The interaction of electron donors and acceptors

Laboratory of Molecular Structure and Spectra, Department of Physics, University of Chicago, Chicago 37, Illinois, U.S.A..

Abstract

This paper reviews some aspects of the theory of the interaction of electron donors and acceptors, with particular reference to 1 : 1 interactions in solution, and also seeks to clarify and broaden some current viewpoints and applications of the theory⁽¹⁾. The classification of donors and acceptors, and the characteristics of various types of donor-acceptor interaction pairs (including ion-pairs, H-bonded complexes, 2-way charge-transfer complexes, reaction intermediates, contact pairs, intramolecular π island interactions, etc.) are first discussed. Two sections are then devoted to the halogen complexes of n donors and of π donors. It is shown that the n donor complexes conform well to predictions of the general theory, the main point of fresh emphasis (whose importance is indicated by Hassel’s work on the geometry of solid complexes) being that some account must be taken of the participation of more than one dative resonance structure in stabilizing the normal state of most complexes. Interesting questions remain concerning the structure and charge-transfer spectra of complexes of the classical benzene-iodine type ; probable answers are discussed. The last section deals, for the interaction of neutral-molecule even-electron donors and acceptors, with potential surfaces for the normal and charge-transfer excited states in each of three limiting cases (W₁ > W₀, W₁ ≈ W₀, and W₁ < W₀, where W₁ and W₀ are the energies of the pure dative and pure no-bond structures respectively), and in each case for the two subcases of strong and weak interaction; the discussion in the third case is applicable also to compounds like NaCl, CH₃NO₃, (CH₃)₃CCl, etc. Photochemistry of contact pairs, the ionogenic effects of polar environments, and electron transfer between low-ionization-potential donors and high-electron-affinity acceptors (with possible relevance to biological systems and semi-conducting solid complexes) are briefly discussed. In discussing H-bonded complexes, new reasons favoring the importance of charge-transfer as against classical electrostatic forces are given (Sec. II).