Planar four-atom molecules

The four normal modes of vibration of planar XY3 molecules are shown in Fig. 2.12 V2, V3, and V4, are infrared-active, and Vj, V3, and V4 are Raman-active. This case should be contrasted with pyramidal XY3 molecules, for which all four vibrations are both 

Structure (a) rather than a three-membered ring C2V structure (b) suggested originally [714]. 

Crystals of UNO3, NaNOs, and KNO3 assume the calcite structure (Sec. 1.31). Nakagawa and Walter [705] carried out normal coordinate analyses on the whole Bravais lattices of these crystals. 

TABLE 2.4b. Vibrational Frequencies of Planar XO3 and Related Compounds in the Crystalline State (cm ) 

Miiller et al. [715] have shown that the half-width of the V3 band of the N03 ion at 1384 cm in KBr pellets is decreased by a factor of 5 when the ion is trapped in the cavity of ionic carcerand compounds such as Kio[HVi2 V6044(N03)2]-14.5H20. UV resonance Raman spectra of the N03 ion in H2O and ethylene glycol are dominated by the Vi, V3, and their overtones and combination bands. The large splitting of the V3 

Some XYj-type halides take the unusual T-shaped structure of C2 symmetry shown below. This geometry is derived from a trigonal-bipyramidal structure in which two equatorial positions are occupied by two lone-pair electrons. Typical examples are CIF3 and Brp3. With the equatorial Y atom represented as Y, the following assignments have been made for these molecules r(XY ), a, 754 and 672 r(XY), B, 683.2 and 597 r( Y), A, 523 and 547 5, A, 328 and 235 S, B, 431 and 347 tt, Bj, 332 and 251.5 cm (for each 

If one of the Y atoms of a planar XY3 molecule is replaced by a Z atom, the symmetry is lowered to C2U. If two of the Y atoms are replaced by two different atoms, W and Z, the symmetry is lowered 10 C. As a result, the selection rules are changed, as already shown in Table 1-12. In both cases, all six vibrations become active in infrared and Raman spectra. Table II-4c lists the vibrational frequencies of planar ZXYj and ZXYW molecules. Although not listed in this table, the infrared spectra of binary mixed halides of boron and aluminum have been measured. The frequencies listed for ihe formate and acetate ions were obtained in aqueous solution. These frequencies are important when we discuss the vibrational spectra of metal salts of these anions (Sec. HI-7). 

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Planar four-atom molecules of the WXYZ, XYZY, and XYYY types have six normal modes of vibration, as shown in Fig. 2.16. All these vibrations are both infrared- and Raman-active. In HXYZ and HYZY molecules, the XYZ and YZY skeletons may be linear (HNCO, HSCN) or nonlinear (HONO, HNSO). In the latter case, the molecule may take a cis or trans structure. Table 2.5d lists the vibrational frequencies of molecules and ions belonging to these types. Normal coordinate analyses have been carried out for HN3 [831] and HONO [832]. 

Table 11-56. Vibrational Frequencies of Planar Four-Atom Molecules (cm" )... 

Figure 5.2 Schematic representation of four atomic molecules (from top to bottom) linear (acetylene), planar (formaldehyde), a-planar (ammonia).

Explain why some four-atom molecules, such as NH3 (ammonia), have a pyramid shape, and other four-atom molecules, such as A1C13 (aluminum chloride), have a triangular-planar shape. 

Vibrational frequencies are reported for other four-atom molecules such as cis-OSOO, which was produced by laser irradiation (193 nm) of the planar SO3 in Ar matrices at 12 K [833], and the HOOO radical, which is probably in cA-conformation in inert gas matrices [834]. Vibrational assignments have also been made for linear four-atom molecules containing CN groups CNCN(isocyanogen) [835], HCCN radical (cyanomethylene), [836] [FCNF] [837], and a pair of HBeCN and HBeNC that were formed by reacting Be atom with HCN in Ar matrices at 6-7 [838],... 

In almost all compounds that have pn-dn bonds, the central atom is connected to four atoms or three atoms and an unshared pair and the bonding is approximately tetrahedral. The pn-dn bond, therefore, does not greatly change the geometry of the molecule in contrast to the normal tc bond, which changes an atom from tetrahedral to trigonal. Calculations show that nonstabilized phosphonium ylids have nonplanar ylidic carbon geometries, whereas stabilized ylids have planar ylidic carbons. ... 

Molecules with a planar coordination figure do not contain planar atoms . Further, tetrahedral atoms , chiral atoms etc. are nonsense. A minimum of four atoms is required for a chiral structure. 

Boron is in Group 3 and so has three electrons in the outer shell. The three Cl atoms contribute one electron each, giving a total of six electrons involved in bonding. So there are three B-Cl bonds and no non-bonding pairs on the boron atom. The shape of the boron trichloride molecule will be trigonal (or trigonal planar) with all four atoms in the same plane. 

Let us now look at the normal modes of vibration of a molecule which is as simple as possible and yet exemplifies all general features ordinarily encountered. The planar ion will serve for this purpose. As a nonlinear four-atomic species, it must have 3(4) -6 = 6 normal modes. In Figure 10.1 we have depicted these vibrations. In each drawing the length of an arrow relative to the length of another arrow in the same drawing shows how much the atom to which it is attached is displaced at any instant relative to the simultaneous displacement of the atom to which the other arrow is attached. 

The xenon atom in XeF4 is bonded to four atoms and has two lone pairs. As you might expect, the lone pairs orient as far away from each other as possible to minimize electronic repulsions, giving the molecule a square planar shape ... 

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