80
2 Actual Potentials of Theoretical Chemistry: What Can Be Obtained
(a)
R
H
H
H
BH 3
R
H
H
BH 2
H
H
Borane HOMO
Borane LUMO
1
1
2
2
1-Butene (R = C 2 H 5 )
HOMO
1-Butene (R = C 2 H 5 )
LUMO
L
L
(b)
Fig. 2.72 a Hydroboration reaction between non-symmetric olefin and borane and b, c HOMOLUMO interactions between 1-butene (R = C 2 H 5 ) and borane based on the calculation by the
HF/3-21G method. L signifies the large lobe
Furthermore, hydroboration (Fig. 2.72a) is another example of regioselectivity,
in which the HOMO lobe of 1-butene becomes the largest at the end-carbon (C 1 ) and
contributes to the interaction with the empty p-AO of B in borane resulting in C 1 –H
bond as shown in Fig. 2.72b. Successive interaction between the borane HOMO (one
of the triply degenerated) lobe and the LUMO lobe of 1-butene at C 2 promotes the
formation of C 2 –B bond. Chemical reaction accompanied with stereoselection is also
possibly interpreted by orbital-interaction examination. One of the typical examples
would be classified into so-called S N 2 reaction of methyl bromide with OH
− accompanied with Walden inversion as seen in Fig. 2.73a. In this reaction, the HOMO of
OH
− attacks the larger LUMO lobe stereoselectively from the backside direction
of methyl bromide at the opposite side of Br as seen in Fig. 2.73b. A concomitant
elimination of Br
− occurs with accompanying of the inversion of ABC atoms named
by Walden inversion giving enantiomeric product as to other ABC substituents.
2.7.1.3 Radical Reactions
It is further noted that chemical reactions involving radical species give a certain
variation in orbital interactions but can be similarly understood. In Fig. 2.74 are shown
the MO energy diagram of a radical molecule and the ordinary closed-shell molecule,
for instance, in which the singly occupied MO (SOMO)-LUMO and the HOMOSOMO interactions possibly take place. However the SOMO-LUMO interaction is
the same with the α-HOMO and the α-LUMO interaction, whereas the HOMOSOMO one with the β-HOMO and the β-LUMO interaction in the DODS pictures
as in Fig. 2.75.
2 Actual Potentials of Theoretical Chemistry: What Can Be Obtained
(a)
R
H
H
H
BH 3
R
H
H
BH 2
H
H
Borane HOMO
Borane LUMO
1
1
2
2
1-Butene (R = C 2 H 5 )
HOMO
1-Butene (R = C 2 H 5 )
LUMO
L
L
(b)
Fig. 2.72 a Hydroboration reaction between non-symmetric olefin and borane and b, c HOMOLUMO interactions between 1-butene (R = C 2 H 5 ) and borane based on the calculation by the
HF/3-21G method. L signifies the large lobe
Furthermore, hydroboration (Fig. 2.72a) is another example of regioselectivity,
in which the HOMO lobe of 1-butene becomes the largest at the end-carbon (C 1 ) and
contributes to the interaction with the empty p-AO of B in borane resulting in C 1 –H
bond as shown in Fig. 2.72b. Successive interaction between the borane HOMO (one
of the triply degenerated) lobe and the LUMO lobe of 1-butene at C 2 promotes the
formation of C 2 –B bond. Chemical reaction accompanied with stereoselection is also
possibly interpreted by orbital-interaction examination. One of the typical examples
would be classified into so-called S N 2 reaction of methyl bromide with OH
− accompanied with Walden inversion as seen in Fig. 2.73a. In this reaction, the HOMO of
OH
− attacks the larger LUMO lobe stereoselectively from the backside direction
of methyl bromide at the opposite side of Br as seen in Fig. 2.73b. A concomitant
elimination of Br
− occurs with accompanying of the inversion of ABC atoms named
by Walden inversion giving enantiomeric product as to other ABC substituents.
2.7.1.3 Radical Reactions
It is further noted that chemical reactions involving radical species give a certain
variation in orbital interactions but can be similarly understood. In Fig. 2.74 are shown
the MO energy diagram of a radical molecule and the ordinary closed-shell molecule,
for instance, in which the singly occupied MO (SOMO)-LUMO and the HOMOSOMO interactions possibly take place. However the SOMO-LUMO interaction is
the same with the α-HOMO and the α-LUMO interaction, whereas the HOMOSOMO one with the β-HOMO and the β-LUMO interaction in the DODS pictures
as in Fig. 2.75.