Molecular orbital calculations organic radical ions

Ab initio and molecular orbital calculations have been used to study die interactions of organic radicals coupled by m-phenylene.9,10 These methods were used to explain the low-lying excitation spectra of radical ions such as (l).9 The tricarbene (2) was also shown to have a high-spin ground state irrespective of the value of die dihedral angle... 

A radical solution to all of the above-mentioned difficulties is to eliminate the solvent medium entirely and to measure structural effects on heteroaromatic reactivity in the gas phase. During the last decade, a revolution has occurred in the experimental and theoretical approaches to understanding gas-phase ion chemistry. This has occurred as the result of the simultaneous development of several experimental methods for studying organic ion-molecule kinetics and equilibria in the gas phase with precision and range of effects equivalent to or even better than that normally obtained in solution and by very sophisticated molecular orbital calculations. The importance of reactivity studies in the gas phase is twofold. Direct comparison of rates and equilibria in gaseous and condensed media reveals previously inaccessible effects of ion solvation. In addition, reactivity data in the gas phase provide a direct evaluation of the fundamental, intrinsic properties of molecules and represent a unique yardstick against which the validity of theoretical estimates of such properties can be adequately assayed. 

In aprotic nonaqueous media, the organic electrochemistry of anodic and cathodic reactions is concerned predominantly with radical-ion chemistry in many cases involving aromatic substances, the radicals are of sufficient stability for them to be characterized spectroscopically by conventional absorption spectrophotometry and by esr spectroscopy. Linear relations are found between the cathodic and anodic half-wave potentials and the ionization potentials or electron affinities determined in the gas phase. The oxidation and reduction potentials can also be related to the theoretically calculated energies of the highest occupied (anodic process) or lowest vacant (cathodic process) molecular orbitals. 

Longuet-Higgins and Salem [1,8] proposed a molecular force field for localized a- electrons and delocalized 77 electrons. Their scheme has become attractive for conjugated polymers [25,26], now in conjunction with semiempirical rather than simple Hiickel calculations. Warshel and Karplus [27] and Hemley et al. [28] combine all-electron calculations of geometry with PPP models for 77 electrons (see also Ref. 21). We will focus exclusively on delocalization or 77-electron contributions using linear response (LR) theory, as illustrated by the AM formalism. LR has been widely applied to vibrational spectra of charge transfer and ion-radical organic crystals [29-32]. The idea is to model shifts due to delocalization relative to some localized reference. The concomitant problem of localized or other contributions is the choice of a reference force field discussed in Section II.B. The solid-state perspective of LR theory is quite compatible with 77-electron or other models based on frontier orbitals. 

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