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 Plenary Lecture Abstract 

Stephen F. Nelsen

   65th Birthday Symposium
Madison, Wisconsin
June 3-4, 2005

Ffrancon Williams

University of Tennessee

Studies of Amines (a) as Nucleophiles in Radiation-Induced Ionic Reactions, and (b) as Electron Donors in the Generation of Aminium Radical Cations Explored by Matrix ESR Spectroscopy

The speaker’s interest in amines began with the use of ammonia as an interceptor of hydrocarbon ions in the radiation chemistry of liquids (J. Am. Chem. Soc.1962, 84, 4355; 1964, 86, 3954).  Together with related work on the use of ammonia and amines in the retardation of radiation-induced cationic polymerization, it eventually became clear that both geminate and free ions are produced by high-energy radiation in liquids.  Whereas the geminate ions are short lived in the nanosecond to picosecond range, the freely diffusing ions that undergo homogeneous second-order ionic recombination have long lifetimes extending into the millisecond region. This latter property was then exploited to obtain the propagation rate constant k_p for the cationic polymerization of isobutylene (J. Am. Chem. Soc.1967, 89, 6539; 1969, 91, 3728). The value of 1.5 x 10⁸ M^–1 s^–1 so determined was a factor of about 10⁴ higher than that obtained through the use of Friedel-Crafts catalysts in more conventional studies, and there has been a vigorous debate over the years as to the absolute magnitude of these k_p rate constants. However, in more recent work, Mayr and his group (Macromolecules 1996, 29, 6104) have reported a value of 6 ± 2 x 10⁸ M^–1 s^–1 in essential agreement with the earlier value.  Also, it has now been shown (Macromolecules 2005, 38, 206) that a previous low value of 0.9 x 10⁴ M^–1 s^–1 in the literature is based on erroneous kinetic assumptions and that a proper analysis of the original data leads to the much higher revised value of 1.8 x 10⁸ M^–1 s^–1, again consistent with the original result from the radiation studies.   The second part of this talk will focus on matrix ESR studies of some amine radical cations and their rearrangements. The range of reactivity is considerable, the photo generation of the very stable N,N,N,^/N,^/-tetramethyl-p-phenylenediamine (TMPD) radical cation at one extreme allowing the direct ESR detection of trapped electrons in organic glasses (J. Chem. Phys.1967, 46, 4982).  At the other end of the reactivity scale, a recent paper (Chem. Eur. J. 2004, 10, 5524) describes the thermal and photochemical rearrangements of n-alkylamine radical cations to their distonic forms.  In particular, this work has for the first time provided ESR evidence for the intramolecular 1,4- and 1,5-hydrogen shifts from carbon to nitrogen that are generally considered to take place in the mechanism of the Hofmann-Löffler-Freytag reaction for the synthesis of pyrrolidine derivatives. Another interesting rearrangement to a distonic radical cation involves a 1,2-hydrogen shift in ionized allylamine to form ·CH₂CH₂CH=NH₂⁺, which is also generated by the ring opening of ionized cyclopropylamine.  Finally, the distorted structure of the tetramethylurea (TMU) radical cation in which one of the NMe₂ groups is twisted out of the molecular plane will be discussed with reference to a second-order Jahn-Teller effect (Rad. Phys.Chem. 2003, 67, 211). The distortion can be explained in terms of the small energy gap between the ²B₁ and ²A₂ states of the ionized TMU within the C_2v structure of the parent molecule. As a result, a mixing of these two states occurs leading to a b₂-type vibronic distortion. This introduces asymmetry and allows one of the NMe₂ groups to rotate such that the axis of its nitrogen p orbital lies in the original TMU molecular plane.  Stabilization is then thought to occur by the interaction of this orbital with the in-plane oxygen p-orbital of the C=O group, resulting in an energy minimum for this twisted form of the TMU radical cation.   

Biosketch

Ffrancon Williams grew up in North Wales, U. K., and obtained a B.Sc. degree in 1949 from University College London.  After a sojourn at the U. K. Atomic Energy Research Establishment, Harwell and at Northwestern University where he worked with Professor Malcolm Dole, he received an external Ph.D. degree in 1960 from the University of London.  He started his academic career at the University of Tennessee in 1961 as an Assistant Professor, and after promotion through the ranks was appointed an Alumni Distinguished Service Professor in 1974.  Dr. Williams was a National Science Foundation Visiting Scientist to Kyoto University, Japan from 1965 to 1966, and was the recipient of a Guggenheim Fellowship in 1972 at The Royal Institute of Technology, Stockholm, and The Swedish Research Councils’ Laboratory, Studsvik.  He has been a Chairman of the Gordon Research Conferences on Radiation Chemistry (1971) and Radical Ions (1984), and has served both as an associate and as a consultant editor of Radiation Research, the journal of the Radiation Research Society.

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