Showing papers in "Annual Review of Physical Chemistry in 1971"

Ion Cyclotron Resonance Spectroscopy

Abstract: Ion cyclotron resonance spectroscopy has met with such wide acceptance that research groups utilizing this relatively new experimental technique now likely outnumber research groups which have used more traditional techniques, such as high pressure mass spectrometry, for the study of ion-molecule reactions. Since the previous review of ion-molecule reactions in Volume 1 9 o f the Annual Review of Physical Chemistry (1) there has been considerable development in the field. This development has been distributed among studies conducted with ion cyclotron resonance spectroscopy, drift tubes, high pressure mass spectrometry, flowing afterglows, tandem mass spectrometry, and beam experiments. Chemical applications of these techniques have been the subject of numerous recent reviews and monographs authored by principals in these investigations. In the present review an attempt will be made to chron­ icle the development of ion cyclotron resonance spectroscopy. Since this is the first review of the subject to appear in these volumes, the present article purports to be not only a review but an introduction as well. Sufficient back­ ground is developed to allow for a critical review of current experimentation in this field of endeavor. Ion cyclotron resonance spectroscopy is finding applications to the study of an increasing number of problems of general chemical interest. Recent developments promise an even wider range of applications. The technique is well suited for the routine study of ion-molecule reactions, which are readily identified using double resonance experiments (2). The identification of an ion-molecule reaction in a double resonance experiment provides useful information relating to the thermochemical properties of both ions and neu­ trals [including acidities (3-8) and basicities (9, 10) determined in the absence of complicating solvation phenomena], the identification of isomeric ion structures (1 1-16), and information concerning reaction mechanisms (17-23). Ion ejection techniques (11, 24) allow for the determination of product ion distributions in ion-molecule reactions even when other processes, such as

Lifetimes in Excited States

Abstract: This review attempts to cover developments within the past year or two up to the end of January 1971. Our understanding is now approaching the level where we can begin to comprehend experimental results in connection with theoretical developments. Nevertheless, though our understanding is just beginning, we are already faced with a large body of data, as well as a large body of theoretical models and discussions. Hence, some arbitrary selection had to be made here to emphasize those contributions which are particularly helpful, admittedly in the eyes of the writers, in demonstrating fundamental principles. Our apologies in this matter are perhaps somewhat mitigated by the presence of some outstanding books and reviews which cover in more complete detail some of the points neglected in this review. In particular, there is the recent treatise by Birks on photophysics (1) as well as the excellent discussion of the triplet state by McGlynn, Azumi & Kinoshita (2) and the book by Becker on fluorescence and phosphorescence (3). Interest in the field has also been furthered by several conferences: in particular, the Conference at Loyola University in Chicago in 1968 (4), the Conference on Radiationless Transitions in Paris in 1969 (5), the Delaware Luminescence Conference in 1969 (6), the EUCHEM Conference in Elmau in 1970, and the Columbus Conference in 1970. At this writing another EUCHEM Conference on Radiationless Processes is announced for April 1971 in Wales. Several reviews have appeared, most notable the one by Henry & Kasha covering theoretical approaches (7), as well as the ones by Birks & Munro (8) and by Jortner, Rice & Hochstrasser (9). This review will limit itself to a discussion of the processes pursuant to the absorption of light and the formation of an excited electronic state. In particu­ lar, singlet states may be formed which then can fluoresce or undergo various nonradiative processes. Also, triplet states can be formed which then phos­ phoresce or undergo other radiationless processes. Radiationless processes would collectively describe the typical case of photochemical reaction, the change in electronic state with the conservation of spin (internal conversion), or the change in electronic state with a change in spin (intersystems crossing), such as singlet to triplet crossing. The processes have characteristic rate constants, or lifetimes (T = k-1), which characterize the rate process and may

Radiation chemistry of oxygenated aqueous solutions.

Abstract: The activity in radiation chemistry in recent years has been so extensive that it seems impossible to review the whole field This review will cover only problems in the radiation chemistry of aqueous solutions Even within this limited field numerous papers have been published as well as several books, symposia, and reviews (1-1 5) Before the techniques of pulse radiolysis were developed about a decade ago, most studies dealt with measurements of yields and relative rate constants The identity of radicals formed in water was inferred from such studies The diffusion model was generally accepted to explain the effect of radiations of various LET, the effect of solutes on G values, and various other related phenomena The subject as it was known in 1 960 was ably summarized by Allen in his book (16) In the last decade more direct kinetic studies of radical-radical reactions and of reactions of radicals with solutes have been carried out These studies have helped to elucidate reaction mechanisms and specifically identify intermediates and have provided a large compilation of rate constants of reactions of various solutes with eaq, OH, and H atoms (17) For many of these intermediates, physical properties such as absorption spectra, ESR spectra, conductivities pKs, etc were de­ termined First I would like to make a few general remarks on the diffusion model and then devote the remaining sections of the review to a summary of the problems and properties of the intermediates found in air-free and air­ saturated aqueous solutions as they are currently known

Infrared Spectra of Free Radicals and Chemical Intermediates in Inert Matrices

Abstract: Free radicals have played a significant role in the development of chemistry during the twentieth century. Since Gomberg's (1) preparation in 1900 of the first free radical, triphenylmethyl-and along with it a challenge to the understanding of valence-free radicals have been used increasingly to explain reaction mechanisms. As early as 1918 Nernst suggested (2) that the photo­ chemical reaction of hydrogen and chlorine involved the individual atoms as intermediates. Further important work of Paneth & Hofeditz in 1929 (3) postulated that alkyl free radicals, produced by thermal decomposition of metal alkyls, were responsible for removing a metal film in the famous metal mirror experiments. Following this early work, the appearance of free radical intermediates in the literature became itself a chain reaction. Free radicals were subsequently proposed as reaction intermediates by numerous prominent scientists. Steacie has reviewed much of this early work (4) in his book Atomic and Free Radical Reactions. Recently Pryor (5) has discussed the mechanistic, synthetic, and industrial significance of free radicals in contemporary organic chemistry. Obviously, the validity of a proposed mechanism is improved by the direct observation of the free radical from a related chemical process. Furthermore, free radicals provide unique tests for theories of chemical bonding. The presence of an unpaired electron in a half-filled orbital supplies an excellent opportunity for electronic interaction with neighboring atoms and groups. Knowledge of the vibrational spectrum of simple free radicals provides insight into the structure and bonding of these interesting chemical species. Free radicals are most simply defined as species containing one or two unpaired electrons. Examples include methyl CH3, the chlorine atom CI, and the lithium atom Li, as well as the stable molecules nitric oxide NO and nitrogen dioxide NOz, each of which contains a single unpaired electron. Diradicals, such as triplet CH2, contain two unpaired electrons. In this dis­ cussion we will use the term radical to refer to species containing a single

Many-Body Theories of the Electronic Structure of Atoms and Molecules

Abstract: Except for the hydrogen atom and the H2 + molecule, the description of the electronic structure of atoms and molecules is a "many-body" problem. Because of the general qualitative success of the orbital concept in quantum chemistry and because of the desirability of obtaining meaningful simplifica­ tion of the Schrodinger equation for atomic and molecular electronic structure calculations, approximate theories based upon independent particle pictures have been developed. All theories based on independent particle models will be excluded from this review. For the purposes of this review Hartree-Fock (HF) self-consistent field theories are not considered to be many-body theories. Similarly the various extended, or projected, Hartree-Fock theories (1-18) can also be considered as independent particle theories and are therefore omitted from the following discussion. Some developments in the area of the theory of the electronic structure of atoms and molecules have been reviewed in a previous volume by Allen (19) and earlier by Golebiewski & Taylor (20). Of these "many-body" theories of atomic and molecular electronic struc­ ture, there are two basic categories. Most of the theories arise from the same basic concepts that provided the independent particle picture, i.e. orbitals, geminals (the electron pair), configuration interaction (el), etc. These con­ cepts are, in general, quite familiar to the physical chemist. However, recently there have been a number of theoretical developments associated with the electronic structure of atoms and molecules which derive their impetus from theories of the many(N)-body problems for which N -+ 00. These many­ body problems have been developed to discuss the properties of infinite nuclear matter, electron gases, superconductors, etc. The mathematical techniques developed for these problems have been applied to the descrip­ tion of the electronic structure of atoms and molecules. Although there are a number of reviews of these many-body techniques (21-26) and their

Aerochemistry of Air Pollution

Abstract: THIS ARTICLE REVIEWS THE PRESENT STATUS OF THE AEROCHEMISTRY OF POLLUTANTS CLASSIFIED BY THE SOURCES: SPACE HEATING, POWER GENERATION, AND TRANSPORTATION. THE PROBLEMS OF INTERACTING SYSTEMS ARE DISCUSSED AND POLLUTION MODELS ARE PRESENTED. CARBON MONOXIDE IS TREATED IN DETAIL AS THE MAJOR POLLUTANT FROM TRANSPORTATION ON A WEIGHT OR VOLUME BASIS. LEAD POLLUTION, NITROGEN OXIDES, AND HYDROCARBONS ARE DISCUSSED IN RELATION TO PHOTOCHEMICAL SMOG. /AUTHOR/

Electron Nuclear Double Resonance

Abstract: Feher introduced the electron nuclear double resonance or ENDOR technique in 1 956 (1) . In the ensuing 1 5 years several hundred ENDOR­ related articles have been published. However, no general review of the subject has appeared. This may be partially due to the fact that ENDOR results Can be thought of as an extension of the usual paramagnetic resonance measure­ ments and consequently are subsumed under the more general categories of magnetic resonance. Therefore, I shall not attempt to discuss in detail the significance or ramifications for a given field of the results of ENDOR measure­ ments. Instead, I shall attempt to cover the entire range of ENDOR experi­ ments from those in organic liquids to those on zero-spin deviations in antiferromagnets. My purpose shall be to illustrate with representative ex­ amples the power and limitations of the technique, to summarize briefly but comprehensively the work that has been done, and hopefully to suggest the potential of the technique for problems still to be solved. The term ENDOR shall be restricted to that class of double resonance experiments in which the effect of a resonant change in the nuclear spin quantization direction (NMR) is observed via its effect on the EPR signal intensity. [This is distinct from an interesting group of related double resonance experiments (Overhauser effect) in which EPR transitions are used to enhance the sensitivity of the NMR detection process itself.] Herein lies the essence of

Photochemistry in the Far Ultraviolet

Abstract: substances. The progress made since then is obviously too vast to be covered in its entirety in the limited space allotted for this review. We therefore decided to concen­ trate on what we cons idered to be some of the more significant developments of the last 2 or 3 years. The rather spectacular increase in the number of far uv radiation studies which appeared in the literature during this period can be ascribed largely to the much wider availability of a variety of far uv sources, some of which are only recently available commercially. Most of the far uv techniques which are of interest to physicists as well as chemists have been reviewed (7, 8). One of the most recent developments is the co nstruction of a