Dispersion interaction, collision-induced

A. Rare-gas Compounds.—The recent preparation of a number of rare-gas compounds has led to a significant number of calculations on these molecules, particularly on diatomic species. The repulsive interaction and dispersion interaction between like rare-gas atoms has been dealt with above. Several years ago Matcha and Nesbet284and Gilbert and Wahl276 presented calculations on the species NeHe, ArHe, andNeAr. The interaction of two dissimilar rare-gas diatoms gives rise to a dipole moment, which leads to far-i.r. collision-induced spectra of rare-gas mixtures. For this reason, the dipole moments of such species are of interest, but dispersion contributions are not described in the HF approximation and have to be computed by an empirical expression ft — B exp (—Rjp). Much more work is needed on this problem. Wahl and co-workers have reported the numerical results of OVC calculations on HeNe,240 but no detailed discussion was given. In cases like these, the HF model should yield reliable interaction potentials, since A and B are both closed systems. 

Macroscopic solvent effects can be described by the dielectric constant of a medium, whereas the effects of polarization, induced dipoles, and specific solvation are examples of microscopic solvent effects. Carbenium ions are very strong electrophiles that interact reversibly with several components of the reaction mixture in addition to undergoing initiation, propagation, transfer, and termination. These interactions may be relatively weak as in dispersive interactions, which last less than it takes for a bond vibration (<10 14 sec), and are thus considered to involve "sticky collisions. Stronger interactions lead to long-lived intermediates and/or complex formation, often with a change of hybridization. For example, onium ions are formed with -donors. Even stable trityl ions react very rapidly with amines to form ammonium ions [41], and with water, alcohol, ethers, and esters to form oxonium ions. Onium ion formation is reversible, with the equilibrium constant depending on the nucleophile, cation, solvent, and temperature (cf., Section IV.C.3). 

In this case, the term cjr includes the dispersion force and higher moment interaction and the capturing cross section will be proportional to This again provides a way to correlate the experimental data of the collision-induced ISC cross section against the physical properties of S and A. [Other types of plots have been proposed by Rossler (1935), Selwyn and Steinfeld (1969), Thayer and Yardley (1972, 1974) and Parmenter (1974).] None of the proposed plots can be expected to meaningfully correlate all the observed data of collision-induced ISC. This is partly because the complex formation may not be fully described by the simple form of V r) given by Eq. (117). Furthermore, P E) may not be completely independent of the physical properties of quenchers. 

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