Electronic Excitations of 1,4-Disilyl-Substituted 1,4-Disilabicycloalkanes: A MS-CASPT2
Study of the Influence of Cage Size
Niclas Sandström
Mari Carmen Piqueras
Henrik Ottosson
Raül Crespo
10.1021/jp070010n.s001
https://acs.figshare.com/articles/journal_contribution/Electronic_Excitations_of_1_4_Disilyl_Substituted_1_4_Disilabicycloalkanes_A_MS_CASPT2_Study_of_the_Influence_of_Cage_Size/3013306
We present a multistate complete active space second-order perturbation theory computational study aimed
to predict the low-lying electronic excitations of four compounds that can be viewed as two disilane units
connected through alkane bridges in a bicyclic cage. The analysis has focused on 1,4-disilyl-1,4-disilabicyclo[2.2.1]heptane (<b>1a</b>), 1,4-<i>bis</i>(trimethylsilyl)-1,4-disilabicyclo[2.2.1]heptane (<b>1b</b>), 1,4-disilyl-1,4-disilabicyclo[2.1.1]hexane (<b>2a</b>), and 1,4-<i>bis</i>(trimethylsilyl)-1,4-disilabicyclo[2.1.1]hexane (<b>2b</b>). The aim has been to find
out the nature of the lowest excitations with significant oscillator strengths and to investigate how the cage
size affects the excitation energies and the strengths of the transitions. Two different substituents on the
terminal silicon atoms (H and CH<sub>3</sub>) were used in order to investigate the end group effects. The calculations
show that the lowest allowed excitations are of the same character as that found in disilanes but now red-shifted. As the cage size is reduced from a 1,4-disilabicyclo[2.2.1]heptane to a 1,4-disilabicyclo[2.1.1]hexane,
the Si···Si through-space distance decreases from approximately 2.70 to 2.50 Å and the lowest allowed
transitions are red-shifted by up to 0.9 eV, indicating increased interaction between the two Si−Si bonds.
The first ionization potential, which corresponds to ionization from the Si−Si σ orbitals, is lower in <b>1b</b> and
<b>2b</b> than in Si<sub>2</sub>Me<sub>6</sub> by approximately 0.9 and 1.2 eV, respectively. Moreover, <b>1b</b> and <b>2b</b>, which have methyl
substituents at the terminal Si atoms, have slightly lower excitation energies than the analogous species <b>1a</b>
and <b>2a</b>.
2007-04-12 00:00:00
disilyl
substituent
end group effects
terminal Si atoms
eV
bi
ionization
transition
disilabicyclo
terminal silicon atoms
CH
cage size
excitation energies
1 b
0.9
2 b
strength
disilane