Sharon G. Lias

Evaluated Gas Phase Basicities and Proton Affinities of Molecules: An Update

The available data on gas-phase basicities and proton affinities of approximately 1700 molecular, radical and atomic neutral species are evaluated and compiled. Tables of the data are sorted (1) according to empirical formula and (2) according to evaluated gas basicity. This publication constitutes an update of a similar evaluation published in 1984.

Evaluated Gas Phase Basicities and Proton Affinities of Molecules; Heats of Formation of Protonated Molecules

The available data on gas phase basicities and proton affinities of molecules are compiled and evaluated. Tables giving the molecules ordered (1) according to proton affinity and (2) according to empirical formula, sorted alphabetically are provided. The heats of formation of the molecules and the corresponding protonated species are also listed.

The critical evaluation of a comprehensive mass spectral library

A description of the methods used to build a high quality, comprehensive reference library of electron-ionization mass spectra is presented. Emphasis is placed on the most challenging part of this project—the improvement of quality by expert evaluation. The methods employed for this task were developed over the course of a spectrum-by-spectrum review of a library containing well over 100,000 spectra. Although the effectiveness of this quality improvement task depended critically on the expertise of the evaluators, a number of guidelines are discussed which were found to be effective in performing this onerous and often subjective task. A number of specific examples of the particularly challenging task of spectrum editing are given.

Structure/Reactivity and Thermochemistry of Ions

Capture Collision Theory.- Collision Theory: Summary of the Panel Discussion.- Optical Studies of Product State Distributions in Thermal Energy Ion-Molecule Reactions.- Crossed-Molecular Beam Studies of Charge Transfer Reactions at Low and Intermediate Energy.- Production, Quenching and Reaction of Vibrationally Excited Ions in Collisions with Neutrals in Drift Tubes.- Kinetic Energy Dependence of Ion-Molecule Reactions: From Triatomics to Transition Metals.- Reactions of Transition Metal Ions with Cycloalkanes and Metal Carbonyls.- Gas Phase Metal Ion Chemistry: Summary of the Panel Discussion.- Dynamics of Dissociation and Reactions of Cluster Ions.- Growing Molecules with Ion/Molecule Reactions.- AB Initio Studies of Interstellar Molecular Ions.- Structures and Spectroscopic Properties of Small Negative Molecular Ions - Theory and Experiment.- AB Initio Calculations on Organic Ion Structures.- Formation of Anions in the Gas Phase.- Proton Transfer Reactions of Anions.- Assignment of Absolute Gas Phase Basicities of Small Molecules.- Kinetics and Equilibria of Electron Transfer Reactions: A? + B = A + B?. Determinations of Electron Affinities of A and B and Stabilities of.Adducts A2? and (A * B)?.- Ion Thermochemistry: Summary of the Panel Discussion.- Entropy-Driven Reactions: Summary of the Panel Discussion.- Organic Ion/Molecule Reactions: Summary of the Panel Discussion.- Experimental and Theoretical Studies of Small Organic Dications, Molecules with Highly Remarkable Properties.- Structure and Reactivity of Gaseous Ions Studied by a Combination of Mass Spectrometric, Nuclear Decay and Radiolytic Techniques.

The fluorescence and photofragmentation of liquid saturated hydrocarbons at energies above the photoionization threshold

Fluroescence quantum yields (φF) are reported for saturated hydrocarbons excited by photon absorption in the liquid phase at energies below and above the photoionization threshold. The quantum yields of fluorescence obtained in the subionization region are in excellent agreement with those reported by Lipsky and colleagues. In the phtotionization region, emission occurs as a result of both charge recombination processes and deactivation of the superexcited molecule to the vibrationally relaxed first excited singlet state. The presence of these two populations of fluorescing species above the ionization threshold is manifested in differences in the slopes of plots of φF as a function of energy above and below the ionization onset. Since, for saturated hydrocarbons, photofragmentation is the only nonradiative channel, the variations in the relative importances of the major modes of fragmentation have been examined as a function of energy for selected molecules. It is observed that the two main dissociative ...

Photolysis of Ethane at 11.6–11.8 eV

The photolysis of ethane, carried out with an argon resonance lamp, has been reinvestigated with the related purposes of (1) measuring quantum yields of all fragments formed in the dissociation of excited ethane and (2) associating these fragments with the primary processes occurring in the photolysis of ethane. These and their relative abundances are C2H6*→C2H6+, 5% →CH4+CH2, 16% →C2H5+H, 41% →C2H4+H2, 26% →CH3+CH3, 15%. These results are compared with conclusions reached in earlier studies on the photolysis of ethane with xenon and krypton lamps in order to determine the effect of energy on the relative probabilities of the primary processes. It is found that direct bond scission increases in importance with increasing energy, while processes involving rearrangement decrease in importance. The radical and molecular fragments formed in the dissociation of excited ethane were determined by (a) analyzing the products formed in C2H6–C2D6–NO (1:1:0.1) mixtures and (b) C2D6in the presence of H2S, which scaven...

Hydride transfer reactions involving saturated hydrocarbons and CCl3+, CCl2H+, CCl2F+, CF2Cl+, CF2H+, CF3+, NO+, C2H5+, sec-C3H7+ and t-C4H9+☆

Abstract The compounds CCl4, CCl3H, CCl3F, CCl2F2, CF3H, CF4, NO, C2H6, neo-C5H12, and C3H8 were ionized by low energy electrons in a pulsed ion cyclotron resonance spectrometer to produce CCl3+, CCl2H+, CCl2F+, CF2Cl+, CF2H+, CF3+, NO+, C2H5+, t-C4H9+, and sec-C3H7+ ions, respectively. In these pure compounds, the respective ions formed are all unreactive, and can be trapped with essentially no loss in the ICR analyzer cell for up to 0.2 s after their formation, at a pressure of 10−7–10−5 torr. When alkanes or cycloalkanes are added to these compounds, the predominant ions formed in each case undergo a hydride transfer reaction with the added alkane. This is the only reaction channel: A+ + RH → AH + R+ (where A+ is the initial ion formed and RH is the added alkane or cycloalkane). By following the abundances of the reactant ions as a function of time in these dilute mixtures, approximately one hundred rate coefficients of hydride transfer reactions of thermal ions (in the range 10−12–10−9 cm3 molecule−1 s−1) have been determined. For several of these ions, it is demonstrated that even when reaction is very exothermic at every site in the molecule the reacting ion exhibits positional selectivity. Detailed examination of the reaction probabilities for these ions reacting with alkanes of different chain lengths and structures leads to the conclusion that the rates of hydride transfer reactions depend in part on the exothermicity of the reaction and in part on the lifetime of the ion-molecule complex. For large reactant ions such as t-C4H9+, steric effects may also play a role. The product ions formed in these hydride transfer reactions undergo further fragmentation to an extent which depends on the exothermicity of the reaction; the most important fragmentation paths correspond to loss of C2C4 olefins from the product alkyl ions.

EFFECT OF PRESSURE IN THE RADIOLYSIS AND PHOTOLYSIS OF METHANE

The photolysis and radiolysis of equimolar CH4–CD4 mixtures were investigated as a function of pressure. The fact that, in the presence of NO, the ethane fraction consists entirely of C2D6, C2D4H2, C2H4D2, and C2H6 in comparable amounts indicates that CH2 and CD2 are produced. The relative yield of these ethanes which are formed by insertion of methylene into methane increases with pressure in both the photolysis and radiolysis. In the radiolysis, the G value reaches a value of 0.35±0.1 at pressures above 15 atm. Information about the effect of pressure on the production of the ethyl ion was obtained by investigating the radiolysis of CH4–C4D10 and CD4—C3H8 mixtures from 1.5 cm to 130 atm. The data indicate that there is a gradual decrease of the ethyl ion yield with increase in pressure while the parent ion yield increases with increase in pressure to a pressure of at least 15 atm.

Effect of Additives on the Ionic Reaction Mechanism in the Radiolysis of Methane

This work describes the reactions of the ions C2H5+ and CH5+ with C3D8, C4D10, C5D12, as well as the reactions of C2D5+ and CD5+ with C3H8. It is shown that ethyl ions undergo a hydride transfer reaction with all higher hydrocarbons even though they are present at a concentration of only 0.01%. The scavengers NO and O2 react with ethyl ions effectively, provided the relative concentration of the higher hydrocarbons is less than that of the free radical scavengers. A proton‐transfer reaction between CH5+ and higher hydrocarbons followed by a rapid decomposition of the protonated hydrocarbon into a carbonium ion and a neutral alkane molecule is shown to occur. From the data it can be deduced that at a methane pressure of 48 cm G(C2H5+) =0.9±0.2 and G(CH5+) =1.9±0.2. It is concluded that in earlier published studies of the radiolysis of methane, the C2H5+ and CH5+ ions reacted with the accumulated products or with the added free radical scavengers, thus excluding the neutralization of these two species.

H2‐Transfer Reactions in the Gas‐Phase Radiolysis of Hydrocarbons

The radiolysis of cyclohexane has been investigated in the presence of varying concentrations of acetylene, ethylene, propylene, butene, and cyclopropane. On the basis of a number of observations, it is concluded that, in all cases, the H2‐transfer reaction CnHm+cyclo−C6H12+→CnHm+2+C6H10+ does take place. No extensive rearrangement occurs in the collision complex. For instance, a transfer of H2 to CD3CDCD2 results exclusively in the formation of CD3CDHCD2H, while an H2 transfer to (CD2)3 leads to the formation of CD2HCD2CD2H. Relative rate constants for the transfer of an H2 molecule to CH3CHCH2, 1‐C4H8, iso‐C4H8, 2‐C4H8, C2H4, and C2H2 are, respectively, 1.00, 0.68, 0.27, 0.10, 0.11, and 0.072.Similar variations in the relative rate constants of the H2−‐transfer reaction CnHm++cyclo−C6H12→CnHm+2+C6H10+ are noted.The following additional information was derived in the course of this study:(1) When cyclopentane or n‐pentane is substituted for cyclohexane in the reaction mixture, the H2‐transfer reaction ag...