ABSTRACT
As represented by carbon, the second row atoms have played a central role in chemistry and provided a variety of compounds. The heavier atoms contain a large number of electrons, for which relativistic effects become also important. Progress in theoretical methods, efficient computer programs and powerful computers have greatly advanced the scope of applicability of ab initio calculations. However, it is important to have a simple conceptual framework before extensive calculations. Because of the remarkable ability of carbon to form hybrid orbitals, the tetrahedral structure of methane is most fundamental in carbon chemistry. Polyhedral carbon compounds such as tetrahedrane, prismane and cubane have long been interesting synthetic targets because of their unique properties and aesthetic appeal due to high symmetry. These are highly strained, as is apparent from their carbon bond angles that deviate greatly from the normal tetrahedral value of 109.5°.
