)t" at 3 keY (6 e'V) is comparable to the Z 5 eV mean internal energy deposited upon the collision-induced dissociative ionization (CIDl) of 3-keY acetone molecules, produced by the fragmentation (C3H60)2H+ (proton-bound dimer) --> C 3H 70+ + C 3H 60 [41]. This similarity reinforces the fact that the internal energy of the nascent molecular ions emerging on NR is basically imparted during the reionization event.…”

Section: Internal Energy Distributionsmentioning

confidence: 96%

“…Oxygen belongs to the "softest" reionization gases used so far. Even higher energies are possible upon reionization with other, harder targets, for example, N 2 or He [41,60].…”

Section: Internal Energy Distributionsmentioning

confidence: 99%

See 1 more Smart Citation

Internal energy distributions of tungsten hexacarbonyl ions after neutralization—Reionization

Beranová

Wesdemiotis

1994

The internal energy distributions P(ε) transferred to W(CO) 6 (+·) during the kiloelectronvolt collisions that occur upon neutralization-reionization (NR) have been estimated based on the relative abundances of the W(CO) 0-6 (+·) products present in NR spectra (thermochemical method). The average internal energy of the incipient {W(CO) 6 (+·) }(*) ions arising after near thermoneutral neutralization with trimethylamine followed by reionization with O2 is ∼9 eV for 8-keV precursor ions and is mainly deposited during reionization, For comparison, the mean internal energy of {W(CO) 6 (+·) }(*) after electron ionization (EO or collisionally activated dissociation (CAD) is ∼ 6 eV. Making the neutralization step endothermic slightly increases the overall excitation gained; however, a large increase in endothermicity (> 16 eV) causes only a modest rise of the average internal energy (<2 eV). The P(ε) curve for NR increases exponentially up to ∼ 6 eV and levels off at higher energies.. showing that the probability of imparting large internal energies (6-17 eV) is high. In sharp contrast, the most probable excitation on CAD is ≤6 eV, and the probability of deposition of larger energies declines exponentially. The mean internal energies after CAD and NR decrease steadily when the kinetic energy is lowered. The structure (minima-maxima) observed in the P(ε) distribution for El, which most likely originates from Franck-Condon factors, is not reproduced in the distributions for NR or high energy CAD, despite the fact that all three methods involve electronic excitation. Because of the large internal energies transferred upon NR, NR mass spectrometry could be particularly useful in the differentiation of ionic isomers with high dissociation but low isomerization thresholds.