Sternlicht, Himan (1963) I. Electron spin resonance line shape of a polycrystalline radical. II. Paramagnetic excitons in molecular crystals. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-12302004-143339
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Electron spin resonance spectra of polycrystalline radicals are complicated by the anisotropic dipolar magnetic interactions between the unpaired electron and the nuclear spins. Although general expressions in the form of multiple integrals can be derived for the polycrystalline line shapes, their complicated nature makes closed form evaluations very difficult to do except in certain limiting cases.
We derive a theorem which enables us to calculate the polycrystalline line shape of an arbitrary polynuclear radical when the external magnetic field is larger than the internal dipolar fields. Our results are applied to polyconjugated hydrocarbons radicals. We find that we can explain a fact heretofore not well understood: namely, the polycrystalline spectra of such radicals can be remarkably similar to those obtained from the liquid state, despite the fact that significant anisotropic dipolar interactions are present in the solid state but not in the liquid.
The possibility of using electron spin resonance to study paramagnetic excited electronic states of aromatic crystals is explored. Two limiting cases, are considered: On the one hand, the excitation energy moves from molecule to molecule by a diffusion process; on the other, the triplet excitation is described by a Frenkel wave exciton. In either case intermolecular propagation of the triplet excitation is assumed to proceed through virtual triplet ionization states.
A. The theory of the spin resonance of triplet wave excitons is developed in some detail. It is shown that large Davydov splittings between the exciton bands require that there be only a weak dependence of the ESR spectra on the [...] vector of the exciton wave, except when [...] is near regions of band degeneracies. Band degeneracies arise from time-reversal symmetry in benzene, naphthalene, and coronene crystals and occur on boundary planes of the first Brillouin zone. A spin Hamiltonian is calculated for these three aromatic crystals for most [...] vectors which are far removed from regions of band degeneracy. Two-line spectra (with no hyperfine structure), representing an average over the molecular sites in the unit cell, are obtained. Scattering of the exciton wave by lattice perturbations can be a mechanism for spin lattice relaxation.
B. The ESR line shape is correlated with the characteristic time, [...], for a diffusing triplet exciton to undergo a random walk between the nontranslatory neighbors of a two site unit cell. A general expression for the line shape is derived by time dependent perturbation treatment. This expression is valid for (1) sufficiently large transfer rates [...]; (2) for all transfer rates provided that significant differences between the spin Hamiltonians of the nontranslatory neighbors only occur to first and second order. Wave and diffusional triplet exciton spectra are compared.
|Item Type:||Thesis (Dissertation (Ph.D.))|
|Degree Grantor:||California Institute of Technology|
|Division:||Chemistry and Chemical Engineering|
|Thesis Availability:||Public (worldwide access)|
|Defense Date:||1 January 1963|
|Default Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Imported from ETD-db|
|Deposited On:||30 Dec 2004|
|Last Modified:||26 Dec 2012 03:15|
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