Example Point Group Assignments

Example Point Group Assignments 3.11.1 Example 1 Conformations of Cyclohexane... 

Figure 2.32. Examples of symmetry group elements and molecular point group assignments.

We have only one example in our list that falls into the S2 groups, as seen in Figure 4.12. A flowchart that summarizes this point group assignment method is given in Figure 4.7, and more examples are given in Table 4.4. 

More detailed questions can be added to these broad categorizations to allow a point group assignment to be made. One strategy is summarized in the flow diagram shown in Figure 3.29, and in the following sections we will apply this approach to some example structures. 

As an example of an AB molecule, we will discuss the planar symmetrical molecule BC1, which belongs to the B point group. First we assign to each chlorine atom a pair of mutually perpendicular... 

Another example is provided by a series of octahedral MX Y6 type ions (n = 0-6), where M is Pt(IV), Os(IV), and Ir(IV), and X and Y are halogens. Preetz and co-workers (4) prepared these mixed-halogeno ions and assigned their IR/Raman bands based on point group symmetry. Table 4-3 shows the point group and classification of IR/Raman-active fundamental vibrations. Figure 4-3 shows the IR/Raman spectra and band assignments of the [PtCl Br6 ]2 series. It should be noted that these ions exhibit v(PtCl),... 

After deciding what symmetry elements are present, a molecule can be assigned to a symmetry category known as a point group, which is based on the collection of symmetry elements the molecule possesses. Table 2.6 shows a summary of some of the most important point groups, lists the symmetry elements present, and shows some examples of each type of structure. 

RetinalS. The structure and photophysics of rhodopsins are intimately related to the spectroscopic properties of their retiny1-polyene chromophore in its protein-free forms, such as the aldehyde (retinal), the alcohol (retinol or vitamin A), and the corresponding Schiff bases. Since most of the available spectroscopic information refers to retinal isomers (48-55), we shall first center the discussion on the aldehyde derivatives. Three bands, a main one (I) around 380 nm and two weaker transitions at 280 nm and 250 nm (II and III), dominate the spectrum of retinals in the visible and near ultraviolet (Fig. 2). Assignments of these transitions are commonly made in terms of the lowest tt, tt excited states of linear polyenes, the spectroscopic theories of which have been extensively discussed in the past decade (56-60). In terms of the idealized C2h point group of, for example, all-trans butadiene, transitions are expected from the Ta ground state to B , A, and A" excited states... 

As an example of the use of two-photon spectroscopy in assigning excited states that are not observed in one-photon UV spectroscopy, the two-photon absorption spectrum of naphthalene is shown in Figure 1.17. Since for the point group all Bg states have a theoretical polarization degree of H = 3/2, the polarization measurement reveals immediately a Bg state near 42,000 cm . In the two-photon absorption spectrum this shows up only as a shoulder, whereas the maximum at 44,500 cm" can be assigned to an Ag state. Neither state is prominent in the one-photon spectrum. 

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