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