Understanding a Bisferrocene Molecular QCA Wire - Field-Coupled Nanocomputing: Paradigms, Progress, and Perspectives - page 307

Many molecules have been presented and analyzed. Most of them are ideal
[ 5 - 7 , 10 , 11 , 16 , 17 , 24 ] and their functionalities have been tested via computer
simulation but never synthesized. Others, instead, are real molecules synthesized
ad hoc for QCA purposes [ 8 , 9 , 18 , 19 , 25 , 26 ] and for some of them even early
preliminary experiments have been carried out.
Fig. 4. The diallyl butane molecule: (A) structure, (B) scheme and (C)-(E) orbital
localization.
The first molecule which has been proposed for QCA computing is the diallyl
butane [ 7 ]. As shown in Fig. 4 , it consists of two allyl groups connected by a
butane bridge. The most suitable form for the molecule is the cationic one, where
a positive charge is free to move inside the molecule. The corresponding unpaired
electron can occupy one of the opposite allyl end-groups, which represent the
dots (circled in Fig. 4 (A) and sketched in Fig. 4 (B)); the tunneling path between
these redox centers is consequently given by the butane bridge. When the charge
tunnels from one end to the other, a different charge configuration of the molecule
is obtained and a different highest occupied molecular orbital (HOMO) is realized
(Fig. 4 (C)-(E)).
A molecular orbital is an eigenfunction of the Hamiltonian operator for a
molecular system, corresponding to a determined value of energy (eigenvalue)
of the molecule. According to quantum mechanics, this function is related to
the spatial probability of finding an electron in a specific region of space with
that energy. Thus, from a molecular orbital the most probable location of an
electron can be evaluated. Moreover, each molecular orbital can be occupied
by two electrons with opposite spin and, in general, all the electrons tend to
arrange themselves in order to fill the orbitals starting from the one with the
lowest energy. In this sense, the HOMO is the last energy level occupied by the
available electrons and it has the highest energy.
In the diallyl butane cation an electron from the HOMO is removed, conse-
quently the HOMO represents the localization of the unpaired electron. The pos-
sible HOMO conformations are shown in Fig. 4 (C)-(E): in Fig. 4 (C) the HOMO
is symmetrically delocalized between the two allyl groups favoring the electron
occupation neither of the top nor of the bottom group: it corresponds to an
undefined state. In Fig. 4 (D) and (E) instead, the HOMO (and thus the charge)
is localized on one of the two dots, so these configurations could represent the
logic states 1 and 0.