The ground and low-lying
excited states of four alternating Si/C
annulenes, H
n
Si
n/2C
n/2 with n =
4, 6, 8, and 10, have been investigated by ab initio molecular orbital
methods and compared to those of their all-carbon and all-silicon
analogues. In the ground state, all of the Si/C-mixed annulenes, except
for the largest 10-membered annulene (H10Si5C5), assume equal-bond-length structures by adopting a
closed-shell electronic structure in the possible highest symmetry.
For the largest H10Si5C5, the trend
of the bond delocalization still remains but the circular structure
is considerably distorted and nonplanar due to severe angle strain.
In the low-lying singlet (S1) and triplet (T1) states, the geometry of the compounds tends to be nonplanar as
the excitations produce silyl radical character. Relative energies
of the T1 and S1 states of the 6-membered ring,
compared to those of the respective ground states (S0),
are higher than those of the 4- and 8-membered rings, suggesting a
special stability for H6Si3C3. The
planar rhombus shape of the formally antiaromatic H4Si2C2 suggests that a synthetic effort is merited.
Bonding analyses are given to support the conclusions reached on the
basis of geometric structures and excited-state energetics.