Competing reactions of the acetone cation radical: RRKM-QET calculations on an ab initio potential-energy surface

J. Am. Soc. Mass Spectrom.

2004

The reaction coordinates of 1,3-H-shifts across double bonds are traced by theory for three reactions, CH 3 C(OH)CH 2 ϩ⅐ (1) 3 CH 3 C(O ϩ⅐ )CH 3 (2), CH 2 C(OH) 2 ϩ⅐ (3) 3 CH 3 CO 2 H ϩ⅐ (4) and CH 3 C(OH)CH 2 ϩ⅐ (1) 3 CH 2 C(OH)CH 3 ϩ⅐ (1Ј), to explore how the need to conserve orbital symmetry influences the pathways for these reactions. In the first and second reactions, prior to the start of the H-transfer the methylene rotates from being in the skeletal plane to being bisected by it. Thus these reactions are neither antarafacial nor suprafacial, but precisely between those possibilities. This stems from a counterbalancing between the need to conserve orbital symmetry and the large distorting forces required to attain an allowed antarafacial transition state. In contrast to the first two reactions, 1 3 1Ј follows a suprafacial pathway. However, this pathway does not violate conservation of orbital symmetry, as it utilizes lower lying orbitals of appropriate symmetry rather than the antisymmetric uppermost occupied allyl-type orbital. Changes in geometry which presumably produce asymmetric vibrational excitation and the unequal losses of methyl that follow 1 3 2, i.e., nonergodic behavior, are also characterized. (J Am Soc Mass Spectrom 2004, 15, 972-981)

Section: Resultsmentioning

confidence: 49%

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confidence: 99%

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confidence: 99%

See 1 more Smart Citation

Circumvention of orbital symmetry restraints by 1,3-H-shifts of enolic radical cations

J. Am. Soc. Mass Spectrom.

2004

“…However, he did not address whether 2 is important in the elimination of ethane from 1. Experimental [8,[17][18][19] and theoretical [20][21][22] results indicate that most alkane eliminations from radical cations in the gas phase take place through [R + OR'] complexes. In particular, a recent high level ab initio study [23] of n-butane and 2-methylpropane ions, adjacent homologs to the ions studied here, demonstrates quite clearly that elimination of methane by those ions is complex-mediated, so this work will proceed from the assumption that alkane eliminations from 1 and 2 are complex-mediated also.…”

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confidence: 99%

A photoionization study of the ion-neutral complexes CH₃CH ⁺CH₃^·CH₂CH₃] and CH₃CH₂CH⁺CH₃^·CH₃ in the gas phase: Formation, H-transfer and C—C bond formation between partners, and channeling of energy into dissociation

Traeger

J. Am. Soc. Mass Spectrom.

1996

Photoionization mass spectrometry was used to investigate the dynamics of ion-neutral complex-mediated dissociations of the n-pentane ion (1). Reinterpretation of previous data demonstrates that a fraction of ions 1 isomerizes to the 2-methylbutane ion (2) through the complex [CH 3CH+CH3'CH 2CH 3] (3), but not through [CH 3CH+CH 2CH 3'CH 3] (4). The appearance energy for C3H~formation from 1 is 66 k] mol-I below that expected for the formation of I1-C3H~and just above that expected for formation of i-C 3H 7 • This demonstrates that the H shift that isomerizes C3H~is synchronized with bond cleavage at the threshold for dissociation to that product. It is suggested that ions that contain n-alkyl chains generally dissociate directly to more stable rearranged carbenium ions. Ethane elimination from 3 is estimated to be about seven times more frequent than is C -C bond formation between the partners in that complex to form 2, which demonstrates a substantial preference in 3 for H abstraction over C-C bond formation. In 1 -+ CH 3CH+CH 2CH 3 +' CH 3 by direct cleavage of the C1-C2 bond, the fragments part rapidly enough to prevent any reaction between them. However, 1 -+ 2 -+ 4 -+ C 4Hs '+ CH 4 occurs in this same energy range. Thus some of the potential energy made available by the isomerization of I1-C 4H 9 in 1 is specifically channeled into the coordinate for dissociation. In contrast, analogous formation of 3 by 1 --+ 3 is predominantly followed by reaction between the electrostatically bound partners. {J Am Soc Mass Spectrom 1996, 7, 73-81) I on-neutral complexes (ionic and neutral fragments held together primarily by noncovalent attractions) are often intermediates in low energy dissociations of ions in the gas phase [1][2][3][4][5]. As the energy in an ion is increased toward the threshold for simple dissociation, partial dissociation to noncovalently bound fragments that can react with each other begins to occur [6][7][8]. Determination of how such reactions depend on energy provides unique information on interactions between ions and neutrals at energies from just below to just above the threshold for separation of the partners-an energy regime that is not readily accessed by other means. Little detailed experimental information exists on such interactions.To understand better the dynamics of complexmediated reactions, we undertook a study of the en- ergy dependence of pertinent dissociations of the isomers ionized n-pentane (1) and 2-methylbutane (2). Scheme I, which is similar to one presented by Wendelboe et al. [9], summarizes the reactions we will characterize for this purpose. It includes three possible mechanisms for alkyl isomerization upon dissociation of 1: dissociation to complexes (species enclosed in brackets in Scheme I and throughout this contribution) followed by isomerization of the alkyl ion partner (reactions c and d) [9], concerted C-C bond cleavage and isomerization (reaction a) [10], and complex formation by isomerization-bond scission (reactions b and e). Simple bond cleavage to form...

“…Whether the 4-heptanone ion eliminates propane via ion-neutral complexes has not specifically been investigated, but numerous experimental 11,12,25 and theoretical 14,15 investigations demonstrate that alkane eliminations from other ketone ions are. First differential PIE curves for the relevant dissociations of ionized 4-heptanone are given in Fig.…”

Section: Alkane Elimination Versus Mclafferty Rearrangementmentioning

confidence: 99%

H-transfer rates in ion-neutral complexes versus those in intact aliphatic ions

Traeger

1999

J. Mass Spectrom.

The relative rates of H‐transfer between partners in ion–neutral complexes were compared with those in intramolecular rearrangements using results of first differential photoionization mass spectrometry measurements. Complex‐mediated H‐transfers are inferred to have rates of the same order as those for intramolecular hydrogen rearrangements, suggesting a similar range of motion of the reactive sites in both types of reactions. It is also concluded that at their fastest H‐transfers take place between the partners in ion–neutral complexes within at most the time of several rotations of the partners in the complexes. Copyright © 1999 John Wiley & Sons, Ltd.