2.7 Chemical Reactions
85
the negative NICS (nuclear-independent chemical shift: see Sect. 2.8.2) value imply
the aromatic characteristics of the TS molecule. There have also been developed a
couple of automated algorithms for the IRC search such as, for instance, the “anharmonic downward distortion following” (ADDF) (Ohno and Maeda 2004) and the
“artificial force induced reaction” (AFIR) (Maeda et al. 2015) methods. Once such
the reaction path on the PES connecting the initial local minimum (reactant), TS,
and the final local minimum (product) is established, the reaction rate constant for
the corresponding elementary reaction can be evaluated by the transition-state theory
(Fernández-Ramos et al. 2006).
2.8 Molecular Design
This section actually shows a couple of examples of molecular design toward
extracting intriguing electronic properties from molecules and polymers. Such material is possibly useful to fabricate molecular devices. The molecular design introduced here utilizes controlling of spin alignment, band-gap value, and fluorescence
efficiency.
2.8.1 High-Spin Molecules
Most of the electronic structures of organic molecules favor the lowest possible spin
multiplicity as the ground state due to the Pauli principle. For instance, molecules
consisting of even numbers of electrons usually become energetically most stable
(ground state) at the singlet spin state, and of odd numbers at the doublet spin state.
Hence it is rather unusual for organic molecules to have high-spin state (see, e.g.,
Fig. 2.80) as the ground state. In other words, ferromagnetic property hardly appears
in organic molecules or polymers except for those containing inorganic elements
with d or f electrons acting as the spin centers. In this sense, theoretical design of
small organic molecules with a stable high-spin state, such as triplet state or higher
ones, is intriguing and rather challenging as the starting point to explore organic
oligomers and/or polymers with higher spin state than the singlet state.
It would be useful to point out here that there are simple rules to predict the stable
high-spin state for non-Kekulé-type molecules that cannot be expressed by classical
Kekulé structures. In alternant hydrocarons let us label the conjugated atoms as
starred and unstarred ones alternatively as shown in Fig. 2.81 so that no two atoms
with the same labels are directly linked. Putting the numbers of starred and unstarred
carbon atoms in an alternant hydrocarbon molecule as n* and n, respectively, it can
be predicted that this molecule has (n* − n) non-bonding MO’s (NBMO’s) and
(n* − n) unpaired electrons as well (Dewar 1969). The total spin number S of the
molecules in Fig. 2.81 is thus expressed by
85
the negative NICS (nuclear-independent chemical shift: see Sect. 2.8.2) value imply
the aromatic characteristics of the TS molecule. There have also been developed a
couple of automated algorithms for the IRC search such as, for instance, the “anharmonic downward distortion following” (ADDF) (Ohno and Maeda 2004) and the
“artificial force induced reaction” (AFIR) (Maeda et al. 2015) methods. Once such
the reaction path on the PES connecting the initial local minimum (reactant), TS,
and the final local minimum (product) is established, the reaction rate constant for
the corresponding elementary reaction can be evaluated by the transition-state theory
(Fernández-Ramos et al. 2006).
2.8 Molecular Design
This section actually shows a couple of examples of molecular design toward
extracting intriguing electronic properties from molecules and polymers. Such material is possibly useful to fabricate molecular devices. The molecular design introduced here utilizes controlling of spin alignment, band-gap value, and fluorescence
efficiency.
2.8.1 High-Spin Molecules
Most of the electronic structures of organic molecules favor the lowest possible spin
multiplicity as the ground state due to the Pauli principle. For instance, molecules
consisting of even numbers of electrons usually become energetically most stable
(ground state) at the singlet spin state, and of odd numbers at the doublet spin state.
Hence it is rather unusual for organic molecules to have high-spin state (see, e.g.,
Fig. 2.80) as the ground state. In other words, ferromagnetic property hardly appears
in organic molecules or polymers except for those containing inorganic elements
with d or f electrons acting as the spin centers. In this sense, theoretical design of
small organic molecules with a stable high-spin state, such as triplet state or higher
ones, is intriguing and rather challenging as the starting point to explore organic
oligomers and/or polymers with higher spin state than the singlet state.
It would be useful to point out here that there are simple rules to predict the stable
high-spin state for non-Kekulé-type molecules that cannot be expressed by classical
Kekulé structures. In alternant hydrocarons let us label the conjugated atoms as
starred and unstarred ones alternatively as shown in Fig. 2.81 so that no two atoms
with the same labels are directly linked. Putting the numbers of starred and unstarred
carbon atoms in an alternant hydrocarbon molecule as n* and n, respectively, it can
be predicted that this molecule has (n* − n) non-bonding MO’s (NBMO’s) and
(n* − n) unpaired electrons as well (Dewar 1969). The total spin number S of the
molecules in Fig. 2.81 is thus expressed by