Triatomic Molecules and Anions

Thermal electron attachment to nitrous oxide has been studied for more than 75 years. In spite of this extensive experimental and theoretical work the adiabatic electron affinity of N20 remains uncertain. The reported electron affinities are 0.0 0.1 eV, 0.22 0.2 eV an upper limit of 0.76 0.1 eV is determined by PES [92, 98-103]. By fitting the ECD data to an expanded kinetic model, the data can be attributed to two states. The Ea obtained from the ECD data are —0.17 0.05 eV for the linear anion and 0.40 0.15 eV for the bent anion. The larger uncertainty in the ECD value results because the transition temperature to dissociative electron attachment cannot be determined. If the calculated curve is extended to the highest temperature, the Ea is 0.5 eV and the E value is 0.5 eV. If it is terminated as shown in the second curve at approximately 350 K, the Ea is 0.3 eV and the E1 is 0.4 eV. The best value of the AEa is thus 0.40(15) eV. 

The transition for the linear anion to the linear neutral has a lower VEa. The anion curves are both M 2) and Mc 2) curves. There is also a D(0)/Dc(0) curve, which leads to S( -) at about 3.6 eV. Similar curves can be drawn to the dissociation limit of CS( ) + S. The electron affinity of CS is uncertain with one value at 0.20(2) eV and the other at 1.6(3) eV. Using 1.6 eV and the same bond orders as utilized for the two lower negative-ion curves, we obtain two low-lying curves a M( 1) and M(0). The D 0) curve gives CS( ) at an electron energy of 6.2 eV. 

The charge density is automatically calculated when one of the molecules is selected in HYPERCHEM. 

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The electron affinities of several triatomic molecules and the azide radical have been evaluated and are supported by the CURES-EC method. The molecules that are linear in the neutral and bent in the anion have been emphasized. The ECD data support two negative-ion states. Morse potential energy curves for N20 and CS2 have been constmcted for the linear and bent ions. The relative energies of the anion and neutrals for C02, COS, CS2, and N20 were presented to explain the electron attachment data. The electron affinities of the SF molecules n = 1 to 6 were evaluated and the largest values assigned to the ground state. 

It may be helpful to consider a simple two-dimensional illustration - a linear triatomic molecule, such as the triiodide anion, which has been observed in very many different crystal environments. Assuming linearity (which holds in practice quite closely for triiodide ions in crystals), we completely describe the structure of any given example of a triiodide ion by the two interatomic distances, rfi and di- A collection of triiodide ions is then described by a collection of points in the plane spanned by the two corresponding basis vectors, pi and pi (Figure 1.5). It is evident that the points corresponding to observed structures lie close to a rather well defined curve. We can interpret this curve in different ways, some of which are described in later Chapters (see especially Chapter 5). For example, we can regard it as the reaction path for the exchange reaction I2 -I-1 I +12. 

The reactions of the gold anion Au- and of the di- and triatomic gold cluster monoanions Au2- and Au3- with CO were studied in a radio-frequency octopole ion trap experiment at cryogenic temperatures. Au- shows no affinity for CO, but the two cluster anions absorb up to two CO molecules. Particular stability has been ascribed to [Au3(CO)2]-, for which the binding energy has been estimated from thermolysis rate coefficients.292... 

Ault and coworkers have utilized salt-molecule reactions to produce a number of novel triatomic and other anions in isolated environments [411]. For example, the linear symmetric [FHF] ion was produced in Ar matrices via codeposition of CsF vapor with HF gas diluted in Ar [399] ... 

Among the triatomic ligands (incl. neutral molecules such as HjO, SO2, and CO2) the anions N02, N3 , NCO", NCS, NCSe , and NCTe are well-known as ambidentate ligands. These anions have two or three donor atoms and can coordinate with a metal atom at one or the other position. Furthermore, they can serve not only as unidentate ligands but also as bridging ligands, connecting two or more metal atoms in various ways. This chapter focusses on the thiocyanate anion the various coordination modes will be shown. 

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