Electronic spectra, diatomics

Figure 7.1 8 Vibrational progressions and sequences in the electronic spectrum of a diatomic molecule...

Until recently, only estimates of the Hartree-Fock limit were available for molecular systems. Now, finite difference [16-24] and finite element [25-28] calculations can yield Hartree-Fock energies for diatomic molecules to at least the 1 ghartree level of accuracy and, furthermore, the ubiquitous finite basis set approach can be developed so as to approach this level of accuracy [29,30] whilst also supporting a representation of the whole one-electron spectrum which is an essential ingredient of subsequent correlation treatments. 

This chapter consists of the application of the symmetry concepts of Chapter 2 to the construction of molecular orbitals for a range of diatomic molecules. The principles of molecular orbital theory are developed in the discussion of the bonding of the simplest molecular species, the one-electron dihydrogen molecule-ion, H2+, and the simplest molecule, the two-electron dihydrogen molecule. Valence bond theory is introduced and compared with molecular orbital theory. The photo-electron spectrum of the dihydrogen molecule is described and interpreted. 

There are many reasons that one might want to record, assign, and interpret the electronic spectrum of a diatomic molecule. These include qualitative (which molecular species are present) and quantitative (what is the number density of a known quantum state of a known molecule) analysis, detection of trace constituents (wanted, as in analysis of ore samples for a precious metal, or unwanted, as in process diagnostics where specific impurities are known to corrupt an industrial process), detection of atmospheric pollutants, monitoring of transient species to optimize a combustion process by enhancing efficiency or minimizing unwanted byproducts, laboratory determinations of transition frequencies and linestrengths of interstellar molecules, and last but certainly not least, fundamental studies of molecular structure and dynamics. 

To return to chemistry, Fig. 6 shows schematic potential energy curves for an electronic spectrum of a cold isolated diatomic molecule. There is no possibility for IVR, but the spectrum (Fig. 7) clearly has a broad envelope which, judging from the spacings,... 

The same approach can be used to understand the chemical bonding in polyatomic molecules and polymers. The electronic spectrum of a polymer can be described as the sum of the spectra of individual diatomic components (Fig. 3). Since the orbital... 

The important point about the electronic spectrum is that we gain information about any vibration in the molecule, even though the fundamental frequency of that vibration might not appear in the infrared spectrum. For example, the vibrational frequency of homonuclear diatomic molecules does not appear in the infrared, since the oscillating dipole moment is absent the bands resulting from that vibration do show up in the electronic spectrum. 

The character of the diatomic skeleton is still visible in the H AB systems with many hydrogen atoms. In essence the hydrogens modify only the stability of the various diatomic MOs as a result of mixing between hydrogen AOs and the diatomic partners. Depending on the geometrical arrangement, they can preferentially influence one MO over the others, and this not only leads to a predictable shift in the electronic spectrum of the H AB system relative to that of the isoelectronic AB but also causes the appearance of various structural isomers. 

There are several recent experimental studies on the CeO diatomic molecule. Schall et al. (1986) have studied CeO using the sub-doppler Zeeman spectroscopy. Again, the ligand-field model is so successful in explaining the observed spectra due to the ionic nature of the diatomic lanthanide oxide. Linton et al. (1979, 1981, I983a,b) as well as Linton and Dulick (1981) have studied the electronic spectrum of CeO using absorption, emission as well as laser spectroscopic method. There are many 0-0 bands for... 

Only the two most important stable binary oxides, P4O6 and P4O10, are considered, but many others are known, even the diatomic species PO, whose electronic spectrum has been characterized. 

Kasai and McLeod (57, 58) also studied a series of bimetallic diatomics, AgM (M = Mg, Ca, Sr, Be, Zn, Cd, or Hg), by ESR spectroscopy. For all of these species, the hyperfine coupling to the Ag nucleus was found to be isotropic. It was shown that the unpaired electron resides in an orbital resulting essentially from an anti-bonding combination of the valence s orbitals of the Ag and M atoms. A typical spectrum is shown in Fig. 13. 

The excited molecules normally release their energy by spontaneous emission of fluorescence, terminating not only in the initial ground state but on all vibronic levels of lower electronic states to which transitions are allowed. This causes a fluorescence spectrum which consists, for instance, in the case of an excited singlet state in a diatomic molecule, of a progression of either single lines (A/ = 0 named Q-lines) or of doublets (A7 = 1 P- and i -lines) ... 

The Fe(III)-NO complex of NPl is EPR silent (Fig. 3) because it contains an odd-electron (ferriheme) center bound to the odd-electron diatomic NO 24), which creates a FeNO center. The NMR spectrum of NPl Fe(III)-NO is that of a diamagnetic protein 85). However, whether the electron configuration is best described as Fe(II)-NO+ or antiferro-magnetically coupled low-spin Fe(III)-NO- is not completely clear, even though the infrared data 49) discussed earlier (Fig. 7) are consistent with the former electron configuration. Thus, as a prelude to planned detailed studies of the Mossbauer spectra of the nitrophorins and their NO complexes, we have reported the Mossbauer spectrum of the six-coordinate complex of OEPFe(III)-NO 86). 

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