2-mm Wave Band EPR Spectroscopy of Condensed Systems

By V.I. Krinichnyi

This is the first book to summarize the problems of using modern high-resolution 2-mm wave band EPR spectroscopy in an interdisciplinary field for the investigation of various condensed systems. The material is well illustrated and the applications are as diverse as possible. The main subjects included are: unique characteristics of 2-mm EPR spectroscopy and appropriate experimental techniques, dynamics and polarity of radical microenvironment in model and biological systems, and the nature of charge carriers and charge transfer mechanisms in organic polymer semiconductors.

          Table of Contents


      Last years are characterized by a vast development of EPR spectroscopy of millimeter and submillimeter ranges, which seems to be more promising in the studies of condensed systems. However, this development faces some difficulties associated mainly with a weak and complicated element base (SHF elements, including a cavity, cryogenic equipment, etc.), sample preparation procedure and appropriate investigation methods.
      By the moment of its appearance in 60-ies, the idea of the broadening of EPR spectroscopy frequency range seemed to be mainly an intellectual game without concrete theoretical and practical future application. It was stipulated mainly by fragmentary investigations of specific objects in millimeter EPR range, which could not enable the full and clear arguments, proving the necessity of the development of EPR spectroscopy, directed towards the increase of registration frequency. However, the elaboration and creation of the first multifunctional universal 2-mm wave band EPR spectrometer at Russian Institute of Chemical Physics allowed the successful investigation of various condensed systems (solutions, polymers, etc.), in which complex molecular and relaxation processes proceed, including slow anisotropic motions, cross-relaxation, etc. This became the reason for the rise of the enthusiasm among chemists, physicists and biologists, which was accompanied by an explosive development of various method's application. However, the present tendency consists in that the main advantage of 2-mm EPR spectroscopy is not its successful application but the possibility to obtain a qualitatively new information on the well-known compounds and to understand various phenomena, from specific interactions and correlated relaxation in condensed media to charge transfer in biological systems and polymer semiconductors. This new appriciation of the essence of quantum mechanical phenomena lies far out of the frames of electron paramagnetic resonance as a special discipline and with no doubt will promote the breakthrough in other fields of biology, physics and chemistry.
      The present book appears as a monograph on the application of high-frequency 2-mm EPR spectroscopy to the study of physical-chemical properties of various condensed systems and their interpretation from the standpoint of modern conceptions of molecular physics. The methods of measurements at 2-mm wave band EPR, considered here found their application in the investigation of both simple (solutions, ion crystals) and complex (biopolymers, enzymes, conducting polymers, ion-radical salts, etc.) condensed systems, and their potential is not yet exhausted. 2-mm wave band EPR spectroscopy enables the profound investigation of the structure, dynamics, other specific characteristics of radical centers and their local environment, and elementary charge transfer processes in these systems.
      The monograph contains mainly the original results, obtained by the author during the latest decade, of the investigation of various model, biological and other high-molecular weight compounds by 2-mm wave band EPR spectroscopy. Therefore a special attention is paid to the description of various practical applications of the method in the study of liquids and solids.
      The first Chapter follows the Introduction, which appears as a brief survey of the main stages of millimeter EPR spectroscopy development. The Chapter presents a concise summary of theoretical fundamentals of EPR spectroscopy, and a body of mathematics, necessary for the interpretation of experimental results. It considers the most important magnetic resonance parameters of paramagnetic centers, the processes of spin relaxation and the factors, affecting it and among them are considered various experimental approaches of EPR spectroscopy, such as steady-state signal saturation, the saturation transfer method, the method of spin label and probe, which is involved for the investigation of various properties of condensed systems.
      The success of 2-mm EPR spectroscopy is attributed mainly to a high spectral resolution over g-factor. Therefore Chapter II describes the reason for the choice of 2-mm wave band for registering EPR spectra of organic radicals, and the advantages of 2-mm wave band EPR spectroscopy are manifested, by using organic peroxide and nitroxide radicals in model systems. This Chapter contains the description of the development of steady-state saturation of spin-packets, microspin label and probe, macrospin probe, and saturation transfer methods, applied to 2-mm wave band registration, which enables a more accurate and complete analysis of dynamic and relaxation properties of radical microenvironment in condensed systems.
      Chapter III starts with the analysis of the restrictions of common EPR method in the study of biological systems and exhibits the data, confirming the advantages of 2-mm EPR spectroscopy in investigating the structure, dynamics and polarity of radical microenvironment in real biological objects.
      And finally, Chapter IV contains the discussion of the principle results, obtained by using 2-mm wave band EPR, at studying structural and electrodynamic peculiarities of the known conducting compounds, such as conjugated polymers, ion-radical salts and high-temperature superconductors.
      In order to satisfy the wishes of a wide range of readers, the book demonstrates the examples of experimental investigation of various classes of compounds, that probably appears as a certain incompleteness of the study of some systems.
      Although this monograph is devoted almost extremely to EPR spectroscopy, it should be emphasized, that it might be considered only as one of numerous useful and fruitful methods on the general background of physics and chemistry of condensed media. All the results, obtained with this method, attain the whole significance only being combined with the data, obtained by other methods. The results, presented in Sections II.D, III.B and IV.A may be considered as an example.
      The author hopes that this book could be useful for radio spectroscopists and investigators of the neighboring branches of science, such as molecular biology, radiation and photochemistry, organic and analytical chemistry, liquid and solid state physics, and for the students, specializing chemistry, physics and biology.
      The author is very grateful to his teachers Professor Ya.S. Lebedev and Professor G.I.Likhtenstein, who promoted the development of his interests in the classic and millimeter EPR spectroscopy. The author expresses his gratitude to his colleagues, who contributed to carrying out the experiments and took part in the discussion. The author is especially grateful to O.Ya.Grinberg, A.A. Dubinski, A.V.Kulikov, L.M.Goldenberg, I.B.Nazarova, S.D.Cheremisov, H.-K.Roth, K.Luders, F.Lux, A.E.Pelekh, S.A.Brazovskii, L.I.Tkachenko, and O.N.Efimov. The author is also grateful to A.V.Lebedeva for her help in Figures preparation.