摘要:

Bonding electron density need not be distributed symmetrically about the line-of-centres of bound atoms. This review describes how recent ligand-field analysis has been able to detect such circumstances and other forms of misdirected valency. Eight groups of analyses, comprising detailed studies of 25 individual complexes, involve quantitative reproductions of paramagnetic susceptibilities, electron spin resonance g2tensors, and ‘d-d’ transition energies, absorbances and rotatory strengths. These careful studies provide a showcase for the reach and power of contemporary ligand-field models as well as a level of detailed insight into transition-metal bonding that is not readily available by other means. The examples given here are prefaced with brief reviews of the parametric structures of the ligand-field method. These refer to the cellular ligand-field models for both energies and intensities of electronic transitions. An indication of how calculations of magnetic properties, transition energies, spectral intensities and circular dichroism—all within a d” basis—are implemented in practice is included.

ISSN:0144-235X    

DOI:10.1080/01442359009353247

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Our recent studies and developments undertaken aimed at spectral characterization of open-shell cations are outlined. The analyses of the rotational structure in the laser excitation spectra of the halocyanide and diacetylene cations have enabled their complete rsgeometries to be inferred. These should serve as a benchmark for calculations in open-shell cations containing either a heavy atom of many atoms. A new direction pursued aims at spectral characterization of fragment ions. The first success is the identification of the [Btilde]4Σu−—[Xtilde]4Σg−transition of C2+, which was observed initially in absorption in a 5 K neon matrix and then in the gas phase by laser excitation. The stimulated emission pumping approach was also used to probe higher vibrational levels of the [Xtilde]4Σg−state. In the studies of new ions by laser methods, two obstacles are encountered. One concerns the uncertainty in the energy region of the transitions sought and the other is the identification of the carrier. We have therefore been developing methods to provide such information. In one, mass-selected ions are co-deposited with neon to form a matrix at 5 K. The electronic absorption spectrum of the trapped ions can then be measured. The first results illustrate the potential of this new technique. In a gas-phase approach, mass-selected ions are investigated by two-photon absorption within a triple-quadrupole instrument. One laser photon induces a transition between bound states of the ion and the second photon leads to fragment ion production. By monitoring the fragment ions as a function of either photon wavelength, both bound-bound and bound-pre-dissociative transitions can be studied. Such results have been obtained on CS2+and CO2+.

ISSN:0144-235X    

DOI:10.1080/01442359009353248

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor