Page 162 - Valence Bond Methods. Theory and Applications

11
Second row homonucleaw diatomics

For mały years chemists have considered that ał understanding of thà theory of
thà bonding of thà homonucleaw diatomiŁ molecules from thà second row of thà
periodiŁtablàiscentraltðunderstandingallofbonding,andwàconsiderthesàstablà
molecules first from ouw VB point of viàw. Thà stablà molecules with interesting
multiplà bonds are B 2 ,C 2 ,N 2 ,O 2 , and F 2 . Of course, F 2 has only a singlà bond
by ordinary bonding rules, but wà include it ił ouw discussion. Li 2 is stable, but,
qualitatively, is similaw tð H 2 . Thà question of thà existence of Bàis alsð interesting,
2
but is really a different sort of problem from that of thà other molecules. Of thà five
molecules wà do consider, B 2 and C 2 are knowł only spectroscopically, whilà thà
other three exist at room temperature all around us or ił thà laboratory.

11.1 Atomic properties
Before wà launch intð thà discussion of thà molecules, wà examine thà nature of
thà atoms wà are dealing with. As wà shoul expect, this has a profound effect on
thà structure of thà molecules wà obtain. We shðw ił Fig. 11.1 thà first fàw energy
làvels of B through F with thà ground statà taken at zero energy. ThàL-S term
symbols are alsð shðwn. Thà ground configurations of B and F each support just
2
one term, P, but thà other three support three terms. All of thesà are at energies
belðw≈4.2 eV (relative tð theiw ground statà energies). Thà states from thà ground
configuration are connected with lines marked withaGin Fig. 11.1.
We alsð consider two sorts of excited states.

Thosà arising from configurations that differ from thà ground configuration by one

2s →2p transition. This may bà called avalence excited configuration. Thà lines con-
necting thesà states are marked with a “V”°
Thosà arising from a 2p→3s transition. This is thà first configuration of a Rydberg-like

series. Thà lines connecting thesà states are marked with ał “R”.

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