摘要:

A summary of the basic principles and information content of the spectroscopic methods most often used to determine the electronic structure of transition metal complexes is presented. The methods covered are electron paramagnetic resonance (EPR), electronic absorption (associated with ligand-field and charge-transfer transitions), x-ray absorption, and photoelectron spectroscopies. The higher resolution information available through EPR (hyperfine coupling) and electronic absorption (vibronic coupling and bandshape analysis) spectroscopies is also considered. Attention throughout this Comment is focused on the relatively simple examples of the square planar and distorted tetrahedral CuCl4= complexes.

ISSN:0260-3594    

DOI:10.1080/02603598408080072

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

The Experiment1-In the EPR experiment the total spin (S=½) of the2B1gground state is perturbed by the application of an external magnetic field H that produces a Zeeman splitting of the Ms=± ½ levels equal to gβH. Here p is the Bohr magneton which is a constant equal to 4.67 × 10−1cm−1/gauss, and g is the quantity of experimental and theoretical interest. The EPR experiment is usually performed with a fixed input microwave energyhv(∼9 GHz=0.30 cm−1forXband and 35 GHz=1.16 cm−1forQ-band microwave sources). When the scanning magnetic field results in a Zeeman splitting equal to the fixed microwave energy, absorption will occur [Figure II-1 and Eq. (II-l)]. Assuming for now a value of g=2,resonance occurs at a magnetic field of ∼3300 gauss for an X-band microwave source. For enhanced resolution the EPR spectrum is plotted as the first derivative (Figure II-2). The crossover point is at the magnetic field associated with the g value (=hv/βH) of the ground state.

ISSN:0260-3594    

DOI:10.1080/02603598408080073

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

In EPR spectroscopy we focused on the2B1gground state wavefunction. This was accomplished by perturbing it with an external magnetic field and observing transitions between the Zeeman-split ground state components using radiation in the ∼0.3 cm−1region.

ISSN:0260-3594    

DOI:10.1080/02603598408080074

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Basic Principles-When the absorption spectrum of CuCl4=is extended to energies higher than those associated with the d → d transitions, new, extremely intense absorption bands are observed. These are Laporteallowed charge-transfer transitions which result from optical excitation of an electron from the valence 3porbitals on the chloride ligands into the half-occupieddx2-y2orbital on the copper ion. Interpretation of the energy and intensity of these ligand-to-metal charge transfer transitions requires that the energy level diagram in Figure III-1 now be extended to include the valence orbitals on the four chloride ligands.

ISSN:0260-3594    

DOI:10.1080/02603598408080075

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

A. Basic Principles Figure V-1 contains a more extended region of the molecular energy level diagram forD4h-CuCl4-. If the absorption spectrum is scanned to energies higher than those of the charge-transfer transitions, we would next expect to observe transitions associated with an electron being excited from the 3dorbitals into the 4sand 4punoccupied levels on the copper. Since the copper 4sand 4pare valence orbitals which can mix with the ligand 3pand copper 3dorbitals, their direct spectroscopic study is quite important in evaluating their participation in bonding. Unfortunately, the spectroscopic region above ∼55,000 cm−1(∼7 eV where 1 eV=8066 cm−1) is quite difficult to probe experimentally. Since most molecules absorb strongly in this region, it can be difficult to find a spectroscopically transparent host and counter-ion. In addition, a VUV (vacuum ultraviolet) spectrometer is required. The most useful continuous energy source of photons in this range and into the x-ray region is synchrotron radiation from a storage ring (as described in Section B). The ring is maintained at ultrahigh vacuum (UHV) with Be windows transmitting photons at energies above ∼3000 eV. In the region between 7 and 3000 eV, Be windows absorb, thus the experiment must be contained in UHV. Few systematic absorption studies on inorganic complexes have been performed in this region.1Above 3000 eV, core electrons are excited at energies which are well separated for different atoms, and transitions on a specific atom of interest can be observed. Studies in this x-ray region will probe the open valence 4sand 4p(as well as the highest energy half-occupied 3dlevels on the copper, but at lower resolution (∼1.5 eV as compared to ∼0.0 1 eV in vacuum UV spectroscopy) due to intrinsic lifetime broadening as well as lower monochromator resolution at these high energies.

ISSN:0260-3594    

DOI:10.1080/02603598408080076

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Photoelectron spectroscopy (PES) provides a powerful probe of the energy level diagram of a metal complex, down to binding energies on the order of 1000 eV (Figure VI-l).1This method complements the methods discussed in preceding sections in that rather than measuring the energy and number of photons absorbed due to excitation of electrons into unoccupied bound states or the continuum, PES measures the kinetic energy (Ek) and number of electrons ejected upon photoexcitation into the continuum (referred to as photoemission). For a photon of fixed energy hv, the kinetic energy of the ejected electron is given by the Einstein relation,Most research in PES is directed toward determining the binding energies (E,) of electrons in the energy levels of a metal complex. Usually, the photon sources available for PES have fixed values of hv and thus photoelectron spectroscopy is subdivided into two fields, XPS (x-ray photoelectron spectroscopy) and UPS (ultraviolet photoelectron spectroscopy), based on the type and thus energy regime of the source used. (With the availability of continuously tunable synchrotron radiation, this distinction is becoming less clear.)

ISSN:0260-3594    

DOI:10.1080/02603598408080077

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

As observed in Section 11, Figure 11–5, the metal ion can also have a nuclear spin which couples to the electron spin to produce a hyperfine splitting of the EPR spectrum.I This coupling requires an additional term in the electron spin Hamiltonian which is given by:Here 1 is the nuclear spin quantum number which has (2I+ 1)zcomponents, M1=I,I-I, … -I. Throughout this section we assume the reasonable limit of Hhan > > Hhyperfine. Therefore Eq. VII- la produces a perturbation correction to each Zeeman level given byFor the simplest example ofS=½,I=½, Figure II-1 must be expanded to include the effects of Eqs. VII-1, resulting in Figure VII- 1.

ISSN:0260-3594    

DOI:10.1080/02603598408080078

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

From Section III, forD4h-CuCl4=, the parity forbiddenness of the d → d transitions in this centrosymmetric complex is overcome by vibronic or Herzberg-Teller coupling to odd parity vibrations of the nuclear framework. In addition, absorption bands in metal complexes can be greatly broadened (relative to the < 1 cm−1expected on the basis of their lifetimes) due to coupling of the electronic transitions to distortions of the molecular framework. Both effects require two new terms in the molecular Hamiltonian which relate to nuclear motion.

ISSN:0260-3594    

DOI:10.1080/02603598408080079

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

In completing this overview of the basic principles and information content of different methods in inorganic spectroscopy, a few final comments seem appropriate. First, a fairly consistent experimental and theoretical picture of the ground state of CuCl4−is developing. The molecular wavefunction can he viewed as having approximately 65% Cudx2−y2character, with the rest being delocalized onto the 3pzorbitals of the chloride ligands. A small amount of C1 3salso contributes (∼3%), as does a few percent Cu 4sand Cu 4pin lower symmetry geometries. This description of covalency in CuCl4−should permit relative estimates of covalency in other complexes as the ligands and/or metal ion are varied.

ISSN:0260-3594    

DOI:10.1080/02603598408080080

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

This is a scanned image of the original Editorial Board page(s) for this issue.

ISSN:0260-3594    

DOI:10.1080/02603598408080071

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor