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Über dieses Buch

The work presented here is a result of an extended collaboration with a number of coworkers and guests. Particularly, I would like to thank Dr. P. Burkhard and Dr. W. Strub for their careful work performed for their Ph. D. thesis and Dr. M. Heming for his brilliant ideas and his dedication. Very fruitful and stimulating were collaborations with our guests, i. e. with G. A. Brinkman and P. W. F. Louwrier from NIKHEF-K in Amsterdam, B. C. Webster, M. J. Ramos and D. McKenna from the University of Glasgow, M. C. R. Symons, D. Geeson and C. J. Rhodes from the University of Leicester, S. F. J. Cox and C. A. Scott from the Rutherford Appleton Laboratory in Chilton, and R. De Renzi and M. Ricco from the University of Parma. Many invaluable discussions with friends and competitors in the field helped to address new viewpoints and to define new goals. I shall not forget my teacher and director of the radical chemistry group, Prof. H. Fischer, whom I wish to thank for his interest and active support and for the great liberty he allowed me for the planning and organization of the project. Last but not least, I thank my dear wife Hanny and our children Christian, Martin and Andrea who suffered, without complaint, daddy's absence for so many hours.

Inhaltsverzeichnis

Frontmatter

Chapter 1. Introduction

Abstract
The principle motivation of natural sciences is the desire to learn about the nature of matter and about its interaction with forces, with the main aibeing to understand fundamentally where we all come from and what it is that keeps nature going. In this century, scientists have not only set off to investigate the macroscopic world of the universe as a whole, they have also succeeded in penetrating the microscopic world to learn more about the structure of molecules, of atoms and their nuclei, and they even break up nuclei to find the most elementary building blocks of matter. A major breakthrough for a detailed understanding of the microworld occurred in the twenties when the concepts of quantum mechanics were introduced. They mark the beginning of the victorious evolution of spectroscopic techniques which are today the most widespread tools to investigate the microworld Thereby, matter is subjected to electromagnetic radiation spanning a range of more than ten orders of magnitude in wavelength between X-rays and radio waves. The response of matter to these waves is often very specific, so that it allows detailed insight into its structure and dynamics.
E. Roduner

Chapter 2. Experiments employing muons

Abstract
‘Natural’ muons are produced in the following way: light nuclei, mainly protons, from primary cosmic rays fall on the earth with high energy. In collisions with molecules in the upper atmosphere they interact with their nuclei and trigger the production of new particles. One of them, the positive pion, decays with a mean lifetime of 26 ns into a positive muon and a neutrino:
$$ {\pi ^{ + }} \to {\mu ^{ + }} + {\nu _{\mu }} $$
(2.1)
The pion is a spin-zero particle, but the neutrino has spin \( \frac{1}{2} \) with negative helicity, i.e. with its spin pointing in the direction opposite to its momentum in the centre-of-mass system. In order to conserve angular momentum the muon must also have spin \( \frac{1}{2} \) and negative helicity.
E. Roduner

Chapter 3. Theory

Abstract
Free muons, or muons in diamagnetic environments, are characterized by the Hamiltonian, in units of ħ:
$$ \hat{H} = - {\omega _{\mu }}\hat{I}_{z}^{\mu } $$
(3.1)
where ωμ = 2πBγμ = B·0.085161 Mrad·s-1·G-1 is the muon Larmor angular frequency and γμ is the is the muon gyromagnetic ratio. The transition observed in transverse fields at ωμ is between the two Zeeman states, \( {m^{\mu }} = \pm \frac{1}{2} \).
E. Roduner

Chapter 4. The cyclohexadienyl radical

Abstract
The cyclohexadienyl radical, C6H7, was often observed by ESR of irradiated benzene [45,23]. Similarly, C6D6H was detected upon X-irradiation of benzene-d6 in adamantane [46]. The muonated analogues, C6H6Mu and C6D6Mu, were among the first organic radicals detected by μSR [8,47]. Corresponding FT-μSR spectra are shown for different magnetic fields in Figure 4.1. Only the radical lines are displayed. As in all μSR spectra of organic liquids, there is a further strong line at a low frequency corresponding to ω D for muons in diamagnetic environments. As a function of field the radical lines behave as described in Table 3.2 and on page 23. They are placed symmetrically around \( \frac{1}{2}{A_{\mu }} \) (broken line). In high fields, here 3000 G, there are two lines. The sum of the two frequencies gives the muon-electron hyperfine coupling constant directly. We obtain Aμ = 514.6 MHz for C6H6Mu and Aμ = 520.0 MHz for C6D6Mu, which corresponds to reduced coupling constants
$$ A_{\mu }^{'} = {A_{\mu }} \cdot {\mu _{p}}/{\mu _{\mu }} = 0.3142 \cdot {A_{\mu }} $$
(4.1)
of 161.7 MHz and 163.4 MHz, respectively. At 1000 G the lines of C6H6Mu split into doublets by 1.23 MHz, which is slightly less than in a less accurate early determination [26]. The splitting increases to 2.5 MHz at 700 G and 4.9 MHz at 500 G. Based on perturbation treatment (page 23) this corresponds to about 150 MHz for the coupling constant of a proton. Accurate numerical.
E. Roduner

Chapter 5. Substituent effects on hyperfine coupling constants

Abstract
27 monosubstituted benzenes ØX were measured either as pure liquids or in concentrated solutions. We use toluene as a model case to discuss the observed features and the assignment procedure. Three spectra are shown in Figure 5.1. At 3 kG three pairs of lines are observed (top). They correspond to radicals with coupling constants of A’μ = 153.8 MHz, 155.9 MHz, and 160.0 MHz, i.e. within 5% the same as the value of 161.7 MHz obtained for C6H6Mu in benzene. This is obviously typical for cyclohexadienyl type radicals. As was demonstrated for C6H6Mu one expects line splitting in a field of 1 kG when the second methylene hydrogen is H, and no splitting when it is D. This is seen for o-dideutero toluene (middle) where the absence of splitting of the two intense lines identifies the ortho Mu adduct. For p-deutero toluene (bottom) the smallest two lines are unsplit and therefore assigned to the para adduct. By default, the lines of intermediate intensity are assigned to the meta isomer. The ipso adduct is not observed. Because it would have no H in the methylene position it would not show splitting in a field of 1 kG.
E. Roduner

Chapter 6. The process of radical formation

Abstract
Possible distribution processes of muons between different chemical states at the end of its thermalization track were sketched in Section (1.3.3) and in Figure 1.1. We now investigate them more in detail for benzene.
E. Roduner

Chapter 7. Distribution of muons in substituted benzenes

Abstract
Muon polarization values are given in Table 7.1. P D values range between 0.15 for unsubstituted benzene and 0.76 for thiophenol. They increase in the series X = F < Cl < Br < I and in X = CH3 < CH2C1 < CHCl2 < CHCl3, and they are higher for α-chloro toluenes than for chlorobenzene. They correlate with C — Cl bond dissociation energies and parallel the efficiency of the reaction of these compounds with electrons via dissociative capture [97].
E. Roduner

Chapter 8. Radical reactions

Abstract
A reaction is said to involve a secondary kinetic isotope effect when no bond to the isotopic atom is broken or formed in the rate-determining step. A significant effect can be expected only when there is some force constant change between reactant and transition state for normal modes which involve the isotope. This can arise from an alteration in the type of bonding, such as a change in hybridisation, or it can be a change in non-bonding interactions of the isotopic atom, as in steric effects [19].
E. Roduner

Chapter 9. Summary and review

Abstract
Throughout this work, we have used cyclohexadienyl type radicals to exemplify how the μSR techniques work, and what their potential and limitations are. We now summarize these results and supplement them with observations made with other types of radicals. We shall see that the problems encountered are similar in most cases, but some nice new features are worth mentioning.
E. Roduner

Backmatter

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