Chen The electron capture detector

xiv FOREWORD

results. This text is also of extreme value to investigators involved in the study of electron attachment to molecules by alternative methods.

I compliment the authors on a job well done. I hope that the text will accomplish its objective giving the analytical chemist the information to operate the ECD optimally, based upon fundamental principles, and to use the results in the evaluation of parameters associated with the thermal attachment to molecules.

xvi PREFACE

This book is based on the reactions of thermal electrons with molecules. The ECD, negative-ion chemical ionization (NICI) mass spectrometry, and polarographic reduction in aprotic solvents methods are used to determine the kinetic and thermodynamic parameters of these reactions. The chromatograph gives a small pure sample of the molecule. The temperature dependence of the response of the ECD and NIMS is measured to determine fundamental properties. The ECD measurements are verified and extended by correlations with half-wave reduction potentials in aprotic solvents, absorption spectra of aromatic hydrocarbons and donor acceptor complexes, electronegativities, and simple molecular orbital theory.

Chemistry is a central science, but there are many diverse areas of study within it. Since the quantities determined using the ECD are important to all areas of chemistry, this book should be of interest to a variety of scientists, including environmental, analytical, physical, organic, and inorganic chemists. An untapped application is the study of thermal electron reactions with biological molecules. This is especially important since the equipment for the application of these tools is relatively inexpensive and now commercially available.

The present book was partially inspired by a conversation the authors had with Dr. Alan Marchand. When reminded that 1997 was the 100th anniversary of the ‘‘discovery of the electron,’’ he laughed out loud. He noted that it was a bit presumptuous of modern scientists to state that the electron had been discovered. According to Marchand it is and always has been a fundamental particle, but indeed it remains the only member of the triad—the electron, proton, and neutron—that has not been further divided. Once we established that this groundwork had already been established, an interesting discussion of the ECD and its use in determining the fundamental properties of the reaction of thermal electrons with molecules evolved. Professor Marchand was unaware of the efforts in this field and believed that the only method of measuring electron affinities was powerful and expensive mass spectrometers. From this conversation came the impetus for our book.

2 SCOPE AND HISTORY OF THE ELECTRON

material only published in dissertations. This is especially important now that it is possible to access or order dissertations online [2]. In Chapter 4 the theoretical background for the tools used to obtain new information are presented. Chapter 5 and Appendix III describe the experimental and data reduction procedures. Procedures that have been used to obtain complementary data are discussed in Chapter 6. Chapter 7 examines the procedures for consolidating diverse data. The next five chapters present representative evaluations of data for atoms, homonuclear diatomic molecules, small inorganic molecules, organic molecules, environmental pollutants, and biologically significant molecules. A more complete list of evaluated molecular electron affinities is given in the appendices.

1.2GENERAL SCOPE

In the first half of the twentieth century, positive-ion molecule reactions and the interaction of hyperthermal electrons with molecules were emphasized. Some thermal electron molecule reactions in flames and electron swarms were investigated [3]. Prior to 1950 only the electron affinities of hydrogen and the halogen atoms had been measured. A 1953 review on electron affinities noted

The determination of further electron affinities is not an easy matter; it is possible that the electronic equilibrium method could be extended to a few more elements, but at the temperatures involved, molecules and radicals would be decomposed. The only reasonable hope of estimating the electron affinities of radicals would seem to lie in a study of the appearance potential of negative ions, and the determination of their kinetic energies, although it must be borne in mind that a careful search of the mass spectrum of methane has failed to reveal the existence of a CH3(-)ion. [4]

Fortunately, this dire prediction was not realized.

In the late 1960s several major advances were made in the study of thermal electron reactions. These were based on the ECD, the extension of the ‘‘magnetron’’ method of studying electron molecule reactions to determine equilibrium constants for electron molecule reactions, and the invention of high-pressure thermal electron negative-ion sources for mass spectrometry [5–7]. Electron swarms were also used to determine rate constants for thermal electron reactions [8, 9]. The electron affinities of molecules were measured using electron and alkali metal beams [10, 11]. Relative electron affinities were obtained from the direction of the reaction of a negative ion with a molecule [12, 13]. Other major advances were photodetachment and photoelectron spectroscopy [14–17].

This book is based on the study of reactions of thermal electrons with molecules using the ECD, negative ion chemical ionization (NICI) mass spectrometry in the gas phase and polarographic reduction in aprotic solvents [18]. Only the complementary studies related to our research are considered here.

Until recently, there were few theoretical methods available to test experimental results. With the developments in computer software and hardware, it is now easy

to compare theory and experiment. The use of multiconfiguration configuration interaction (MCCI), in a post-self-consistent field calculation improves the agreement to chemical accuracy. The systematic use of MCCI has been assigned the acronym CURES-EC. It appropriately offers a cure for the electron correlation problem in semi-empirical calculations. The CURES-EC method has been successfully applied to over 200 electron affinities, including those for purines and pyrimidines [19, 20].

The physical properties determined using the ECD are important to different areas of chemistry. Analytical chemistry deals with ‘‘how much and what’’ are involved in a chemical reaction. Expressed differently, it establishes what we refer to as the QQQ: quantitation, qualitative identification, and the quality of the results. The determination of the electron affinities of the chlorinated biphenyls, dioxins, and phenols and the prediction of the response of the ECD and NIMS are important to qualitative and quantitative analyses of environmental pollutants [21]. Polarographic reduction in solutions likewise gives accurate and precise qualitative and quantitative results. The quality of the analyses is expressed by the random and systematic uncertainties in the reported values. These are obtained from the same principle of weighted least squares used to obtain information from ECD data. Wentworth has described the application of the general least-squares procedure to chemical problems [22, 23].

The experimental and theoretical methods of measuring or calculating the kinetic and thermodynamic properties of reactions of thermal electrons are important to physical chemistry. The ECD method will be compared to other methods. The CURES-EC procedure can be applied to other energetic quantities. We present a method to consolidate diverse data into ‘‘pseudo-two-dimensional’’ potential energy curves for these reactions [9, 10].

The biochemical applications involve the electronic nature of the components of DNA and proteins, especially the charge distributions, electron affinities, and gas phase acidities of purines and pyrimidines and amino acids. The role of electron reactions in diverse areas such as cancer, electron conduction, and sequence recognition all depend on fundamental energetic properties such as electron affinities and solution energies. We explain nonadiabatic experimental data from radiation chemistry by excited anionic states of biological molecules [24].

Inorganic chemistry concerns molecules of all the atoms. The electron affinities of atoms, small molecules, and radicals and their relationship with the Periodic Table, electronegativities of elements, Morse curves of diatomic anions, and the energies of ion molecule reactions and bond energies are inorganic problems we have considered. Ionic radii can be estimated using potential energy curves.

The reactions of organic molecules in solution are related to gas phase electron affinities and electronegativities. Anions are often intermediates in such reactions. The electron conduction of polymers is related to the electron affinities of the components. The theoretical calculations of electron affinities of aromatic hydrocarbons and the effect of substitution on electrons affinities and gas phase acidities are important to organic chemistry. Pseudo-two-dimensional Morse potentials have been used to represent the dissociation of organic molecules and their anions [18].

4 SCOPE AND HISTORY OF THE ELECTRON

A major objective of this book is to evaluate the reported values of molecular electron affinities and their errors and to assign them to specific states. Prior to 1970 the magnetron and ECD methods were used to measure the majority of gas phase molecular electron affinities. An extensive compilation of unevaluated experimental, empirical, and theoretical electron affinities of atoms, molecules, and radicals was published before 1990 [9]. The electron affinities measured in the gas phase are now available on the Internet but have not been evaluated [26]. The molecular Ea in this list is defined and evaluated in Appendix IV. Values that are significantly lower than the selected values will be assigned to excited states. Semi-empirical calculations and the CURES-EC technique support these assignments. Unpublished electron affinities and updated electron affinities from charge transfer complex data and half-wave reduction potentials are given in Appendix IV.

1.3HISTORY OF THE ELECTRON

The first recorded study of the electron dates back to 600 B.C when a Greek philosopher, Thales of Miletus, observed that amber, when rubbed with a cloth, attracted bits of feathers and the pith of plants. In 1600 William Gilbert, an English physician, characterized static electricity and used the Latin word for amber, electrum, to characterize substances that behave like amber. In 1646 Sir Thomas Browne, another English physician, first used the word ‘‘electricity,’’ deriving it from the Greek word elektron for amber. In the early to middle 1700s an English scientist Stephen Gray and a French scientist Charles-Franc¸ois Du Fay identified two types of electricity of opposite signs so that those with like signs repelled each other, while those of opposite signs attracted each other. In the mid-1700s Benjamin Franklin used the terms ‘‘positive’’ and ‘‘negative’’ to describe the two types of electricity. In 1752 he performed his famous kite experiment, which led to the discovery of the lightning rod, the first invention to protect man from the hazards of electricity.

The late 1700s witnessed the work of the French physicist Charles-Augustin de Coulomb, and the Italian physicists Luigi Galvani and Alessandro Volta; their research led to development of the battery and the first source of a steady current of electricity. In the next half-century the studies of the Danish physicist Hans Christian Ørsted, French physicist Andre´-Marie Ampe`re, German school teacher Georg Simon Ohm, and German physicist Thomas Johann Seebeck related electricity to magnetism and heat. These concepts were then put into practice by Michael Faraday, the English physicist, and Joseph Henry, an American contemporary. Faraday recognized the fundamental nature of the electron and its relationship to electrochemistry. The events surrounding ‘‘the discovery of the electron’’ began in 1833, when Faraday postulated the following two laws of electrolysis:

1.The chemical power of a current of electricity exists in direct proportion to the absolute quantity of electricity that passes through it.

2.The equivalent weight of bodies is simply those quantities that contain equal quantities of electricity.

Scottish physicist James Clerk Maxwell, who developed the theory of electricity and magnetism in the late 1800s, proposed that Faraday’s laws required that ‘‘one molecule’’ of positive and negative electricity is involved in electrolysis. Irish physicist George Johnstone Stoney believed in this ‘‘molecule of electricity’’ and set out to measure the definite quantity of electricity, called e1. He suggested that if this unit of electricity was adopted, it would represent a very important step in our study of molecular phenomena. In 1891 Stoney referred to these charges as ‘‘electrons.’’ The controversy of the day then revolved around whether these electrons were waves or particles.

The relationships to electromagnetic waves postulated by the German physicist Heinrich Rudoph Hertz led to the work of the English physicist Sir Joseph John Thomson in 1897, which is often linked to the actual discovery of the electron [26]. The measurement of the e/m and m of the corpuscles called electrons by Thompson settled this controversy. Electrons were at least particles, but other studies suggested that they were also electromagnetic radiation. Thompson described his conclusions as follows:

If, in the intense electric field in the neighbourhood of the cathode, the molecules of the gas are dissociated and are split up, not into the ordinary chemical atoms, but into these primordial atoms, which we shall for brevity call corpuscles; and if these corpuscles are charged with electricity and projected from the cathode by the electric field, they would behave exactly like the cathode rays. They would evidently give a value of (e/m) which is independent of the nature of the gas and its pressure, for the carriers are the same whatever the gas may be. . . . Thus on this view we have in the cathode rays matter in a new state, a state in which the subdivision of matter is carried very much further than in the ordinary gaseous state: a state in which all matter—that is, matter derived from different sources—is of one and the same kind; this matter being the substance from which all the chemical elements are built up. [27]

In the next decade the physicists Townsend, Charles Wilson, Sir John Planck, Ernest Rutherford, Johannes Geiger, Max Begeman, and finally Robert Millikan estimated the charge on an electron, the quantity e1, and by inference, its mass. Millikan deserves the greatest credit because his measurements proved that e1 is a fundamental and invariant unit of charge. Subsequently, experiments by Planck introduced the two fundamental quantum constants, h and k, now known as Planck’s constant and Boltzman’s constant, respectively. Planck recognized that k is the gas constant for one molecule that gives the value of Avogardro’s number N0.

This led to the important discoveries of Albert Einstein and the quantum revolution at the heart of modern chemistry. Modern chemical pioneers Gilbert Newton Lewis, Linus Pauling, and Mulliken elucidated the fundamental concepts of the electron as the heart of chemical bonding [1, 28, 29]. The role of the electron in modern society is much too extensive to chronicle. Radio, television, nuclear weapons and power are but a few applications. Indeed, the ultimate importance of the electron is easily realized if we return to the idea that all matter contains electrons.

6 SCOPE AND HISTORY OF THE ELECTRON

Methods of studying the reactions of molecules with thermal electrons have not been concisely described. This book is written with the hope that it will encourage others to use these techniques to add to the knowledge of the reactions of thermal electrons as chemical reagents. Indeed, we explore the role of the electron as the zeroth member of the Periodic Table and use the ECD as the focal point of these investigations.

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14.Branscomb, L. M. In Photodetachment in Atomic and Molecular Processes, edited by D. R. Bates, New York: Academic Press, 1962, p. 100.

15.Mead, R. D.; Stevens A. E.; and Lineberger, W. C. ‘‘Photodetachment in Negative Ion Beams,’’ in Gas Phase Ion Chemistry, Vol. 3, edited by M. T. Bowers. New York: Academic Press, 1984, p. 214.

16.Drzaic, P. S.; Marks, J.; and Brauman, J. I. ‘‘Electron Photodetachment in Gas Phase Ions,’’ in Gas Phase Ion Chemistry, Vol. 3, edited by M. T. Bowers. New York: Academic Press, 1984, p. 168.

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