The dissociation energetics and kinetics of energy-selected gas-phase ions of C(4)H(6), C(4)H(9)Br, C(8)H(20)Si, C(6)H(16)Si and C(4)H(12)Si studied by threshold photoelectron photoion coincidence.

摘要

Experimental determinations of ion dissociation energetics and kinetics are made using the threshold photoelectron-photoion coincidence (TPEPICO) technique. The objects of study are the energy-selected gas-phase ions of 1,3-butadiene, 3-methylcyclopropene, 2-bromobutane, tetraethylsilane, triethylsilane and diethylsilane.; Vacuum ultraviolet photoelectron spectroscopy is the basis for this coincidence technique. Through the analysis of time-of-flight mass spectra of ions which are formed in coincidence with nearly zero kinetic energy (threshold) electrons, the dissociation behavior of energy-selected ions is revealed. In this way, ionization energies, dissociation energies, and microcanonical dissociation rate constants are obtained which can be compared to the most up-to-date theoretical predictions.; However, the accurate determination of 0 K thermochemistry and reaction rate constants can only be made by explicit consideration of the reactant ion internal energy distribution. Several approaches for dealing with this complication are presented.; Fundamental features of the ionic systems under study can also be tested, to some degree, using the transition state theory and ab initio molecular orbital predictions. For example, the data can be explained in terms of potential energy surfaces with stable structures connected to each other by "tight" rearrangement transition states and to dissociation products by "loose" dissociative transition states. This is a result of the powerful statistical rate expression of Rice, Ramsperger, Kassel, and Marcus (RRKM).; In addition, reaction mechanisms and energetics can be understood in great detail with the help of ab initio molecular orbital calculations. Some interesting reaction mechanisms which appear to be important for the observed reactions include H or {dollar}rm Hsp+{dollar} tunneling through a transition state barrier, rearrangement of reactants to less reactive lower-energy structures, and barrierless reactions. Details of these features are given in the text.