HOMO-LUMO gap and Charge transfer mechanism –
Molecular orbital analysis can provide very important information on
electronic structure. The ability of molecules and clusters to
participate in chemical reaction depends upon the energy gap which is
known as HOMO [Highest Occupied Molecular Orbital] LUMO [Lowest
Unoccupied Molecular Orbital] gap. The wide energy gap of clusters
also decides the optical polarizability of the molecule[4]. A large value of HOMO-LUMO gap always
indicate the closed shell electronic configuration[1-4] and the ability to take jump from lower
state to higher state. The Egap of
TM@Ge12 [TM = Co, Pd, Tc, and Zr] clusters are
summarizes in table 3.
The results show that doping of 4d transition metal like Tc, Zr, and Pd
have relatively large gap in compare to 3d transition metal Co. Here we
obtain 1.96eV [Tc@Ge12], 1.96eV
[Pd@Ge12], 1.86eV [Zr@Ge12], and
0.97eV [Co@Ge12] HOMO-LUMO gap for the most stable
clusters. The stability of these clusters can be defined using closed
shell electronic configuration and the contribution of π and σ bond. The
energy gap of Tc and Pd doped germanium cluster is biggest among those
of considered groups. On the off chance that we see the enhanced
geometries of Tc and Pd doped germanium confine clusters, we found that
both structure are impeccable hexagonal prism closed shell structure. It
can likewise be finished up by the figure that there is π-π bond
arrangement at the focal point of the HP ring and σ bond is framing at
the ring side means among the germanium atoms. So also, in the Pd doped
germanium cluster display 58 valence electron which is the sign of magic
number. For Zr@Ge12 cluster, there are six down spin
LUMO states of Ge12 confine that impeccably connect
covalently with valence state electrons which is fundamentally the same
as past examination by Vijay kumar et al. [19].
For our case the 4 valence electrons [4d25s2] of Zr totally share bonding with germanium
enclosure and give huge HOMO-LUMO hole. It additionally shows that the
HOMO and LUMO is roughly confined on the whole molecule. Because of this
enormous energy gap, LUMO can scarcely gain electron from closed shell
HOMO. So the enormous estimation of energy gap show lower reactivity in
compound and photochemical procedure with electron move[59]. Based on above investigation, the cluster
with huge energy can gap seen as building blocks of the novel materials.
In the next part of discussion, we describe the charge transfer
mechanism by calculating the natural bond orbital analysis[43-44]. Since, among all the TM metals, studied
in this work, only Pd (2.2) is more electronegative than germanium
(2.01) on Pauling scale. It means the charge will always transfer from
Pd to Ge atoms and in other TM atom; the charge will transfer from Ge
atom to TM (Tc, Zr, and Co) atoms. The natural population analysis
precisely determined the distribution of electrons in various sub shell
of their atomic orbitals.
It is worth mentioning that the many body system properties like
electronic geometry, dipole moment, polarizability are influenced by the
atomic charges [60]. It can be seen from table 4,
the most electronegative charge of -1.883e accumulated for Tc, whereas
all germanium atoms gained positive charge. The electrostatic point of
view tells us that most electronegative atoms have tendency to donate
electron and electropositive atoms have inclination to accept electrons,
it means here charge is transferring from Tc atom to germanium cage.
Similarly, Pd atom plays a donor role in
Pd@Ge12 cluster. In the
Zr@Ge12 system, the charge accumulated by the
Zr atom is -2.871 which again donate the electrons to
Ge12 cage. The case of Co doped
Ge12 cage is quite different. Here the charge
is transferring from Ge12 cage to TM metal Co
atom.