A photoionization study of the ion-neutral complexes CH3CH (+)CH 3 (·) CH 2CH 3] and CH 3CH 2CH (+)CH 3 (·) CH 3 in the gas phase: Formation, H-transfer and C-C bond formation between partners, and channeling of energy into dissociation

Two-Body Metastable Dissociation of n-Pentane and n-Hexane Triplet Dications in Intense Femtosecond-Laser Fields

Mass spectra of n-pentane and n-hexane ionized through femtosecond-laser pulses were measured using a time-of-flight mass spectrometer. Fragment ions ejected with large kinetic energies were identified as side peaks in which a two-body dissociation pathway, C₅H₁₂⁺⁺ → C₂H₅⁺ + C₃H₇⁺, was identified for n-pentane, and two for n-hexane, C₆H₁₄⁺⁺ → C₂H₅⁺ + C₄H₉⁺ and C₃H₇⁺ + C₃H₇⁺, based on momentum matching of the fragments. The two-body dissociation pathways were observed when the polarization direction of the linearly polarized laser light was perpendicular to the molecular axis. However, when the polarization direction was parallel to the molecular axis or the laser light was circularly polarized, these signals were weak or difficult to identify. These results suggest that the two-body dissociation pathways are caused by nonsequential double ionization (NSDI), which begins with ionization from the π-type second highest occupied molecular orbital (HOMO-1) via the laser electric field perpendicular to the molecular axis rather than bonding the σ-type HOMO. Quantum chemical calculations show that the dication has a triplet metastable state with the same formula as the neutral state (i.e., ³[CH₃-(CH₂)_n-CH₃]⁺⁺). Therefore, the relevant two-body dissociation channels open through transition states with the (HOMO)¹(HOMO-1)¹ electron configuration and the estimated kinetic energy release values correlate with those observed.

Determination of absolute photoionization cross-sections of alkanes and cyclo-alkanes

Absolute photoionization and dissociative photoionization cross-sections of eleven n-alkanes (n-pentane, n-hexane, n-heptane, n-nonane, n-decane, n-undecane, n-dodecane, n-tridecane, n-tetradecane, n-pentadecane and n-hexadecane), three cyclo-alkanes (cyclopentane, methylcyclohexane and trans-decahydronaphthalene) and iso-octane were measured for photon energies from the ionization thresholds to 11.5 eV. The measurements were performed with the binary-liquid-mixture method utilizing the photoionization cross-sections of benzene as a calibration standard. The ionization energies of n-alkanes and cyclo-alkanes were also calculated at the B3P86/6-31 + +G(d,p) level and by the G3B3 method.

Elimination of H2 from CH3CH=N+HCH3: a synchronous, concerted 1,4-H2 elimination

Most H2 eliminations from cations in the gas phase are formally 1,1- or 1,2- processes. Larger ring size H2 eliminations are rare and little studied. Thus, whether the 6-center, 1,4- elimination CH3CH=N+HCH3-->CH2=CHN+H=CH2+H2 is concerted and synchronous, as indicated by isotope effects and predicted by conservation of orbital symmetry, is a significant question. This reaction is characterized here by application of QCI and B3LYP theories. CH bond-breaking and H-H bond-making in this reaction are found by theory to be highly synchronized, consistent with previously established isotope effects and in contrast to "forbidden" 1,2-eliminations from organic cations in the gas phase. This reaction is made feasible by its conservation of orbital symmetry, the energy supplied by formation of the H-H bond, and a favorable geometry of the ion for eliminating H2.

Why CH3CH3+* formation competes with H* loss from CCCO C3H6O+* isomers

How formation of CH3CH3+* competes with H* loss from C3H6O+* isomers with the CCCO framework has been a puzzle of gas phase ion chemistry because the first reaction has a substantially higher threshold and a supposedly tighter transition state. These together should make CH3CH3+* formation much the slower of the two reactions at all internal energies. However, the rates of the two reactions become comparable at about 20 kJ x mol(-1) above the threshold for CH3CH3+* formation. It was recently shown that losses of atomic fragments increase in rate much more slowly with increasing internal energy than do the rates of competing dissociations to two polyatomic fragments. This occurs because fewer frequencies are substantially lowered in transition states for the former type of reaction than for the latter. The resulting lower transition state sums of states cause the rates of dissociations producing atoms as fragments to increase much more slowly than competing processes with increasing energy. Here we show that this is why CH3CH3+* formation competes with H* loss from CH3CH2CHO+*. These results further establish that the dependence on energy of the rate of a simple unimolecular dissociation is usually directly related to the number of rotational degrees of freedom in the products, a newly recognized factor in determining the dependence of unimolecular reaction rates on internal energy.

Contributions of C3H6O+. ions with the oxygen on the middle carbon to gas phase ion chemistry

The numerous ways in which studies of C3H6O+. ions with the oxygen on the second carbon have added to our knowledge of gas phase ion chemistry are reviewed. The enol form of this ion (1) first attracted interest during early investigations of the mechanism of the McLafferty Rearrangement and later in characterizing the double McLafferty Rearrangement. Next, it was found that 1 isomerizes to the higher energy acetone ion (2). This discovery sparked studies of the relative stabilities of ionized and neutral enol and ketone species. It also led to the discovery that 1-->2 surmounts a substantial barrier, and that 2 dissociates faster than the excess energy deposited in it by the isomerization can become randomly distributed; i.e., dissociation following 1-->2 is nonergodic. This fact is manifested by a more abundant loss of the methyl formed by isomerization and a greater associated translational energy release. Propene oxide and methyl vinyl ether ions also appear to ionize to 2 and to dissociate nonergodically. Methane is eliminated from 2 through a methyl-acetyl ion complex. Characterization of this reaction by photoionization mass spectrometry, ab initio theory, and RRKM calculations helped to establish that complex-mediated alkane eliminations are generally confined to a region just above threshold. At higher energies, because attractions between ions and nonpolar neutrals are weak, simple dissociation is too fast for complex-mediated H-transfer to compete with it. Studies of the collision-induced dissociations of 2 demonstrate that the first electronically excited A state of 2 is very long-lived and efficiently releases its energy into translational energy when the ion collides with a neutral at low impact energies. Finally, ion-molecule reactions of 2 and acetone-containing ion clusters in the gas phase are described.

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

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.

Ab initio characterization of ring-opening H-transfer in ionized cyclopentanone: Similarity to ion-neutral complex-mediated alkane eliminations

Ab initio procedures were used to characterize the transition states for α-cleavage and subsequent H-transfer starting with ionized cyclopentanone (1). The objective was to determine whether the parts of the ion disconnected by the bond cleavage remain in close association through the H-transfer. The transition state for C-C bond cleavage is close in energy to the resulting distonic ion, which is a stable species, and far in geometry from any transition state involving H-transfer. We find no evidence for any concertedness or "quasiconcertedness" in this reaction. Ring-opening H-transfer in 1 and ion-neutral complex-mediated alkane eliminations are compared.