PEPICO

Results of a PEPICO study of the dissociation dynamics of 2-bromobutane ions have been analysed with tunnelling-corrected RRKM statistical theory using vibrational frequencies obtained from ab initio MO calculations. It has been concluded that the slow rate of loss of HBr, to form the but-2-ene ion, occurs via a concerted mechanism in which tunnelling is a feature of the proton transfer. 

To date, by far the best quantitative studies of unimolecular dissociations of ions with a well-defined energy distribution have been those using the photoelectron photoionization technique (PEPICO). These reactions have been discussed in detail elsewhere, most notably by Baer. ... 

Our first TRPD study, on chlorobenzene ion, concerned a system where the rate-energy curve from PEPICO was well established, and the point of this work was to confirm that TRPD results were consistent with expectations. Most of our subsequent TRPD work has resulted in new thermochemical information, or in major strengthening of existing values, and will be worth discussing here from a specifically thermochemical point of view. 

The study of cyanobenzene ion treated an ion whose thermochemistry was already well studied by PEPICO. However, the TRPD result was at a substantially lower internal energy, so that the measured dissociation rate of 5 x 10 s at 4.10 eV internal energy was slower by more than an order of magnitude than the slowest dissociations probed by PEPICO. By extension of the known rate-energy curve nearer to threshold, this added quantitative confidence to the RRKM extrapolation and considerably strengthened the g value for Equation (5) of 3.02 eV assigned... 

When TRPD measurements are combined with PEPICO results, the dissociation rate-energy curve for styrene ion is known over perhaps the largest of any polyatomic ion s range. ° Simple RRKM theory gives an excellent fit (as does Klots thermodynamic formulation), and an extrapolated of 2.43 eV is derived. The thermochemistry for this dissociation to benzene ion plus acetylene [Equation (16)] is very well known from independent heats of formation, giving a calculated... 

Some confusion about toluene ion dissociation thermochemistry has arisen from the PEPICO rate-energy curve reported by Bombach, R. Dannacher, J. Stadelmann, J.-T. J. Am. Chem. Soc. 1983,105,4205, which is not in agreement with other observations and apparently suffered from an experimental artifact. The two-charmel analysis offered in their paper was qualitatively but not quantitatively correct See Refs. 5, 19. 

PEPICO, see Photoelectron photoion coincidence spectroscopy Peptides... 

Photoionization always produces two species available for analysis the ion and the electron. By measuring both photoelectrons and photoions in coincidence, the kinetic electron may be assigned to its correlated parent ion partner, which may be identified by mass spectrometry. The extension of the photoelectron-photoion-coincidence (PEPICO) technique to the femtosecond time-resolved domain was shown to be very important for studies of dynamics in clusters [131, 132]. In these experiments, a simple yet efficient permanent magnet design magnetic bottle electron spectrometer was used for photoelectron... 

For all the dissociating halotoluenes examined, the formation of tight transition states typical of rearrangement reactions is confirmed by the negative S% values found by fitting the dissociation rate vs internal energy curves, calculated on the basis of the RRKM theory, with the experimental PEPICO data145. 

Kinetic energy releases resulting from HjfDJ elimination from C2H4 and C2D4 obtained using PEPICO... 

Combination of PES with mass spectrometry in a coincidence system leads to an extremely powerful technique called photoelectron-photoion coincidence (PEPICO) spectroscopy, which renders it possible to examine dissociations of energy-selected molecular ions. 

A typical PEPICO apparatus is shown in Figure 1, where an acceleration region followed by a drift tube is used to determine the ionic species. The coincidence condition is achieved by using electron and ion signals as start and stop inputs, respectively, to a time-to-pulse height converter whose output is sent to a multichannel pulse height analyser. 

By using this PEPICO apparatus a great amount of dynamical information can be... 

FIGURE 1. Block diagram of a PEPICO experiments. Reproduced by permission of Academic Press... 

The result of another type of PEPICO experiment is the breakdown diagram, which is a plot of the fractional abundance of the ions formed by the dissociation of energy-selected parent ions as a function of the photon energy. It is also represented as a plot of branching ratios to the various dissociation products as a function of the precursor ion internal energy. The breakdown diagrams are obtained directly from a series of PEPICO mass spectra at several photon energies. 

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