Boron trifluoride symmetry

Although all molecules are in constant thermal motion, when all of their atoms are at their equilibrium positions, a specific geometrical structure can usually be assigned to a given molecule. In this sense these molecules are said to be rigid. The first step in the analysis of the structure of a molecule is the determination of the group of operations that characterizes its symmetry. Each symmetry operation (aside from the trivial one, E) is associated with an element of symmetry. Thus for example, certain molecules are said to be planar. Well known examples are water, boron trifluoride and benzene, whose structures can be drawn on paper in the forms shown in Fig. 1. 

Normalization, 6 Normal modes, 240-244 of benzene, 438-439 of boron trifluoride, 281, 290 of carbon dioxide, 242, 248, 262, 265 of ethylene, 291 and group frequencies, 266-268 IR active, 457 Raman active, 457 and symmetry, 246-249,430-439 of water, 431-437 Normal operator, 108 Nuclear g factor, 3 24 Nuclear magnetic moments, 323-325 Nuclear magnetic resonance, 129-130, 323-366... 

Boron trifluoride has a trigonal-planar structure. Formulate the bonding in terms of molecular orbitals for the Dsjj symmetry. In addition, construct wave functions for three equivalent sp2 hybrid orbitals, using the 2px, 2p, and 2s boron valence orbitals, which may be used to form three localized bonds with the three fluorines. Compare and contrast the molecular-orbital and the hybrid-orbital descriptions. 

Thus, IBr has a nonzero dipole moment, which points from the Br atom to the I atom, (b) Because fluorine is more electronegative than boron, each B—F bond in BF3 (boron trifluoride) is polar, and the three bond moments are equal. According to the VSEPR model, however, BF3 should be trigonal planar and the symmetry of the trigonal planar shape means that the three bond moments exactly cancel one another ... 

Let us examine some simple types of molecules for symmetry planes. Carbon dioxide, a typical linear molecule, has an infinite number of symmetry planes through the molecular axis, the line extending from one O atom through the C atom to the other O atom. It also has a plane through the carbon atom perpendicular to the molecular axis (Fig. 19.24a). Boron trifluoride, a typical planar triangular molecule, has, in addition to the molecular plane, three planes each passing through the B atom and one of the F atoms (Fig. 19.24b). 

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