Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

150 EXPERIMENTAL METHODS FOR FAST REACTIONSI IVI IwA"MFig. 17. ESR absorption line (a) and its first derivative (b).an estimation of the kinetics of a reaction. Generally in a given radical the unpairedelectron interacts with species of finite nuclear spin (e.g., 'H, 14N) with the resultthat the absorption has a hyperfine structure whose nature depends on this interaction.This has the beneficial resdt that it is usually possible to identify with certaintythe species whose reaction is being followed.Again, it is possible to describe the shape of a spectral line mathematically; ashas been noted, the shape of a spectroscopic line associated with a transition betweena pair of states is affected by the mean lifetime of the states. Lorentz was the&-st to relate the lifetime and linebreadth, and more recently other authors haveextended this work. Application of the Uncertainty Principle gives the result, if theline shape is assumed to be Lorentzian, that Am = l/nz where Am is the linebroadeningat half-height and z is the average lifetime of the radical under observation.(In fact, the shape of the curve in the case of ESR is probably modified slightlyby the exchange proeess, a factor which somewhat reduces the accuracy of themethod.)The negative ion of naphthalene can be made in a suitable solvent, such as dimethoxyethane(DME), by treatment with an alkali metal in the absence of oxygen.This stable free radical-ion has an ESR spectrum with hyperfine structure. If morenaphthalene is added, a broadening of the spectral lines occurs which is attributedto the reaction(naphthalene)- + (naphthalene)(naphthalene) + (naphthalene)-(N-+N * N+N-).