1.2 Factors to determine molecular structures
1.2 Factors to determine molecular structures
Even in the structural formulas 2~5, these can not reflect the real structure of those molecules, since those formulas drawn on a sheet of paper can describe only the order of atoms in molecules.
Then, what kind of parameters could define a molecular structure? It can be defined by only atomic distance r AB for diatomic molecules, and for triatomic molecules, it can be defined by three parameters two atomic distances r AB, r AC and bond angle BAC. Both an atomic distance and a bond angle are the specific parameters of atoms that form the bonds, and they have own values which can be predicted by the chemical bond theory.
For tetra atomic molecules, three atomic distances r AB, r BC, r CD and two bond angles ABC, BCD cannot be enough to define the geometry of a molecule. Even though these parameters would be determined, the geometry could not be defined until it is determined that whether those four atoms placed on the identical plane or not. If these are not on the identical plane, it must be decided which atom is away from the plane.
The geometry of 10 can finally be defined by the sixth parameter dihedral angle f. A dihedral angle is defined as an angle between the plane 1 including atom ABC and the plane 2 including BCD. It is necessary to introduce a new parameter dihedral angle, which is driven from neither the atomic distance nor the bond angle, to determine the structure of tetraatomic molecule. This fact suggests us two following things.
Figure 1.1 Dihedral angle
1) Molecules are three-dimensional, and are not always arranged on the two-dimensional sheet of a paper.
2) Molecular structures are not defined by atoms or by types of bonding in the molecules.
Needless to say, bond angles play the main role in determining molecular structures. Therefore atomic orbitals of a carbon atom, which is the factor controlling bond angles in organic compound, will be learned in the next section.