Selection Rules and Molecular Structure

Photochromic reactions of the heterocyclic fulgides are also in accordance with the Woodward-Hoffmann selection rules. The molecular structures have dramatic effects on the quantum yields of photoinduced ring-closure and ring-opening reactions of fulgides. The photochemical reactions of furyl fulgides are shown in Scheme 35 and the quantum yields are summarized in Table 4.29.44 45 104... 

Electric-quadrupole transition, 123,127 Electromagnetic radiation, 114-117. See also Radiation, electromagnetic Electromagnetic spectrum, 115 Electronic energy, 57,64,148 Electronic spectra, 130, 296-314 of diatomics, 298-306 and molecular structure, 311 of polyatomics, 71-72, 73, 75, 306-314 selection rules for, 297-301, 306-307 Electronic structure of molecules, 56-76 Electron spectroscopy for chemical analysis (ESCA), 319-320 Electron spin resonance (ESR), 130, 366-381... 

Chemical symmetry has been noted and investigated for centuries in crystallography which is at the border between chemistry and physics. It was more physics when crystal morphology and other properties of the crystal were described. It was more chemistry when the inner structure of the crystal and the interactions between its building units were considered. Later, descriptions of molecular vibrations and the establishment of selection rules and other basic principles happened in all kinds of spectroscopy. This led to another uniquely important place for the symmetry concept in chemistry with practical implications. 

Additional information on electronic structure may be obtained from the x-ray emission spectra of the SiOj polymorphs. As explained in Chapter 2, x-ray emission spectra obey rather strict selection rules, and their intensities can therefore give information on the symmetry (atomic or molecular) of the valence states involved in the transition. In order to draw a correspondence between the various x-ray emission spectra and the photoelectron spectrum, the binding energies of core orbitals must be measured. In Fig. 4.12 (Fischer et al., 1977), the x-ray photoelectron and x-ray emission spectra of a-quartz are aligned on a common energy scale. All three x-ray emission spectra may be readily interpreted within the SiO/ cluster model. Indeed, the Si x-ray emission spectra of silicates are all similar to those of SiOj, no matter what their degree of polymerization. Some differences in detail exist between the spectra of a-quartz and other well-studied silicates, such as olivine, and such differences will be discussed later. 

X-ray emission spectra of solids and molecules are methods of measuring electronic structure of matter [1-5]. The x-ray emission spectra reflect the occupied electronic structure as shown in Fig. 1, while the x-ray absorption spectra reflect the unoccupied molecular orbitals (MO). These x-ray spectra repre nt local (L) and partial (P) electron density of states (DOS) because of the electric dipole selection rule, and thus the x-ray spectroscopy is a powerful tool to study the electronic structure of matter. The development of... 

Molecular structure may often be inferred for simple molecules (up to 10 atoms) by correlating an assumed geometric structure with the selection rules for that structure. This process can be learned by a student without knowing much about the quantum mechanics or group theory that underlies it, but it does take time and practice. The selection rules tell us which modes of vibration are permitted to appear in the infrared and Raman spectrum. Thus, from the actual infrared and Raman spectral patterns and those implied by the selection rules for the assumed structure, the structure may be proven or disproven. The process is often like working a crossword puzzle. Bits of information are gathered from several analytical and spectroscopic methods and then fitted properly into place to obtain the structure of the molecule. 

In the area of the elucidation of molecular structure the Raman method has proved particularly useful. The application of group theory leads to a set of selection rules for Raman and infrared spectra on the basis of a proposed molecular model. One can then determine the total number of theoretical frequencies which should appear in Raman and infrared spectra. This is then compared with the observed spectra. The procedure can be repeated with other models until a good match between theoretical and experimental spectra is obtained. The method is illustrated by Table III. On the basis of the selection rules, Lord picked structure D h as the most likely for the molecule IF . Recently x-ray work on IF has been published [ ]. 

This spectrum is called a Raman spectrum and corresponds to the vibrational or rotational changes in the molecule. The selection rules for Raman activity are different from those for i.r. activity and the two types of spectroscopy are complementary in the study of molecular structure. Modern Raman spectrometers use lasers for excitation. In the resonance Raman effect excitation at a frequency corresponding to electronic absorption causes great enhancement of the Raman spectrum. 

We now turn to electronic selection rules for syimnetrical nonlinear molecules. The procedure here is to examme the structure of a molecule to detennine what synnnetry operations exist which will leave the molecular framework in an equivalent configuration. Then one looks at the various possible point groups to see what group would consist of those particular operations. The character table for that group will then pennit one to classify electronic states by symmetry and to work out the selection rules. Character tables for all relevant groups can be found in many books on spectroscopy or group theory. Ftere we will only pick one very sunple point group called 2 and look at some simple examples to illustrate the method. 

The enzymes are protein molecules having globular structure, as a rule. The molecular masses of the different enzymes have values between ten thousands and hundred thousands. The enzyme s active site, which, as a rule, consists of a nonproteinic organic compound containing metal ions of variable valency (iron, copper, molybdenum, etc.) is linked to the protein globule by covalent or hydrogen bonds. The catalytic action of the enzymes is due to electron transfer from these ions to the substrate. The protein part of the enzyme secures a suitable disposition of the substrate relative to the active site and is responsible for the high selectivity of catalytic action. 

The dipole and polarization selection rules of microwave and infrared spectroscopy place a restriction on the utility of these techniques in the study of molecular structure. However, there are complementary techniques that can be used to obtain rotational and vibrational spectrum for many other molecules as well. The most useful is Raman spectroscopy. 

In the investigations of molecular adsorption reported here our philosophy has been to first determine the orientation of the adsorbed molecule or molecular fragment using NEXAFS and/or photoelectron diffraction. Using photoemission selection rules we then assign the observed spectral features in the photoelectron spectrum. On the basis of Koopmans theorem a comparison with a quantum chemical cluster calculation is then possible, should this be available. All three types of measurement can be performed with the same angle-resolving photoelectron spectrometer, but on different monochromators. In the next Section we briefly discuss the techniques. The third Section is devoted to three examples of the combined application of NEXAFS and photoemission, whereby the first - C0/Ni(100) - is chosen mainly for didactic reasons. The results for the systems CN/Pd(111) and HCOO/Cu(110) show, however, the power of this approach in situations where no a priori predictions of structure are possible. 

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