UV-Visible and Infrared Spectroscopic Methods in Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 196

Another type of molecular orbitals (the p orbital) may be formed from two
p-orbitals by a lateral overlap (i.e., shoulder-to-shoulder) as shown in Figure 8.7a.
This is termed a p bond (p-orbital) . Since bonds consisting of occupied p -orbitals
are weaker than s bonds, p -bonding between two atoms occurs only when a
s bond has already been established. Thus, p -bonding is generally found only
as a component of double and triple covalent bonds. Analogous to s bond,
a shoulder-to-shoulder overlap of two out-of-phase p orbitals will form a
p antibonding molecular orbital (p*) .
A hybrid orbital forms when two or more atomic orbitals of an isolated
atom mix. The shapes and orientations are different from those of atomic
orbitals in isolated atoms. This is called orbital hybridization . For example, since
carbon atoms (electron configuration: 1s 2 ,2s 2 ,2p 2 ) involved in double bonds
have only three bonding partners, they require only three hybrid orbitals to
contribute to three sigma bonds. A mixing of the 2s-orbital with two of the 2p
orbitals gives three sp 2 hybrid orbitals, leaving one of the p-orbitals unused.
Two sp 2 hybridized carbon atoms are then joined together by sigma and
p -bonds (a double bond) as shown in Figure 8.7b.

π∗ antibonding molecular orbital

(a)

2p atomic
orbital

2p atomic
orbital

+

π bonding molecular orbital

(b)

π bond

σ bond

p orbital

+

sp 2 hybrid orbital
sp 2 carbon atom sp 2 carbon atom

carbon-carbon double bond

Figure 8.7 Formation of p and p* bond: (a) The formation of p orbital from two p-orbitals and
(b) The formation of s- and p-molecular orbitals from two sp 2 hybridized carbon atoms

UV Absorption and Electronic Transitions

When molecules are exposed to UV or visible radiation, three types of electronic
transitions occur as a result of excitation of outer electrons in the valence shell
(i.e., the electrons that form bonds in a molecule). These include: (1) transitions
involving p , s , and n electrons, where n denotes a nonbonding orbital,