Trigonal planar molecule symmetry

The example of COj discussed previously, which has no vibrations which are active in both the Raman and infrared spectra, is an illustration of the Principle of Mutual Exclusion For a centrosymmetric molecule every Raman active vibration is inactive in the infrared and any infrared active vibration is inactive in the Raman spectrum. A centrosymmetric molecule is one which possesses a center of symmetry. A center of symmetry is a point in a molecule about which the atoms are arranged in conjugate pairs. That is, taking the center of inversion as the origin (0, 0, 0), for every atom positioned at (au, yi, z ) there will be an identical atom at (-a ,-, —y%, —z,). A square planar molecule XY4 has a center of symmetry at atom X, whereas a trigonal planar molecule XYS does not possess a center of symmetry. 

A proper axis of symmetry, denoted by Cn, is an axis around which a molecule is rotated by 360°ln to produce an equivalent configuration. The trigonally planar molecule BF3 may be set up so that the molecular plane is contained by the xy Cartesian plane (that containing the x and y axes) and so that the z Cartesian axis passes through the centre of the boron nucleus, as is shown in Figure 2.1. 

Determine the point-group symmetries of the following molecules (1) O2 (2) HCl (3) ethylene (C2H4) (4) (Z)-l,2-difluoroethylene (5) (E)-l,2-difluoroethylene (6) AICI3, a trigonal-planar molecule (7) tetrachloromethane (CCI4) (8) dichloromethane (CH2CI2) (9)... 

Syiltmetry Elements and Operations. A symmetry operation is a transformation of a body such that the final position is physically indistinguishable from the initial position, and the distances between ill pairs of points in the body are preserved. For example, consider the trigonal-planar molecule BF3 (Fig. 12.1a), where for convenience we have numbered the fluorine nuclei. If we rotate the molecule counterclockwise by 120° about an axis through the boron nucleus and perpendicular to the plane of the molecule, the new position will be as in Fig. 12.1b. Since in reality the fluorine nuclei are physically indistinguishable from one another, we have carried out a symmetry operation. The axis about which we rotated is an example of a synunetry element. Synunetry elements and symmetry operations are related but different things, which are often confused. A symmetry element is a geometrical entity (point, line, or plane) with respect to which a symmetry operation is carried out. 

The chemistry of propylene is characterized both by the double bond and by the aHyUc hydrogen atoms. Propylene is the smallest stable unsaturated hydrocarbon molecule that exhibits low order symmetry, ie, only reflection along the main plane. This loss of symmetry, which implies the possibiUty of different types of chemical reactions, is also responsible for the existence of the propylene dipole moment of 0.35 D. Carbon atoms 1 and 2 have trigonal planar geometry identical to that of ethylene. Generally, these carbons are not free to rotate, because of the double bond. Carbon atom 3 is tetrahedral, like methane, and is free to rotate. The hydrogen atoms attached to this carbon are aUyflc. 

The NF3 molecule has two extra electrons, compared to BF3, which would occupy an anti-bonding 7t-type 2a/ orbital if it had a trigonally planar shape. The C1F3 molecule with another two electrons would make use of the anti-bonding or-type 4a,7 orbital if the molecule were to be trigonally planar. It is to avoid use of anti-bonding orbitals that they adopt different symmetries. 

Figure 6.8 A diagram showing the interaction of the 4a/ and 282" MOs of trigonally planar NF3 permitted by a distortion of the molecule, giving a shape of lower symmetry, i.e. trigonally pyramidal...

The MO treatment of S03 would make use of a diagram such as Figure 6.6 modified by incorporating the two sulfur 3d orbitals that could take part in the trigonally planar bonding as 7t orbitals, i.e. the 3dv and 3dv. orbitals which have e" symmetry. The extra e" bonding orbitals would accommodate four bonding 7t electrons, which would stabilize the molecule and ensure that the S-O bond order was 2. 

It has been pointed out88 that the angle —in pyrimidines is about 11° less than the angle —C—. Also, in -triazine, C8N Ha, which is a planar molecule with trigonal symmetry, the two angles have the Values84 113.2° 0.4° and 126.8° 0.4°, respectively, with difference — 13.6°, and in s-tetrazine, they have the values81... 

SOLUTION (a) According to the VSEPR model, BF3 is a trigonal planar AX3 molecule. This shape has a symmetry in which all three bond dipoles cancel. It is listed in Fig. 3.10 and classified there as nonpolar (see also 6). (b) According to the VSEPR model, the 03 molecule is angular, with one lone pair on the central O atom (37). Two positions around that central O atom are linked to O atoms, but the third is a lone pair, making it an AX2E molecule. The dipoles do not cancel as a result, the molecule is polar. This example shows that a homonuclear polyatomic molecule can be polar. 

The existence of a dipole moment tells us something about the molecular geometry or symmetry, and vice versa. For example, the existence of a dipole moment for H20 and NH3 implies that the former cannot be linear and the latter cannot be trigonal planar. Indeed, it is not difficult to show that molecules that possess dipole moments belong only to point groups C , Cs, or Cm. 

The possible stroctures 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, symmetry) or linear (sp hybrid, D j, symmetry) molecules three will form pyramidal (p. C3,) or trigonal planar (jp2, Dj,) molecules four will usually form tetrahedral (sp, TJ) or square planar (dip, 4 ,) hve usually form a trigonal bipyramidal (Dyf,), more rarely a square pyramidal (C4,.) molecule (both dsp hybrids, but using different orbitals, see Table 6.2) and six groups will usually form an octahedral molecule (d sp, Oi,). 

T2.6 The planar from of NHj molecule would have a trigonal planar molecular geometiy with the lone pair residing in nitrogen s p orbital that is perpendicular to the plane of the molecule. This structure belongs to a Djh point group (for discussion of molecular symmetry and point groups refer to Chapter 6). Now we can consult Resource Section... 

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