The Transoid, Ortho, and Gauche Conformers of Decamethyl-n-tetrasilane, n-Si4Me10: Electronic Transitions in the Multistate Complete Active Space Second-Order Perturbation Theory Description

Abstract

Multistate complete active space second-order perturbation theory (MS-CASPT2) is used to improve earlier descriptions of the low-energy valence excited states of the transoid, ortho, and gauche conformers of decamethyl-n-tetrasilane, n-Si4Me10, using a generally contracted basis set of atomic natural orbitals (ANOs) at a ground-state geometry optimized in the second-order Møller−Plesset perturbation theory (MP2) approximation with Dunning's correlation consistent triple-ζ basis set (cc-pVTZ) on the silicon atoms and the 6-31G* and 6-31G basis sets on the carbon and hydrogen atoms, respectively. Relative energies, relative free energies, and mole fractions of the transoid, ortho, and gauche conformers are reported at various temperatures. CASPT2 ionization potentials for the three conformers are calculated and found to agree with photoelectron spectra. The first eight valence excited states of each conformer are computed and analyzed in terms of natural orbitals. Most of the important excitations are due to electron promotions from the σ1 HOMO orbital. However, in the ortho and gauche conformers, promotions from the σ2 HOMO-1 orbital play a significant and previously unrecognized role. The MS-CASPT2 simulation of the low-energy region of the absorption spectra of the transoid, ortho, and gauche conformers agrees with the UV absorption spectrum of the transoid conformer of n-Si4Me10 and with the conformationally constrained models of the ortho and gauche tetrasilanes, MeSi[(CH2)5]2Si−Si[(CH2)5]2SiMe and (SiMe2)4(CH2)3, respectively. The UV absorption spectra of n-Si4Me10 conformer equilibrium mixtures at room temperature and 77 K are interpreted. An analysis of the dependence of the nature of electronic excitation on the skeletal dihedral angle confirms the main features of the previously developed picture, which described it in terms of avoided crossings of both B and A symmetry states. These cause the states to interchange their σσ* and σπ* character upon going between the two planar limits, syn and anti. However, the new results show that at small dihedral angles the situation is more complicated because of additional avoided crossings.