2.2 Chemical Bonds
21
Table 2.4 NLMO representation of water molecule (starting from the CMO’s obtained by
DFT/B3LYP/6-31G**)
NLMO No.
Occupancy
% from the parent NBO (%)
Characteristics
1
2.00000
99.9516
σ(O–H)
Ψ = 0.8586(sp 3.45 ) O +
0.5122s H
2
2.00000
99.9516
σ(O–H)
Ψ = 0.8586(sp 3.45 ) O +
0.5122s H
3
2.00000
99.8547
σ(lone pair at O)
Ψ = 0.9993(sp 0.80 ) O +
0.0270(sp 0.49 ) H
4
2.00000
99.8949
π(lone pair at O)
Ψ = p O
5
2.00000
99.9958
Core(at O)
Ψ = s O
π(lone pair at O) -7.868 eV
σ(lone pair at O) -16.705 eV
σ(O-H) -19.286 eV
σ*(O-H) 12.866 eV
σ(O-H) -19.286 eV
σ*(O-H) 12.866 eV
Fig. 2.21 Two bonding, two lone pairs, and two antibonding NBO patterns of water molecule with
orbital energies calculated by DFT/B3LYP/6-31G** (sequentially comparable with Table 2.3)
many chemical bonds, however, the experimental measurement of bond energy
becomes rather difficult. On the other hand, the theoretical enumeration of bond
dissociation energies is yet possible as far as the target bond is clear.
Let us consider the energy of the bond A–B between two moieties A and B in a
molecule. The most formal and straightforward definition of the bond energy E(A–B)
is defined by the dissociation energy, that is,