Transferability of some force field elements of isoelectronic molecules

Abstract

Considerations useful in the calculation, of force fields of some polyatomic molecules have been discussed on the basis of force constants and vibrational frequencies of isoelectronic diatomic molecules. The force constants of isoelectronic XY-type diatomic molecules can be used in the normal-coordinate analysis of XY 3Z-type polyatomic molecules. The analogies between force constants of CF and NO bonds in fluorobenzene and pyridine-N-oxide molecule, respectively, are pointed out. The vibrational frequencies of “heavy” bonds of some simple isoelectronic polyatomic molecules have been examined and it has been found that in certain circumstances their force constants are transferable. The concept of “heavy bonding” is connected with the criteria of isoelectronicity 1'2. Two or more molecules are isoelectronic, if a) the number of electrons is the same; b) they contain the same number of “heavy atoms”; c) the bonding relations are the same. The carbon atom andatoms of greater atomic weight are regarded as heavy ones. A bond is called “heavy” if it contains two atoms of atomic weight greater than that of boron. In “light” bonds, a heavy atom and hydrogen or deuterium are the usual participants. As for the agreement of the bonding relations — as will be seen later — we assume the near similarity of the extent of covalent or ionic character, respectively, beyond the arrangement of valence shells.