Pyramidal Four-Atom Molecules

The four normal modes of vibrations of a pyramidal XYj molecule are shown in Fig. II-3. These four vibrations are both infrared and Raman active. Table II-3a lists ihe fundamental frequencies of XHj-type molecules. Several bands marked by an asterisk are split into two by inversion doubUng. As is shown in Fig. II-4, two configurations of the XH, molecule are equally probable. If the potential barrier between them is small, the molecule may resonate between the two structures. As a result, each vibrational level splits into two levels (positive and negative). Transitions between levels of dilTerent sign are allowed in the infrared spectrum, whereas those between levels of ihe same sign are allowed in the Raman spectrum. The transition between the two levels at u = 0 is also observed in the microwave region (v = 0.79 cm ). If the 

As is seen in Table Il-3a, and overlap or are close in most compounds. The presence of the hydronium (H3O ) ion in hydrated acids has been confirmed by observing its characteristic frequencies. For example, it was shown from infrared specira that H2PtCl6-2H20 should be formulated as (H30)2[PtCl6], For normal coordinate analysis of pyramidal XH, molecules, see Refs. 293-295. 

Substiiution of one of the Y atoms of a pyramidal XYj molecule by a Z atom lowers the symmetry from Cj to C. Then the degenerate vibrations split into two bands, and all six vibrations become infrared and Raman active. The relationship between C3 and C is shown in Table II-3d. Table ll-3e lists the vibrational frequencies of pyramidal ZXY2 molecules. Simon and Paet-zold made an extensive study of the vibrational spectra of selenium compounds. The ZXYW-type molecule belongs to the C, point group, and all six vibrations are infrared and Raman active. The vibrational spectra of OSClBr and [XSnYZ] (X, Y, Z a halogen) have been reported. 

Splittings due to inversion doubling were also observed for the V4 of [578], and and Vi and V3 of ND3 [579]. Optical isomers may be separated if the three Y groups are not identical and the inversion barrier is sufficiently high. 

Reaction of CH3I with Na (or K) atoms in N2 matrices at 20 K [575] produces an ion-pair complex CH3 Na+ (or CH3 K+), of C31, symmetry. The v(Na—CH3) and v(X—CH3) vibrations were observed at 298 and 280cm respectively. Vibrational spectra of the (NH3)2 dimer in Ar matrices show that two NH3 molecules are bonded via a very weak hydrogen bond [584]. 

Substitution of one of the Y atoms of a pyramidal XY 3 molecule by a Z atom lowers the symmetry from C31, to Then the degenerate vibrations split into two bands, and all 

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Fig. 4.5 The average bond energy per atom (in units of the magnitude of the dimer bond integral 0 ) as a function of the electron count N for three-, four-, and five-atom molecules. The pentagon and square pyramid five-atom molecules have been omitted for clarity. (After Shah and Pettifor (1993).)...

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. 

The practical independence of Li-0 and Li-D distances with temperature shows the strong orientational correlation of water molecules around Li it is most likely that an average orientation of the coordinated water molecules is such that the four atoms in a Li -D20 unit is pyramidal. The strong orientational correlation of the bound water molecules rationalizes the anisotropic reorientational motion of the water molecules in the hydration shell found by nuclear magnetic relaxation data of supercooled LiCl solutions. The evolution of the secondary hydration shell of Li may be a hint of nucleation of ice, glass transition, and partial recovery of hydrogen bonds in the supercooled state. 

Once the Lewis structure has been determined, it is possible to know the shape of the molecule or ion. The most important piece of information needed to determine the shape is the total number of groups around the central atom, where a group could be another atom or a lone pair. A central atom connected to one or two other atoms is linear. When the central atom is connected to three atoms, the shape is trigonal planar. When the central atom is connected to four atoms, the shape is tetrahedral. When the central atom is connected to five atoms the shape is trigonal bipyramidal (two triangular-based pyramids joined at the base). When the central atom is connected to six atoms, the shape is octahedral. Other shapes are possible when atoms are replaced with lone pairs. 

By the addition of one 0 atom to the four kinds of pyramidal ions and molecules of the previous section we have ... 

Although both BF3 molecules and NH3 molecules have four atoms, the BF3 molecules are planar, and NFi3 molecules are pyramidal. Why ... 

Compare Figures 9.1 and 9.2 to notice the difference between NF3 and CCl4. The CCI4 molecule is tetrahedral because the four atoms bonded to the carbon are disposed at the four apexes of a tetrahedron around the central atom. The NF3 molecule is pyramidal because the three atoms bonded to nitrogen lie at the base of a trigonal pyramid. 

The possible structures may be classified in terms of the coordination number of the central atom and the symmetry of the resulting molecule (Fig. 6.17). Two groups about a central atom will form angular (p2 orbitals, C2, symmetry) or linear (sp hybrid, symmetry) molecules three will form pyramidal (p>, Cj,) or trigonal planar (sp>, Dy,) molecules four will usually form tetrahedral (sp>, T,j) or square planar (c/sp>, O4 ) five usually form a trigonal bipyramidal (D3/,), more rarely a square pyramidal (C4 J molecule (both dsp hybrids, but using different orbitals, see Table 6.2) and six groups will usually form an octahedral molecule (d spi, 0,). 

Let us begin by diagramming the movements for the nonlinear molecule NH3. Since there are four atoms, there should be 3V — 6 or six fundamentals. The structure of NH3 is pyramidal, with the N atom at the apex and the three H atoms forming the base. The vibrations for NH3 are shown in Figure 4-14. Only four of the six motions are shown since two of them (V3 and V4) are two-dimensional... 

Pyramidal XY3 (C3J molecules have four normal modes of vibration (Figure 7) (Table 5), all infrared and Raman active, which have been used, for example, to confirm the presence of the hydronium (H30 ) ion in hydrated acids. Substitution of one of the Y atoms of a pyramidal XY3 molecule by a Z atom lowers the symmetry from to Q. Then the degenerate vibrations split into two bands [v,(XY) (XY) + V3(XY) (5d(YXY) <53(YXY) + (5a(YXZ)] ((5 = bending) and all six vibrations... 

Next we will consider molecules that have both bonded and nonbonded pairs of electrons in the valence shell of the central atom. Water and ammonia have four electron pairs around the central atom. Some of the electron pairs in water and ammonia are bonded to hydrogen atoms, but the central atom also has unshared electron pairs. VSEPR theory describes the distribution of bonded and nonbonded electron pairs. However, molecular structure is defined by the positions of the nuclei. The four pairs of electrons in both water and ammonia are tetrahedrally arranged around the central atom. Water, with only three atoms, is angular, and ammonia, with four atoms, is pyramidal (Figure 1.7). 

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