Electron-pair bound states

The free valencies of a crystal can form pairs, each such pair wandering through the crystal as an entity until it breaks up. Such formations are well known in the theory of the solid state. A pair of opposite valencies in an ionic crystal (electron - - hole bound by Coulomb interaction) forms what is called a Mott exciton. A pair of like valencies (election + electron or hole + hole bound by exchange interactions) forms a so-called doublon. Such formations have recently been investigated, 12, IS). 

Charge transfer states (CT) are often found in molecular systems side by side with excitonic states. CT states describe polar nonconducting states bound by coulomb interaction of the electron-hole pairs. CT states may be ionized with localization of the charges on definite molecules. 

Similar excited states have been observed for diatomic molecules of the alkali metals. They may be interpreted as involving a molecule-ion, such as Li, with a one-electron bond, plus a loosely-bound outer electron. The internuclear distances are about 0.3 A greater than for the corresponding normal states 2 2.94 A lor Lij (2.672 A for Lit), 3.41 A for Na (3.079 A for Nat), and 4.24 A for K (3.923 A for Kt). The values of the bond energies for the one-electron bonds, as indicated by the vibrational levels, are about 60 percent of those for the corresponding electron-pair bonds. 

The NO+ and NO- modes of coordination differ by two electrons in terms of formal charge. Interconversion of these two bonding modes becomes feasible when the bound metal ion possesses two complementary oxidation states. It has been proposed that this interconversion, which corresponds to an intramolecular redox reaction, represents a unique and facile way to achieve coordinative unsaturation at the metal center with the nitrosyl acting as an electron pair reservoir (201). Interconversion of linear and bent nitrosyls has been reported in the unusual complex Ru(NO)2C1-(PPh3)2+ that possesses one linear and one bent nitrosyl, structure (39) (202). 

Results of protoly tic reactions of hydrocarbons in superacid media were interpreted by Olah as indication for the general electrophilic reactivity of covalent C H and C—C single bonds of alkanes and cycloalkanes. The reactivity is due to the tr-donor ability of a shared electron pair (of cr-bond) via two-electron, three-center bond formation. Consequently, the transition state of the reaction, is of three-center bound pentacoordinate carbonium ion nature [Eq. (5.5)]. 

The first pulse is used to excite a pair of states in an electronic state supporting j bound states, and the second pulse is used to dissociate the system by deexciting it I back to the ground state, above the dissociation threshold. 

Transition of oxygen molecules on Au(110) from the physisorbed to chemisorbed state has been achieved by UV radiation at 28 K.1 This probably excites them from the triplet to the singlet 1Ag state in which all electrons are paired although the bond length is almost unchanged, the dissociation energy (396 kJ mol-1) is much decreased, and it appears that in this state it is more easily chemisorbed, probably without dissociation. Bombardment of this surface by 0+ ions led on TPD to recognition of a number of bound states.1... 

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