2.2 Chemical Bonds
17
HOMO -8.641 eV
HOMO-1 -8.655 eV
HOMO -6.250 eV
HOMO-1 -8.719 eV
(a)
(b)
Fig. 2.15 Selected CMO patterns of a butane and b butadiene molecules with their orbital energies
calculated by DFT/B3LYP/6-31G** after the structural optimization
et al. 1988). For instance, NBO’s of butane and butadiene are shown in Fig. 2.16, by
which specific bond pictures become somewhat clearer than those by the CMO’s.
That is, the two bonding NBO’s of butane in Fig. 2.16a are doubly degenerated and
mainly localized between the 3C and the attached two H atoms, respectively, representing two σ(C–H) bonds. There are two more degenerate NBO’s localized between
2C and the attached two H atoms, which are omitted here. The two bonding NBO’s
(a)
HOMO NBO -12.890 eV
HOMO NBO -12.890 eV
(b)
HOMO NBO -7.132 eV
HOMO NBO -7.133 eV
1
2
3
4
1
2
3
4
Fig. 2.16 Selected degenerated NBO patterns of a butane and b butadiene molecules with their
orbital energies calculated by DFT/B3LYP/6-31G** after the structural optimization. Numberings
of carbon atoms are also indicated